SQL is a Standard Language for Accessing and Manipulating Databases

7/29/2019 SQL is a Standard Language for Accessing and Manipulating Databases

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What is SQL?

SQL stands for Structured Query Language SQL lets you access and manipulate databases SQL is an ANSI (American National Standards Institute) standard

What Can SQL do?

SQL can execute queries against a database SQL can retrieve data from a database

SQL can insert records in a database SQL can update records in a database SQL can delete records from a database SQL can create new databases SQL can create new tables in a database SQL can create stored procedures in a database SQL can create views in a database SQL can set permissions on tables, procedures, and views

SQL is a Standard - BUT....

Although SQL is an ANSI (American National Standards Institute) standard, there are manydifferent versions of the SQL language.

However, to be compliant with the ANSI standard, they all support at least the major commands(such as SELECT, UPDATE, DELETE, INSERT, WHERE) in a similar manner.

Note: Most of the SQL database programs also have their own proprietary extensions in additionto the SQL standard!

Using SQL in Your Web Site

To build a web site that shows some data from a database, you will need the following:

An RDBMS database program (i.e. MS Access, SQL Server, MySQL)


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A server-side scripting language, like PHP or ASP SQL HTML / CSS

RDBMS

RDBMS stands for Relational Database Management System.

RDBMS is the basis for SQL, and for all modern database systems like MS SQL Server, IBM

DB2, Oracle, MySQL, and Microsoft Access.

The data in RDBMS is stored in database objects called tables.

A table is a collection of related data entries and it consists of columns and rows.

SQL DML and DDL

SQL can be divided into two parts: The Data Manipulation Language (DML) and the Data

Definition Language (DDL).

The query and update commands form the DML part of SQL:

SELECT - extracts data from a database UPDATE - updates data in a database

DELETE - deletes data from a database INSERT INTO - inserts new data into a database

The DDL part of SQL permits database tables to be created or deleted. It also define indexes(keys), specify links between tables, and impose constraints between tables. The most important

DDL statements in SQL are:

CREATE DATABASE - creates a new database ALTER DATABASE - modifies a database CREATE TABLE - creates a new table ALTER TABLE - modifies a table

DROP TABLE - deletes a table CREATE INDEX - creates an index (search key) DROP INDEX - deletes an index

The SQL SELECT Statement

The SELECT statement is used to select data from a database.


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The result is stored in a result table, called the result-set.

SQL SELECT Syntax

SELECT column_name(s)

FROM table_name

and

SELECT * FROM table_name;

Eg

SELECT * FROM employee

The SQL SELECT DISTINCT Statement

In a table, some of the columns may contain duplicate values. This is not a problem, however,

sometimes you will want to list only the different (distinct) values in a table.

The DISTINCT keyword can be used to return only distinct (different) values.

SQL SELECT DISTINCT Syntax

SELECT DISTINCT column_name(s)

FROM table_name

Eg

SELECT DISTINCT(dept_no) FROM employee;

The WHERE clause is used to filter records.

The WHERE Clause

The WHERE clause is used to extract only those records that fulfill a specified criterion.

SQL WHERE Syntax


FROM table_name

WHERE column_name operator value

SELECT dept_no FROM employee WHERE emp_id=12;


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This is correct:

SELECT * FROM Persons WHERE FirstName='Tove'

This is wrong:

SELECT * FROM Persons WHERE FirstName=Tove

This is correct:

SELECT * FROM Persons WHERE Year=1965

This is wrong:

SELECT * FROM Persons WHERE Year='1965'

Operators Allowed in the WHERE Clause

With the WHERE clause, the following operators can be used:

Operator Description

= Equal

Not equal

> Greater than

< Less than

>= Greater than or equal


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WHERE FirstName='Tove'

AND LastName='Svendson'


WHERE FirstName='Tove'

OR FirstName='Ola'

Combining AND & OR

You can also combine AND and OR (use parenthesis to form complex expressions).

Now we want to select only the persons with the last name equal to "Svendson" AND the firstname equal to "Tove" OR to "Ola":

We use the following SELECT statement:

SELECT * FROM Persons WHERELastName='Svendson'

AND (FirstName='Tove' OR FirstName='Ola')

The ORDER BY keyword is used to sort the result-set.

The ORDER BY Keyword

The ORDER BY keyword is used to sort the result-set by a specified column.

The ORDER BY keyword sort the records in ascending order by default.

If you want to sort the records in a descending order, you can use the DESC keyword.

SQL ORDER BY Syntax


FROM table_name

ORDER BY column_name(s) ASC|DESC

SELECT * FROM employee ORDER BY LastName

ORDER BY DESC Example

Now we want to select all the persons from the table above, however, we want to sort the personsdescending by their last name.


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SELECT * FROM employeeORDER BY LastName DESC

The INSERT INTO statement is used to insert new records in a table.

The INSERT INTO Statement

The INSERT INTO statement is used to insert a new row in a table.

SQL INSERT INTO Syntax

It is possible to write the INSERT INTO statement in two forms.

The first form doesn't specify the column names where the data will be inserted, only their

values:

INSERT INTO table_name

VALUES (value1, value2, value3,...)

The second form specifies both the column names and the values to be inserted:

INSERT INTO table_name (column1, column2, column3,...)

VALUES (value1, value2, value3,...)

INSERT INTO employee VALUES (4,'Nilsen', 'Johan', 'Bakken 2', 'Stavanger')

Insert Data Only in Specified Columns

It is also possible to only add data in specific columns.

The following SQL statement will add a new row, but only add data in the "P_Id", "LastName"and the "FirstName" columns:

INSERT INTO employee (P_Id, LastName, FirstName)

VALUES (5, 'Tjessem', 'Jakob')


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The UPDATE statement is used to update records in a table.

The UPDATE Statement

The UPDATE statement is used to update existing records in a table.

SQL UPDATE Syntax

UPDATE table_name

SET column1=value, column2=value2,...

WHERE some_column=some_value

Note: Notice the WHERE clause in the UPDATE syntax. The WHERE clause specifies whichrecord or records that should be updated. If you omit the WHERE clause, all records will be

updated!

UPDATE employee

SET Address='chembur', City='mumbai'

WHERE LastName='Ray' AND FirstName='Abhishak';

SQL UPDATE Warning

Be careful when updating records. If we had omitted the WHERE clause in the example above,like this:

UPDATE employee

SET Address='chembur', City='mumbai'

All the records of Address,City will be updated with chembur,mumbai

The DELETE statement is used to delete records in a table.

The DELETE Statement

The DELETE statement is used to delete rows in a table.

SQL DELETE Syntax

DELETE FROM table_name


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WHERE some_column=some_value

Note: Notice the WHERE clause in the DELETE syntax. The WHERE clause specifies which

record or records that should be deleted. If you omit the WHERE clause, all records will be

deleted!

DELETE FROM employee

WHERE LastName='Tjessem' AND FirstName='Jakob'

Delete All Rows

It is possible to delete all rows in a table without deleting the table. This means that the tablestructure, attributes, and indexes will be intact:

DELETE FROM table_name

or

DELETE * FROM table_name

Note: Be very careful when deleting records. You cannot undo this statement!

The LIKE operator is used in a WHERE clause to search for a specified pattern in a column.

The LIKE Operator

The LIKE operator is used to search for a specified pattern in a column.

SQL LIKE Syntax


FROM table_name

WHERE column_name LIKE pattern

SELECT * FROM PersonsWHERE City LIKE 's%'

SELECT * FROM Persons

WHERE City LIKE '%s'


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WHERE City LIKE '%tav%'

NOT LIKE

SELECT * FROM PersonsWHERE City NOT LIKE '%tav%'

Using the _ Wildcard

Now we want to select the persons with a first name that starts with any character, followed by

"la" from the "Persons" table.


SELECT * FROM PersonsWHERE FirstName LIKE '_la'


WHERE LastName LIKE 'S_end_on'

The IN Operator

The IN operator allows you to specify multiple values in a WHERE clause.

SQL IN SyntaxSELECT column_name(s)

FROM table_name

WHERE column_name IN (value1,value2,...)

SELECT * FROM employeeWHERE LastName IN ('Hansen','Pettersen')

The BETWEEN operator is used in a WHERE clause to select a range of data between two

values.

The BETWEEN Operator

The BETWEEN operator selects a range of data between two values. The values can benumbers, text, or dates.


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SQL BETWEEN Syntax


FROM table_name

WHERE column_name

BETWEEN value1 AND value2

SELECT * FROM employeeWHERE LastName

BETWEEN 'Hansen' AND 'Pettersen'

NOT BETWEEN:


WHERE LastName

NOT BETWEEN 'Hansen' AND 'Pettersen'

SQL Alias

You can give a table or a column another name by using an alias. This can be a good thing to doif you have very long or complex table names or column names.

An alias name could be anything, but usually it is short.

SQL Alias Syntax for Tables


FROM table_name

AS alias_name

SELECT po.OrderID, p.LastName, p.FirstNameFROM Persons AS p

SQL joins are used to query data from two or more tables, based on a relationship betweencertain columns in these tables.

Different SQL JOINs

Before we continue with examples, we will list the types of JOIN you can use, and the

differences between them.

JOIN: Return rows when there is at least one match in both tables LEFT JOIN: Return all rows from the left table, even if there are no matches in the

right table

RIGHT JOIN: Return all rows from the right table, even if there are no matches in the


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left table

FULL JOIN: Return rows when there is a match in one of the tables

SQL INNER JOIN Keyword

The INNER JOIN keyword return rows when there is at least one match in both tables.

SQL INNER JOIN Syntax


FROM table_name1

INNER JOIN table_name2

ON table_name1.column_name=table_name2.column_name

PS: INNER JOIN is the same as JOIN.

SELECT s.LastName, s.FirstNameFROM student INNER JOIN Orders

ON s.S_Id=college.P_Id

ORDER BY s.LastName

SQL LEFT JOIN Keyword

The LEFT JOIN keyword returns all rows from the left table (table_name1), even if there are no

matches in the right table (table_name2).

SQL LEFT JOIN SyntaxSELECT column_name(s)

FROM table_name1

LEFT JOIN table_name2

ON table_name1.column_name=table_name2.column_name

PS: In some databases LEFT JOIN is called LEFT OUTER JOIN.

SELECT Persons.LastName, Persons.FirstName, Orders.OrderNoFROM Persons

LEFT JOIN OrdersON Persons.P_Id=Orders.P_Id

ORDER BY Persons.LastName

SQL RIGHT JOIN Keyword

The RIGHT JOIN keyword returns all the rows from the right table (table_name2), even if there


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are no matches in the left table (table_name1).

SQL RIGHT JOIN Syntax


FROM table_name1

RIGHT JOIN table_name2ON table_name1.column_name=table_name2.column_name

PS: In some databases RIGHT JOIN is called RIGHT OUTER JOIN.

SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo

FROM PersonsRIGHT JOIN Orders

ON Persons.P_Id=Orders.P_Id

ORDER BY Persons.LastName

The SQL UNION operator combines two or more SELECT statements.

The SQL UNION Operator

The UNION operator is used to combine the result-set of two or more SELECT statements.

Notice that each SELECT statement within the UNION must have the same number ofcolumns. The columns must also have similar data types. Also, the columns in each SELECT

statement must be in the same order.

SQL UNION Syntax

SELECT column_name(s) FROM table_name1

UNION


Note: The UNION operator selects only distinct values by default. To allow duplicate values,use UNION ALL.

SQL UNION ALL Syntax


UNION ALL


PS: The column names in the result-set of a UNION are always equal to the column names in


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the first SELECT statement in the UNION.

SELECT E_Name FROM Employees_NorwayUNION

SELECT E_Name FROM Employees_USA

SQL UNION ALL Example

Now we want to list all employees in Norway and USA:

SELECT E_Name FROM Employees_Norway

UNION ALL

SELECT E_Name FROM Employees_USA

The CREATE DATABASE Statement

The CREATE DATABASE statement is used to create a database.

SQL CREATE DATABASE Syntax

CREATE DATABASE database_name

CREATE DATABASE Example

Now we want to create a database called "my_db".

We use the following CREATE DATABASE statement:

CREATE DATABASE university

Database tables can be added with the CREATE TABLE statement.

The CREATE TABLE Statement

The CREATE TABLE statement is used to create a table in a database.

SQL CREATE TABLE Syntax

CREATE TABLE table_name

(

column_name1 data_type,


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....

)

The data type specifies what type of data the column can hold. For a complete reference of allthe data types available in MS Access, MySQL, and SQL Server, go to our completeData

Types reference.

CREATE TABLE Example

Now we want to create a table called "Persons" that contains five columns: P_Id, LastName,FirstName, Address, and City.

We use the following CREATE TABLE statement:

CREATE TABLE student

(

S_Id int,

LastName varchar(255),

FirstName varchar(255),

Address varchar(255),

City varchar(255)

)

The S_Id column is of type int and will hold a number. The LastName, FirstName, Address, andCity columns are of type varchar with a maximum length of 255 characters.

SQL Constraints

Constraints are used to limit the type of data that can go into a table.

Constraints can be specified when a table is created (with the CREATE TABLE statement) orafter the table is created (with the ALTER TABLE statement).

We will focus on the following constraints:

NOT NULL UNIQUE PRIMARY KEY
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FOREIGN KEY CHECK DEFAULT

By default, a table column can hold NULL values.

SQL NOT NULL Constraint

The NOT NULL constraint enforces a column to NOT accept NULL values.

The NOT NULL constraint enforces a field to always contain a value. This means that you

cannot insert a new record, or update a record without adding a value to this field.

The following SQL enforces the "P_Id" column and the "LastName" column to not accept

NULL values:


(

S_Id int NOT NULL,LastName varchar(255) NOT NULL,


Address varchar(255),City varchar(255)

)

SQL UNIQUE Constraint

The UNIQUE constraint uniquely identifies each record in a database table.

The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a

column or set of columns.

A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.

Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY

constraint per table.

SQL UNIQUE Constraint on CREATE TABLE

The following SQL creates a UNIQUE constraint on the "P_Id" column when the "Persons"

table is created:


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MySQL:

CREATE TABLE student(

S_Id int NOT NULL,

LastName varchar(255) NOT NULL,FirstName varchar(255),


City varchar(255),UNIQUE (S_Id)

)

SQL UNIQUE Constraint on ALTER TABLE

To create a UNIQUE constraint on the "S_Id" column when the table is already created, use the

following SQL:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE student ADD UNIQUE (S_Id)


ALTER TABLE studentADD CONSTRAINT uc_student UNIQUE (S_Id,LastName)

To DROP a UNIQUE Constraint

To drop a UNIQUE constraint, use the following SQL:

MySQL:

ALTER TABLE student DROP INDEX uc_student

SQL PRIMARY KEY Constraint Previous Next Chapter

SQL PRIMARY KEY Constraint
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The PRIMARY KEY constraint uniquely identifies each record in a database table.

Primary keys must contain unique values.

A primary key column cannot contain NULL values.

Each table should have a primary key, and each table can have only ONE primary key.

SQL PRIMARY KEY Constraint on CREATE TABLE

The following SQL creates a PRIMARY KEY on the "S_Id" column when the "student" table iscreated:

MySQL:


(

S_Id int NOT NULL,

LastName varchar(255) NOT NULL,



City varchar(255),

PRIMARY KEY (S_Id)

)



S_Id int NOT NULL,



City varchar(255),

CONSTRAINT pk_student PRIMARY KEY (S_Id,LastName)

)

SQL PRIMARY KEY Constraint on ALTER TABLE

To create a PRIMARY KEY constraint on the "S_Id" column when the table is already created,

use the following SQL:


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ALTER TABLE studentADD PRIMARY KEY (S_Id)

To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEYconstraint

on multiple columns, use the following SQL syntax:


ALTER TABLE student

ADD CONSTRAINT pk_student PRIMARY KEY (S_Id,LastName)

Note: If you use the ALTER TABLE statement to add a primary key, the primary key

column(s) must already have been declared to not contain NULL values (when the table wasfirst created).

To DROP a PRIMARY KEY Constraint

To drop a PRIMARY KEY constraint, use the following SQL:

MySQL:

ALTER TABLE studentDROP PRIMARY KEY

SQL FOREIGN KEY Constraint

A FOREIGN KEY in one table points to a PRIMARY KEY in another table.

Let's illustrate the foreign key with an example. Look at the following two tables:

The "student" table:

S_Id LastName FirstName Address City1 Ram dev mumbai 10 mumbai

2 Svendson Tove mumbai 23 mumbai

3 Pettersen Kari mumbai 20 mumbai

The "Exam_enrollment" table:


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Serial_no Examcode S_Id

1 77895 3

2 44678 3

3 22456 2

4 24562 1

Note that the "S_Id" column in the "Exam_enrollment" table points to the "S_Id" column in the"student" table.

The "S_Id" column in the "student" table is the PRIMARY KEY in the "student" table.

The "S_Id" column in the "student" table is a FOREIGN KEY in the " Exam_enrollment table.

The FOREIGN KEY constraint is used to prevent actions that would destroy links between

tables.

The FOREIGN KEY constraint also prevents that invalid data form being inserted into theforeign key column, because it has to be one of the values contained in the table it points to.

SQL FOREIGN KEY Constraint on CREATE TABLE

The following SQL creates a FOREIGN KEY on the "P_Id" column when the "Orders" table is

created:

MySQL:

CREATE TABLE Exam_enrollment

(serial int NOT NULL,

examcode int NOT NULL,

S_Id int,PRIMARY KEY (serial),

FOREIGN KEY (serial) REFERENCES student(S_Id)

)

To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constrainton multiple columns, use the following SQL syntax:


CREATE TABLE Orders

(

serial int NOT NULL,


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examcode int NOT NULL,

S_Id int,

PRIMARY KEY (serial),CONSTRAINT fk_ Exam_enrollment FOREIGN KEY (S_Id)

REFERENCES student(P_Id)

)

SQL FOREIGN KEY Constraint on ALTER TABLE

To create a FOREIGN KEY constraint on the "P_Id" column when the "Orders" table is already

created, use the following SQL:


ALTER TABLE Exam_enrollmentADD FOREIGN KEY (S_Id)

REFERENCES student(S_Id)

To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint

on multiple columns, use the following SQL syntax:


ALTER TABLE Orders

ADD CONSTRAINT fk_ Exam_enrollment

FOREIGN KEY (S_Id)REFERENCES student(S_Id)

To DROP a FOREIGN KEY Constraint

To drop a FOREIGN KEY constraint, use the following SQL:

MySQL:

ALTER TABLE Exam_enrollment

DROP FOREIGN KEY fk_ Exam_enrollment

SQL CHECK Constraint

The CHECK constraint is used to limit the value range that can be placed in a column.


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If you define a CHECK constraint on a single column it allows only certain values for this

column.

If you define a CHECK constraint on a table it can limit the values in certain columns based on

values in other columns in the row.

SQL CHECK Constraint on CREATE TABLE

The following SQL creates a CHECK constraint on the "P_Id" column when the "Persons" table

is created. The CHECK constraint specifies that the column "P_Id" must only include integers

greater than 0.

My SQL:

CREATE TABLE student(S_Id int NOT NULL,


FirstName varchar(255),Address varchar(255),

City varchar(255),

CHECK (S_Id>0)

)

To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiple

columns, use the following SQL syntax:



(S_Id int NOT NULL,



Address varchar(255),City varchar(255),

CONSTRAINT chk_student CHECK (S_Id>0 AND City='mumbai'))

SQL CHECK Constraint on ALTER TABLE

To create a CHECK constraint on the "P_Id" column when the table is already created, use the


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following SQL:


ALTER TABLE student

ADD CHECK (S_Id>0)

To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiplecolumns, use the following SQL syntax:


ALTER TABLE student

ADD CONSTRAINT chk_student CHECK (S_Id>0 AND City='mumbai')

SQL DEFAULT Constraint

The DEFAULT constraint is used to insert a default value into a column.

The default value will be added to all new records, if no other value is specified.

SQL DEFAULT Constraint on CREATE TABLE

The following SQL creates a DEFAULT constraint on the "City" column when the "student"table is created:

My SQL / SQL Server / Oracle / MS Access:


S_Id int NOT NULL,



City varchar(255) DEFAULT 'mumbai')

SQL DEFAULT Constraint on ALTER TABLE

To create a DEFAULT constraint on the "City" column when the table is already created, use


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the following SQL:

MySQL:

ALTER TABLE student ALTER City SET DEFAULT 'mumbai'

To DROP a DEFAULT Constraint

To drop a DEFAULT constraint, use the following SQL:

MySQL:

ALTER TABLE studentALTER City DROP DEFAULT

SQL CREATE INDEX Statement Previous Next Chapter

The CREATE INDEX statement is used to create indexes in tables.

Indexes allow the database application to find data fast; without reading the whole table.

Indexes

An index can be created in a table to find data more quickly and efficiently.

The users cannot see the indexes, they are just used to speed up searches/queries.

Note: Updating a table with indexes takes more time than updating a table without (because the

indexes also need an update). So you should only create indexes on columns (and tables) thatwill be frequently searched against.

SQL CREATE INDEX Syntax

Creates an index on a table. Duplicate values are allowed:

CREATE INDEX index_name
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ON table_name (column_name)

SQL CREATE UNIQUE INDEX Syntax

Creates a unique index on a table. Duplicate values are not allowed:

CREATE UNIQUE INDEX index_name

ON table_name (column_name)

Note: The syntax for creating indexes varies amongst different databases. Therefore: Check thesyntax for creating indexes in your database.

CREATE INDEX ExampleThe SQL statement below creates an index named "SIndex" on the "LastName" column in the

"Persons" table:

CREATE INDEX SIndex

ON student(LastName)

If you want to create an index on a combination of columns, you can list the column names

within the parentheses, separated by commas:

CREATE INDEX SIndex

ON student (LastName, FirstName)

Indexes, tables, and databases can easily be deleted/removed with the DROP statement.

The DROP INDEX Statement

The DROP INDEX statement is used to delete an index in a table.

DROP INDEX Syntax for MySQL:

ALTER TABLE table_name DROP INDEX index_name


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The DROP TABLE Statement

The DROP TABLE statement is used to delete a table.

DROP TABLE table_name

The DROP DATABASE Statement

The DROP DATABASE statement is used to delete a database.

DROP DATABASE database_name

The TRUNCATE TABLE StatementWhat if we only want to delete the data inside the table, and not the table itself?

Then, use the TRUNCATE TABLE statement:

TRUNCATE TABLE table_name

The ALTER TABLE Statement

The ALTER TABLE statement is used to add, delete, or modify columns in an existing table.

SQL ALTER TABLE Syntax

To add a column in a table, use the following syntax:

ALTER TABLE table_name

ADD column_name datatype

To delete a column in a table, use the following syntax (notice that some database systems don't

allow deleting a column):


DROP COLUMN column_name

To change the data type of a column in a table, use the following syntax:


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ALTER COLUMN column_name datatype

ALTER TABLE student

ALTER COLUMN DateOfBirth year

DROP COLUMN Example

Next, we want to delete the column named "DateOfBirth" in the "student" table.

We use the following SQL statement:

ALTER TABLE student

DROP COLUMN DateOfBirth

Auto-increment allows a unique number to be generated when a new record is inserted into a

table.

AUTO INCREMENT a Field

Very often we would like the value of the primary key field to be created automatically every

time a new record is inserted.

We would like to create an auto-increment field in a table.

Syntax for MySQL

The following SQL statement defines the "S_Id" column to be an auto-increment primary key

field in the "Persons" table:


S_Id int NOT NULL AUTO_INCREMENT,


FirstName varchar(255),Address varchar(255),

City varchar(255),

PRIMARY KEY (S_Id)


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)

MySQL uses the AUTO_INCREMENT keyword to perform an auto-increment feature.

By default, the starting value for AUTO_INCREMENT is 1, and it will increment by 1 for each

new record.

To let the AUTO_INCREMENT sequence start with another value, use the following SQLstatement:

ALTER TABLE student AUTO_INCREMENT=100

SQL Views Previous Next Chapter

A view is a virtual table.

This chapter shows how to create, update, and delete a view.

SQL CREATE VIEW Statement

In SQL, a view is a virtual table based on the result-set of an SQL statement.

A view contains rows and columns, just like a real table. The fields in a view are fields from

one or more real tables in the database.

You can add SQL functions, WHERE, and JOIN statements to a view and present the data as if

the data were coming from one single table.

SQL CREATE VIEW Syntax

CREATE VIEW view_name AS


FROM table_name

WHERE condition

Note: A view always shows up-to-date data! The database engine recreates the data, using theview's SQL statement, every time a user queries a view.
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SQL CREATE VIEW Examples

If you have the Northwind database you can see that it has several views installed by default.

The view "Current Product List" lists all active products (products that are not discontinued)from the "Products" table. The view is created with the following SQL:

CREATE VIEW [Current Product List] AS

SELECT ProductID,ProductName

FROM Products

WHERE Discontinued=No

We can query the view above as follows:

SELECT * FROM [Current Product List]

Another view in the Northwind sample database selects every product in the "Products" table

with a unit price higher than the average unit price:

CREATE VIEW [Products Above Average Price] AS

SELECT ProductName,UnitPrice

FROM Products

WHERE UnitPrice>(SELECT AVG(UnitPrice) FROM Products)


SELECT * FROM [Products Above Average Price]

Another view in the Northwind database calculates the total sale for each category in 1997.

Note that this view selects its data from another view called "Product Sales for 1997":

CREATE VIEW [Category Sales For 1997] AS

SELECT DISTINCT CategoryName,Sum(ProductSales) AS CategorySales

FROM [Product Sales for 1997]

GROUP BY CategoryName



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SELECT * FROM [Category Sales For 1997]

We can also add a condition to the query. Now we want to see the total sale only for the

category "Beverages":

SELECT * FROM [Category Sales For 1997]

WHERE CategoryName='Beverages'

SQL Updating a View

You can update a view by using the following syntax:

SQL CREATE OR REPLACE VIEW Syntax

CREATE OR REPLACE VIEW view_name AS


FROM table_name

WHERE condition

Now we want to add the "Category" column to the "Current Product List" view. We will update

the view with the following SQL:

CREATE VIEW [Current Product List] AS

SELECT ProductID,ProductName,Category

FROM Products

WHERE Discontinued=No

SQL Dropping a View

You can delete a view with the DROP VIEW command.

SQL DROP VIEW Syntax

DROP VIEW view_name

MySQL Date Functions


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The following table lists the most important built-in date functions in MySQL:

Function Description

NOW() Returns the current date and time

CURDATE() Returns the current date

CURTIME() Returns the current timeDATE() Extracts the date part of a date or date/time expression

EXTRACT() Returns a single part of a date/time

DATE_ADD() Adds a specified time interval to a date

DATE_SUB() Subtracts a specified time interval from a date

DATEDIFF() Returns the number of days between two dates

DATE_FORMAT() Displays date/time data in different formats

Definition and Usage

NOW() returns the current date and time.

Syntax

NOW()

Example

The following SELECT statement:

SELECT NOW(),CURDATE(),CURTIME()

SQL IS NULL

How do we select only the records with NULL values in the "Address" column?

We will have to use the IS NULL operator:

SELECT LastName,FirstName,Address FROM studentWHERE Address IS NULL

SQL IS NOT NULL

How do we select only the records with no NULL values in the "Address" column?
http://www.w3schools.com/sql/func_now.asphttp://www.w3schools.com/sql/func_curdate.asphttp://www.w3schools.com/sql/func_curdate.asphttp://www.w3schools.com/sql/func_curtime.asphttp://www.w3schools.com/sql/func_curtime.asphttp://www.w3schools.com/sql/func_date.asphttp://www.w3schools.com/sql/func_date.asphttp://www.w3schools.com/sql/func_extract.asphttp://www.w3schools.com/sql/func_extract.asphttp://www.w3schools.com/sql/func_date_add.asphttp://www.w3schools.com/sql/func_date_add.asphttp://www.w3schools.com/sql/func_date_sub.asphttp://www.w3schools.com/sql/func_date_sub.asphttp://www.w3schools.com/sql/func_datediff_mysql.asphttp://www.w3schools.com/sql/func_datediff_mysql.asphttp://www.w3schools.com/sql/func_date_format.asphttp://www.w3schools.com/sql/func_date_format.asphttp://www.w3schools.com/sql/func_date_format.asphttp://www.w3schools.com/sql/func_datediff_mysql.asphttp://www.w3schools.com/sql/func_date_sub.asphttp://www.w3schools.com/sql/func_date_add.asphttp://www.w3schools.com/sql/func_extract.asphttp://www.w3schools.com/sql/func_date.asphttp://www.w3schools.com/sql/func_curtime.asphttp://www.w3schools.com/sql/func_curdate.asphttp://www.w3schools.com/sql/func_now.asp


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We will have to use the IS NOT NULL operator:

SELECT LastName,FirstName,Address FROM studentWHERE Address IS NOT NULL

MySQL Data Types

In MySQL there are three main types : text, number, and Date/Time types.

Text types:

Data type Description

CHAR(size) Holds a fixed length string (can contain letters, numbers, and special

characters). The fixed size is specified in parenthesis. Can store up to 255

characters

VARCHAR(size) Holds a variable length string (can contain letters, numbers, and special

characters). The maximum size is specified in parenthesis. Can store up to255 characters. Note: If you put a greater value than 255 it will be converted

to a TEXT type

TINYTEXT Holds a string with a maximum length of 255 characters

TEXT Holds a string with a maximum length of 65,535 characters

BLOB For BLOBs (Binary Large OBjects). Holds up to 65,535 bytes of data

MEDIUMTEXT Holds a string with a maximum length of 16,777,215 characters

MEDIUMBLOB For BLOBs (Binary Large OBjects). Holds up to 16,777,215 bytes of data

LONGTEXT Holds a string with a maximum length of 4,294,967,295 characters

LONGBLOB For BLOBs (Binary Large OBjects). Holds up to 4,294,967,295 bytes ofdata

ENUM(x,y,z,etc.) Let you enter a list of possible values. You can list up to 65535 values in anENUM list. If a value is inserted that is not in the list, a blank value will be

inserted.

Note: The values are sorted in the order you enter them.

You enter the possible values in this format: ENUM('X','Y','Z')

SET Similar to ENUM except that SET may contain up to 64 list items and can

store more than one choice

Number types:


TINYINT(size) -128 to 127 normal. 0 to 255 UNSIGNED*. The maximum number of

digits may be specified in parenthesis

SMALLINT(size) -32768 to 32767 normal. 0 to 65535 UNSIGNED*. The maximum number


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of digits may be specified in parenthesis

MEDIUMINT(size) -8388608 to 8388607 normal. 0 to 16777215 UNSIGNED*. The maximum

number of digits may be specified in parenthesis

INT(size) -2147483648 to 2147483647 normal. 0 to 4294967295 UNSIGNED*. The

maximum number of digits may be specified in parenthesis

BIGINT(size) -9223372036854775808 to 9223372036854775807 normal. 0 to18446744073709551615 UNSIGNED*. The maximum number of digitsmay be specified in parenthesis

FLOAT(size,d) A small number with a floating decimal point. The maximum number ofdigits may be specified in the size parameter. The maximum number of

digits to the right of the decimal point is specified in the d parameter

DOUBLE(size,d) A large number with a floating decimal point. The maximum number of

digits may be specified in the size parameter. The maximum number of

digits to the right of the decimal point is specified in the d parameter

DECIMAL(size,d) A DOUBLE stored as a string , allowing for a fixed decimal point. The

maximum number of digits may be specified in the size parameter. The

maximum number of digits to the right of the decimal point is specified inthe d parameter

*The integer types have an extra option called UNSIGNED. Normally, the integer goes from annegative to positive value. Adding the UNSIGNED attribute will move that range up so it starts

at zero instead of a negative number.

Date types:


DATE() A date. Format: YYYY-MM-DD

Note: The supported range is from '1000-01-01' to '9999-12-31'

DATETIME() *A date and time combination. Format: YYYY-MM-DD HH:MM:SS

Note: The supported range is from '1000-01-01 00:00:00' to '9999-12-31

23:59:59'

TIMESTAMP() *A timestamp. TIMESTAMP values are stored as the number of seconds

since the Unix epoch ('1970-01-01 00:00:00' UTC). Format: YYYY-MM-

DD HH:MM:SS

Note: The supported range is from '1970-01-01 00:00:01' UTC to '2038-01-09 03:14:07' UTC

TIME() A time. Format: HH:MM:SS

Note: The supported range is from '-838:59:59' to '838:59:59'

YEAR() A year in two-digit or four-digit format.

Note: Values allowed in four-digit format: 1901 to 2155. Values allowed in


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two-digit format: 70 to 69, representing years from 1970 to 2069

*Even if DATETIME and TIMESTAMP return the same format, they work very differently. Inan INSERT or UPDATE query, the TIMESTAMP automatically set itself to the current date

and time. TIMESTAMP also accepts various formats, like YYYYMMDDHHMMSS,

YYMMDDHHMMSS, YYYYMMDD, or YYMMDD.

SQL Aggregate Functions

SQL aggregate functions return a single value, calculated from values in a column.

Useful aggregate functions:

AVG() - Returns the average value COUNT() - Returns the number of rows FIRST() - Returns the first value LAST() - Returns the last value MAX() - Returns the largest value MIN() - Returns the smallest value SUM() - Returns the sum

SQL Scalar functions

SQL scalar functions return a single value, based on the input value.


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Useful scalar functions:

UCASE() - Converts a field to upper case LCASE() - Converts a field to lower case MID() - Extract characters from a text field

LEN() - Returns the length of a text field ROUND() - Rounds a numeric field to the number of decimals specified NOW() - Returns the current system date and time FORMAT() - Formats how a field is to be displayed

The AVG() Function

The AVG() function returns the average value of a numeric column.

SQL AVG() Syntax

SELECT AVG(column_name) FROM table_name

Eg

SELECT AVG(marks) FROM student

SQL COUNT() Function

The COUNT() function returns the number of rows that matches a specified criteria.

SQL COUNT(column_name) Syntax

The COUNT(column_name) function returns the number of values (NULL values will not be

counted) of the specified column:

SELECT COUNT(column_name) FROM table_name

SQL COUNT(*) Syntax

The COUNT(*) function returns the number of records in a table:

SELECT COUNT(*) FROM student

The MAX() Function


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The MAX() function returns the largest value of the selected column.

SQL MAX() Syntax

SELECT MAX(column_name) FROM table_name

Eg

SELECT MAX(marks) FROM student

SQL MIN() Function

The MIN() Function

The MIN() function returns the smallest value of the selected column.

SQL MIN() Syntax

SELECT MIN(column_name) FROM table_name

SELECT MIN(marks) FROM student

SQL SUM() Function

The SUM() Function

The SUM() function returns the total sum of a numeric column.

SQL SUM() Syntax

SELECT SUM(column_name) FROM table_name

SELECT SUM(marks) FROM student

Aggregate functions often need an added GROUP BY statement.

The GROUP BY Statement

The GROUP BY statement is used in conjunction with the aggregate functions to group the

result-set by one or more columns.


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SQL GROUP BY Syntax

SELECT column_name, aggregate_function(column_name)

FROM table_name


GROUP BY column_name

SELECT Customer,SUM(OrderPrice) FROM Orders

GROUP BY Customer

GROUP BY More Than One Column

We can also use the GROUP BY statement on more than one column, like this:

SELECT Customer,OrderDate,SUM(OrderPrice) FROM OrdersGROUP BY Customer,OrderDate

The HAVING Clause

The HAVING clause was added to SQL because the WHERE keyword could not be used

with aggregate functions.

SQL HAVING Syntax

SELECT column_name, aggregate_function(column_name)

FROM table_name


GROUP BY column_name

HAVING aggregate_function(column_name) operator value

SELECT Customer,SUM(OrderPrice) FROM Orders

GROUP BY Customer

HAVING SUM(OrderPrice)


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CREATE USER Syntax

The general syntax of CREATE USER statement is :CREATE USER user [IDENTIFIED BY [PASSWORD] 'password'].....

In It is used to create a new MySQL account. But for using this statement you need theglobal CREATE USER privilege or the INSERT privilege for mysql database. The CREATE

USER statement creates a new record for each account in user table of mysql database. But if

account is already exists then it occurs the error message. By the IDENTIFIED BY clause wecan give the password to account. If you want to specify a password in plain text the does not

include the PASSWORD keyword. But when you specify the PASSWORD keyword then

password returned as the hashed value by the PASSWORD() function. Each account can benamed by using the same format as for GRANT statement like ?root?@?localhost?. But

when you define only the username as part of the account name then a hostname part of ?%?

is used. Example :

mysql> CREATE USER chandan IDENTIFIED BY'chandan';Query OK, 0 rows affected (0.04 sec)mysql> select user from user;+---------+| user |+---------+| || chandan || || root |+---------+4 rows in set (0.00 sec)

DROP USER Syntax

The general syntax of DROP USER statement is :

DROP USER user [, user] ...

DROP USER statement is used to remove one or more than MySQL account. But for using

this statement you need the global CREATE USER privilege or DELETE privilege. Example:

mysql> DROP USER chandan;Query OK, 0 rows affected (0.69 sec)

mysql> select user from user;+------+| user |+------+| || || root |+------+3 rows in set (0.11 sec)


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This statement is used to delete only that MySQL accounts which have no privileges and it

removes each account only from user table. For removing a MySQL account completely youhave to perform the following steps :

Firstly use SHOW GRANTS statements for determining the account has what type ofprivileges.

Then use REVOKE statement for revoking the privileges that displayed by SHOW GRANTSstatement.

Use DROP USER statement for removing the account.DROP USER statement cannot automatically close any open user session. But, if any open

session user is dropped then this statement does not effect until the session is closed. After

closing the session the user is dropped.

GRANT Syntax

The general syntax of GRANT statement is:

GRANT priv_type [(column_list)] [, priv_type [(column_list)]] ... ON [object_type]

{tbl_name | * | *.* | db_name.*} TO user [IDENTIFIED BY [PASSWORD] 'password'] [,user [IDENTIFIED BY [PASSWORD] 'password']] ... [WITH with_option [with_option] ...]

object_type = TABLE | FUNCTION | PROCEDURE

with_option = GRANT OPTION | MAX_QUERIES_PER_HOUR count |

MAX_UPDATES_PER_HOUR count | MAX_CONNECTIONS_PER_HOUR count |MAX_USER_CONNECTIONS count

By using GRANT statement we can enable the system administrator for creating MySQL

user accounts and for granting the right to from accounts. But for using the GRANT

statement you need the GRANT OPTION privilege and you also required the privileges

which you are granting. The REVOKE statement is used to relate and enable theadministrator for removing the account privileges. But when grant tables hold the privilege

rows which contain the mixed case database or the table name and the

lower_case_table_name system variable is set to non-zero value then REVOKE statementcannot used for revoking these privileges.

Privileges can be granted at several levels:

Global levelGlobal level privileges are applied to all databases on a given server. These type of privileges

are stored in the user table of mysql database. Ex ? GRANT ALL ON *.* and REVOKE ALL ON

*.*;

Database levelDatabase level privileges are applied to all objects in a given database. These type of

privileges are stored in the db and host tables of the mysql databases. Ex ? GRANT ALL ON


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database_ name.* and REVOKE ALL ON database_name.*

Table levelTable level privileges are applied to all columns on a given table. These type of privileges are

stored in the table_priv table of the mysql database. EX ? GRANT ALL ON

database_name.table_name and REVOKE ALL ON database_name.table_name.

Column levelColumn level privileges are applied to single column on a given table. These type of

privileges are stored in columns_priv table of mysql database. And at the time of using

REVOKE statement you have to specify the same column name that were granted.

Routine levelRoutine level privileges like CREATE ROUTINE, EXECUTE, ALTER ROUTING and GRANT

privileges are applied to stored routines. These type of privileges can be granted at global

and database level. Except CREATE ROUTINE, rest of these privileges can be granted at

routine level for particular routines and they are stored in the procs_priv table of mysql

database.

The object_type clause was included in the version of MySQL5.0.6. This clause can be

defined as TABLE, FUNCTION or PROCEDURE when the following object is a table,function or procedure.

priv_type can be specified as any of the following :

Privilege Meaning

ALL [PRIVILEGES] Sets all simple privileges except GRANT OPTION

ALTER Enables use of ALTER TABLE

CREATE Enables use of CREATE TABLE

CREATE USEREnables use of CREATE USER, DROP USER, RENAME

USER, and REVOKE ALL PRIVILEGES.

CREATE VIEW Enables use of CREATE VIEW

DELETE Enables use of DELETE

DROP Enables use of DROP TABLE

INSERT Enables use of INSERT

SELECT Enables use of SELECT


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UPDATE Enables use of UPDATE

USAGE Synonym for ?no privileges?

GRANT OPTION Enables privileges to be granted

The privileges like PROCESS, FILE, REPLICATION CLIENT, RELOAD, REPLICATION

SLAVE, SHUTDOWN, SHOW DATABASES, SUPER privileges are administrative

privileges which can only be granted globally. And other privileges can also be granted

global or more specific levels. Example for granting the global privilege :

mysql> CREATE USER raj@localhost IDENTIFIED BY'raj';Query OK, 0 rows affected (0.00 sec)mysql> select user from user;+---------+| user |+---------+| || chandan || || raj || root |+---------+5 rows in set (0.00 sec)mysql> GRANT ALL ON *.* TO raj@localhost;Query OK, 0 rows affected (0.00 sec)

In usernames Mysql does not support wildcards. Anonymous users are defined by inserting

in user table of mysql database with User=?? or creating a user account with an empty namewith the GRANT statement. By executing the following query of any anonymous users :

mysql> SELECT HOST, USER FROM USER WHEREUSER='';+-----------+------+| HOST | USER |+-----------+------+| % | || localhost | |+-----------+------+2 rows in set (0.00 sec)

By using following statement you can delete the local anonymous user account :

mysql> DELETE FROM USER WHERE HOST='localhost'AND User='';Query OK, 1 row affected (0.00 sec)mysql> FLUSH PRIVILEGES;Query OK, 0 rows affected (0.06 sec)

The WITH GRANT OPTIONS clause is used to provide the ability to user for providing to


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other users any privileges. But you have to careful about who is providing you the GRANT

OPTION privilege because two users that have the different privileges can be able to join

privileges. By the GRANT OPTION you can only assign only those privilege which yourselfyou have.

The MAX_QUERIES_PER_HOUR count, MAX_UPDATES_PER_HOUR count, andMAX_CONNECTIONS_PER_HOUR count options is used to limit the total number of

queries, updates and logins, a user can perform these between specified one hour period. But

if count is 0 then there is no limitation for that user. The MAX_USER_CONNECTIONScount option is used to limit the number of simultaneous connections, which the account can

make.

RENAME USER Syntax

The general syntax of RENAME USER statement is:RENAME USER old_user TO new_user [, old_user TO new_user] ...

The RENAME USER statement is used to rename the existing MySQL user accounts but forusing this statement you need the global CREATE USER privilege or the UPDATE

privilege. But if old account does not exists or the new account exists then it occurs the error.

Example :

mysql> RENAME USER chandan TO chand;Query OK, 0 rows affected (0.80 sec)mysql> SELECT User FROM User;+-------+| User |+-------+| |

| chand || raj || root |+-------+4 rows in set (0.16 sec)

REVOKE Syntax

The general syntax of REVOKE statement is :

REVOKE priv_type [(column_list)] [, priv_type [(column_list)]] ... ON [object_type]{tbl_name | * | *.* | db_name.*} FROM user [, user] ...

REVOKE ALL PRIVILEGES, GRANT OPTION FROM user [, user] ...

The REVOKE statement is used to enable the system administrator for revoking the

privileges from MySQL accounts but for using this statement you need the GRANT

OPTION privilege and you also need the privileges that you are revoking. All level ofprivileges and allowable priv_type values we have discussed above.

But when grant tables hold the privilege rows which contain the mixed case database or the


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table name and the lower_case_table_name system variable is set to non-zero value then

REVOKE statement cannot used for revoking these privileges. It will be necessary to

manipulate the grant tables directly.

By using following statement you can revoke all privileges for the name user. Example :

mysql> REVOKE ALL PRIVILEGES, GRANT OPTION FROM chand;Query OK, 0 rows affected (0.01 sec)mysql> SHOW GRANTS FOR chand \G;*************************** 1. row***************************Grants for chand@%: GRANT USAGE ON *.* TO 'chand'@'%'IDENTIFIED BY PASSWORD '*A59F8074680E742CC90A8595EFD7D1404FC8ED2F'1 row in set (0.00 sec)

SET PASSWORD Syntax

The general syntax of SET PASSWORD statement is:SET PASSWORD [FOR user] = PASSWORD('some password')

The SET PASSWORD statement is used to assign a password to existing user. If you are notusing FOR clause then its set the password for the current user. Any client using non

anonymous account and it is connected with the server can change the password for that

account. But if you are using FOR clause then it sets the password for a specified account oncurrent server host but for this you must have the UPDATE privilege. Example :

mysql> SELECT User, Password FROM User;+-------+-------------------------------------------+| User | Password|+-------+-------------------------------------------+| root | *81F5E21E35407D884A6CD4A731AEBFB6AF209E1B|| ||| raj | *7A5773507B1A6F85B4954BC90D6FB55416B0DCF8|| chand | *DD13F1F66054912AB8F82CA33BBDEE9E442582DB|+-------+------------------------------------------

-+4 rows in set (0.00 sec)mysql> SET PASSWORD FOR chand=PASSWORD('chand2');Query OK, 0 rows affected (0.00 sec)mysql> SELECT User, Password FROM User;+-------+-------------------------------------------+| User | Password|+-------+------------------------------------------


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-+| root | *81F5E21E35407D884A6CD4A731AEBFB6AF209E1B|| ||| raj | *7A5773507B1A6F85B4954BC90D6FB55416B0DCF8|| chand | *8CCD023D22DCD5607CD453A60598EF23B29DCA6B|+-------+-------------------------------------------+4 rows in set (0.00 sec)

Writing Subqueries

Posted on: March 13, 2008 at 12:00 AM

A subquery can be defined as a query within a query. In other words, any query results that we

reuse in another query. Subquery is known as nestee queries or subselects also. Subqueries don?t

include any new functionality but the queries are more readable

Writing Subqueries

A subquery can be defined as a query within a query. In other words, any query results thatwe reuse in another query. Subquery is known as nestee queries or subselects also.

Subqueries don?t include any new functionality but the queries are more readable with using

subqueries rather than of joins.

We will describe you the subqueries with the help of following tables :

mysql> SELECT * FROM Client;+------+---------------+----------+| C_ID | Name | City |+------+---------------+----------+| 1 | A K Ltd | Delhi || 2 | V K Associate | Mumbai || 3 | R K India | Banglore || 4 | R S P Ltd | Kolkata || 5 | A T Ltd | Delhi |

| 6 | D T Info | Delhi |+------+---------------+----------+6 rows in set (0.08 sec)mysql> SELECT * FROM Products;+---------+-------------+------+----------+| Prod_ID | Prod_Detail | C_ID | price |+---------+-------------+------+----------+| 111 | Monitor | 1 | 7000.00 || 112 | Processor | 2 | 11000.00 || 113 | Keyboard | 2 | 1200.00 |
http://www.roseindia.net/mysql/mysql5/flow-control-constructs.shtmlhttp://www.roseindia.net/mysql/mysql5/index.shtmlhttp://www.roseindia.net/mysql/mysql5/stored-procedures-and-functions.shtmlhttp://www.roseindia.net/mysql/mysql5/flow-control-constructs.shtmlhttp://www.roseindia.net/mysql/mysql5/index.shtmlhttp://www.roseindia.net/mysql/mysql5/stored-procedures-and-functions.shtmlhttp://www.roseindia.net/mysql/mysql5/flow-control-constructs.shtmlhttp://www.roseindia.net/mysql/mysql5/index.shtmlhttp://www.roseindia.net/mysql/mysql5/stored-procedures-and-functions.shtml


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| 114 | Mouse | 3 | 500.00 || 115 | CPU | 5 | 15500.00 |+---------+-------------+------+----------+5 rows in set (0.00 sec)

There are 3 basic types of subqueries in SQL:

Predicate Subqueries - extended logical constructs in the WHERE (and HAVING) clause. Scalar Subqueries - standalone queries that return a single value; they can be used

anywhere a scalar value is used.

Table Subqueries - queries nested in the FROM clause.All subqueries must be enclosed in parentheses.

Predicate Subqueries

Predicate Subqueries can be used in the HAVING and WHERE clause only because both are

special logical construct. These subqueries must retrieve one column.

IN SubqueryThe IN subquery tests if a scalar values match with the single query column value in any

subquery result row. The general syntax is :

Value_1 [NOT] IN (query_1)

In the following example we are getting the list of clients that are available in

Products table also. Example :

mysql> SELECT * FROM ClientWHERE C_ID IN

-> (SELECT C_ID FROMProducts);+------+---------------+----------+| C_ID | Name | City|+------+---------------+----------+| 1 | A K Ltd | Delhi|| 2 | V K Associate | Mumbai|

| 3 | R K India |Banglore || 5 | A T Ltd | Delhi|+------+---------------+----------+4 rows in set (0.00 sec)


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In the following example we are getting the list of clients that are not available in Products

table also. Example :

mysql> SELECT * FROM Client

WHERE C_ID NOT IN-> (SELECT C_ID FROMProducts);+------+-----------+---------+| C_ID | Name | City |+------+-----------+---------+| 4 | R S P Ltd | Kolkata || 6 | D T Info | Delhi |+------+-----------+---------+2 rows in set (0.01 sec)

Quantified SubqueriesA quantified subquery can use the all comparison operators for several types of tests. The

general syntax is :

Value_1 {=|>|=| SELECT * FROM Client WHERE C_ID=ANY(SELECT C_ID FROM Products);+------+---------------+----------+| C_ID | Name | City |+------+---------------+----------+| 1 | A K Ltd | Delhi || 2 | V K Associate | Mumbai || 3 | R K India | Banglore || 5 | A T Ltd | Delhi |+------+---------------+----------+4 rows in set (0.00 sec)

Exists SubqueriesThe EXISTS subquery is used to tests whether a subquery returns at least one row or

a qualifying row exists. The general syntax is :

Exists (query_1)

Any EXISTS subquery should contain an outer reference. It must be a correlated

subquery. Example :


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mysql> SELECT * FROM Client-> WHERE EXISTS-> (SELECT * FROM Products WHERE

Client.C_ID=Products.C_ID);+------+---------------+----------+| C_ID | Name | City |+------+---------------+----------+| 1 | A K Ltd | Delhi || 2 | V K Associate | Mumbai || 3 | R K India | Banglore || 5 | A T Ltd | Delhi |+------+---------------+----------+4 rows in set (0.00 sec)

Scalar Subqueries

The Scalar Subquery is a subquery which returns a single value. A Scalar subquery can be

used almost anywhere a single column value can be used. The subquery have to reference

only one column in the select list. It must not retrieve more than one row. When subquery

retrieve one row then the value of select list column becomes the value of the ScalarSubquery. Example :

mysql> SELECT (SELECT Name FROM Client WHEREC_ID=1);+----------------------------------------+| (SELECT Name FROM Client WHERE C_ID=1) |+----------------------------------------+| A K Ltd |+----------------------------------------+1 row in set (0.00 sec)mysql> SELECT (SELECT C_ID FROM Products WHEREC_ID=2) FROM Client;ERROR 1242 (21000): Subquery returns more than 1rowmysql> SELECT (SELECT C_ID FROM Products WHEREC_ID=1) FROM Client;+------------------------------------------+| (SELECT C_ID FROM Products WHERE C_ID=1) |+------------------------------------------+| 1 || 1 || 1 || 1 || 1 || 1 |

+------------------------------------------+6 rows in set (0.01 sec)

Table Subqueries

Table subqueries are used in the FROM Clause , replace the table name. These subqueriescan have correlation name also. Example :


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mysql> SELECT Client.*,Price-> FROM Client, Products-> WHERE Client.C_ID=Products.C_ID-> AND Price>1000;

+------+---------------+--------+----------+| C_ID | Name | City | Price |+------+---------------+--------+----------+| 1 | A K Ltd | Delhi | 7000.00 || 2 | V K Associate | Mumbai | 11000.00 || 2 | V K Associate | Mumbai | 1200.00 || 5 | A T Ltd | Delhi | 15500.00 |+------+---------------+--------+----------+4 rows in set (0.06 sec)

Using Single Value Subqueries

Firstly we will start with a simple query :

mysql> SELECT MAX(Price) FROM

Products;+------------+| MAX(Price) |+------------+| 15500.00 |+------------+1 row in set (0.60 sec)

The above example retrieve only a single value and its representing the maximum Price ofthe Product. In this example we used a MySQL Function MAX() that finds the greatest

values in a specified column.

Single ? value subqueries is used to return a single column value and then they are typicallyused for comparison. For Example :

mysql> SELECT * FROM Client c,Products p WHEREc.C_ID=p.C_ID

-> AND p.Price=(SELECT MAX(Price) FROMProducts);+------+---------+-------+---------+-------------+------+----------+| C_ID | Name | City | Prod_ID |Prod_Detail | C_ID | price |+------+---------+-------+---------+-------------+------+----------+

| 5 | A T Ltd | Delhi | 115 | CPU| 5 | 15500.00 |+------+---------+-------+---------+-------------+------+----------+1 row in set (0.02 sec)

In the above example we are getting the detail of products that have the highest price and the

client details also.


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SQL is a Standard Language for Accessing and Manipulating Databases

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