sql w3

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 many different 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 addition to 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)

A server-side scripting language, like PHP or ASP

SQL

HTML / CSS

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 collections of related data entries and it consists of columns and rows.

Below is an example ofa table called "Persons":

P_IdLastNameFirstNameAddressCity

1HansenOlaTimoteivn 10Sandnes

2SvendsonToveBorgvn 23Sandnes

3PettersenKariStorgt 20Stavanger

The "Persons" table contains three rows (one for each person) and five columns (P_Id, LastName, FirstName, Address, and City).

Another example is a table called "Orders":

O_IdOrderNoP_Id

1778953

2446783

3224561

4245621

The "Orders" table contains five rows (one for each order) and three columns (O_Id, OrderNo, P_Id).

Notice that the relationship between the two tables above is the "P_Id" column (which refers to the persons in the "Persons" table without using their names).

The data in RDBMS is stored into group of tables, which might or might not be related.

Database Tables

A database most often contains one or more tables. Each table is identified by a name (e.g. "Customers" or "Orders"). Tables contain records (rows) with data.

Below is an example ofa table called "Persons":





The table above contains three records (one for each person) and five columns (P_Id, LastName, FirstName, Address, and City).

SQL Statements

Most of the actions you need to perform on a database are done with SQL statements.

The following SQL statement will select all the records in the "Persons" table:

SELECT * FROM Persons

In this tutorial we will teach you all about the different SQL statements.

Keep in Mind That...

SQL is not case sensitive

Semicolon after SQL Statements?

Some database systems require a semicolon at the end of each SQL statement.

Semicolon is the standard way to separate each SQL statement in database systems that allow more than one SQL statement to be executed in the same call to the server.

We are using MS Access and SQL Server 2000 and we do not have to put a semicolon after each SQL statement, but some database programs force you to use it.

SQL DML and DLL

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 table.

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

SQL SELECT SyntaxSELECT column_name(s)

FROM table_name

and

SELECT * FROM table_name

Note: SQL is not case sensitive. SELECT is the same as select.

An SQL SELECT Example

The "Persons" table:





Now we want to select the content of the columns named "LastName" and "FirstName" from the table above.

We use the following SELECT statement:

SELECT LastName,FirstName FROM Persons

The result-set will look like this:

LastNameFirstName

HansenOla

SvendsonTove

PettersenKari

SELECT * Example

Now we want to select all the columns from the "Persons" table.



Tip: The asterisk (*) is a quick way of selecting all columns!






Navigation in a Result-set

Most database software systems allow navigation in the result-set with programming functions, like: Move-To-First-Record, Get-Record-Content, Move-To-Next-Record, etc.

Programming functions like these are not a part of this tutorial. To learn about accessing data with function calls, please visit our ADO tutorial or our PHP tutorial.

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 SyntaxSELECT DISTINCT column_name(s)

FROM table_name

SELECT DISTINCT Example






Now we want to select only the distinct values from the column named "City" from the table above.


SELECT DISTINCT City FROM Persons


City

Sandnes

Stavanger

The WHERE Clause

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

SQL WHERE SyntaxSELECT column_name(s)

FROM table_name

WHERE column_name operator value

WHERE Clause Example






Now we want to select only the persons living in the city "Sandnes" from the table above.



WHERE City='Sandnes'





Quotes Around Text Fields

SQL uses single quotes around text values (most database systems will also accept double quotes).

Although, numeric values should not be enclosed in quotes.

For text values:

This is correct:

SELECT * FROM Persons WHERE FirstName='Tove'

This is wrong:

SELECT * FROM Persons WHERE FirstName=Tove

For numeric values:

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:

OperatorDescription

=Equal

Not equal

>Greater than

=Greater than or equal

10000

Returns this result

CompanySUM(Amount)

W3Schools12600

The SELECT INTO Statement

The SELECT INTO statement is most often used to create backup copies of tables or for archiving records.

SyntaxSELECT column_name(s) INTO newtable [IN externaldatabase]

FROM source

Make a Backup Copy

The following example makes a backup copy of the "Persons" table:

SELECT * INTO Persons_backup

FROM Persons

The IN clause can be used to copy tables into another database:

SELECT Persons.* INTO Persons IN 'Backup.mdb'

FROM Persons

If you only want to copy a few fields, you can do so by listing them after the SELECT statement:

SELECT LastName,FirstName INTO Persons_backup

FROM Persons

You can also add a WHERE clause. The following example creates a "Persons_backup" table with two columns (FirstName and LastName) by extracting the persons who lives in "Sandnes" from the "Persons" table:

SELECT LastName,Firstname INTO Persons_backup

FROM Persons

WHERE City='Sandnes'

Selecting data from more than one table is also possible. The following example creates a new table "Empl_Ord_backup" that contains data from the two tables Employees and Orders:

SELECT Employees.Name,Orders.Product

INTO Empl_Ord_backup

FROM Employees

INNER JOIN Orders

ON Employees.Employee_ID=Orders.Employee_ID

A view is a virtual table based on the result-set of a SELECT statement.

What is a View?

In SQL, a VIEW is a virtual table based on the result-set of a SELECT 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 a single table.

Note: The database design and structure will NOT be affected by the functions, where, or join statements in a view.

SyntaxCREATE VIEW view_name AS

SELECT column_name(s)

FROM table_name

WHERE condition

Note: The database does not store the view data! The database engine recreates the data, using the view's SELECT statement, every time a user queries a view.

Using Views

A view could be used from inside a query, a stored procedure, or from inside another view. By adding functions, joins, etc., to a view, it allows you to present exactly the data you want to the user.

The sample database Northwind has some 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 from the Northwind sample database selects every product in the Products table that has a unit price that is 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 example view from 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


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'

Data types and ranges for Microsoft Access, MySQL and SQL Server.

Microsoft Access Data Types

Data typeDescriptionStorage

TextUse for text or combinations of text and numbers. 255 characters maximum

MemoMemo is used for larger amounts of text. Stores up to 65,536 characters. Note: You cannot sort a memo field. However, they are searchable

ByteAllows whole numbers from 0 to 2551 byte

IntegerAllows whole numbers between -32,768 and 32,7672 bytes

LongAllows whole numbers between -2,147,483,648 and 2,147,483,6474 bytes

SingleSingle precision floating-point. Will handle most decimals 4 bytes

DoubleDouble precision floating-point. Will handle most decimals8 bytes

CurrencyUse for currency. Holds up to 15 digits of whole dollars, plus 4 decimal places. Tip: You can choose which country's currency to use8 bytes

AutoNumberAutoNumber fields automatically give each record its own number, usually starting at 14 bytes

Date/TimeUse for dates and times8 bytes

Yes/NoA logical field can be displayed as Yes/No, True/False, or On/Off. In code, use the constants True and False (equivalent to -1 and 0). Note: Null values are not allowed in Yes/No fields1 bit

Ole ObjectCan store pictures, audio, video, or other BLOBs (Binary Large OBjects)up to 1GB

HyperlinkContain links to other files, including web pages

Lookup WizardLet you type a list of options, which can then be chosen from a drop-down list4 bytes

MySQL Data Types

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

Text types:Data typeDescription

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 to 255 characters. Note: If you put a greater value than 255 it will be converted to a TEXT type

TINYTEXTHolds a string with a maximum length of 255 characters

TEXTHolds a string with a maximum length of 65,535 characters

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

MEDIUMTEXTHolds a string with a maximum length of 16,777,215 characters

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

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

LONGBLOBFor BLOBs (Binary Large OBjects). Holds up to 4,294,967,295 bytes of data

ENUM(x,y,z,etc.)Let you enter a list of possible values. You can list up to 65535 values in an ENUM 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')

SETSimilar to ENUM except that SET may contain up to 64 list items and can store more than one choice

Number types:Data typeDescription

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 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 to 18446744073709551615 UNSIGNED*. The maximum number of digits may be specified in parenthesis

FLOAT(size,d)A small 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

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 in the d parameter

*The integer types have an extra option called UNSIGNED. Normally, the integer goes from an negative to positive value. Adding the UNSIGNED attribute will move that range up so it starts at zero instead of a negative number.

Date types:Data typeDescription

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 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. In an 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 Server Data Types

Character strings:Data typeDescriptionStorage

char(n)Fixed-length character string. Maximum 8,000 charactersn

varchar(n)Variable-length character string. Maximum 8,000 characters

varchar(max)Variable-length character string. Maximum 1,073,741,824 characters

textVariable-length character string. Maximum 2GB of text data

Unicode strings:Data typeDescriptionStorage

nchar(n)Fixed-length Unicode data. Maximum 4,000 characters

nvarchar(n)Variable-length Unicode data. Maximum 4,000 characters

nvarchar(max)Variable-length Unicode data. Maximum 536,870,912 characters

ntextVariable-length Unicode data. Maximum 2GB of text data

Binary types:Data typeDescriptionStorage

bitAllows 0, 1, or NULL

binary(n)Fixed-length binary data. Maximum 8,000 bytes

varbinary(n)Variable-length binary data. Maximum 8,000 bytes

varbinary(max)Variable-length binary data. Maximum 2GB

imageVariable-length binary data. Maximum 2GB

Number types:Data typeDescriptionStorage

tinyintAllows whole numbers from 0 to 2551 byte

smallintAllows whole numbers between -32,768 and 32,7672 bytes

intAllows whole numbers between -2,147,483,648 and 2,147,483,647 4 bytes

bigintAllows whole numbers between -9,223,372,036,854,775,808 and 9,223,372,036,854,775,807 8 bytes

decimal(p,s)Fixed precision and scale numbers.

Allows numbers from -10^38 +1 to 10^38 1.

The p parameter indicates the maximum total number of digits that can be stored (both to the left and to the right of the decimal point). p must be a value from 1 to 38. Default is 18.

The s parameter indicates the maximum number of digits stored to the right of the decimal point. s must be a value from 0 to p. Default value is 05-17 bytes

numeric(p,s)Fixed precision and scale numbers.

Allows numbers from -10^38 +1 to 10^38 1.

The p parameter indicates the maximum total number of digits that can be stored (both to the left and to the right of the decimal point). p must be a value from 1 to 38. Default is 18.

The s parameter indicates the maximum number of digits stored to the right of the decimal point. s must be a value from 0 to p. Default value is 05-17 bytes

smallmoneyMonetary data from -214,748.3648 to 214,748.3647 4 bytes

moneyMonetary data from -922,337,203,685,477.5808 to 922,337,203,685,477.58078 bytes

float(n)Floating precision number data from -1.79E + 308 to 1.79E + 308.

The n parameter indicates whether the field should hold 4 or 8 bytes. float(24) holds a 4-byte field and float(53) holds an 8-byte field. Default value of n is 53.4 or 8 bytes

realFloating precision number data from -3.40E + 38 to 3.40E + 384 bytes

Date types:Data typeDescriptionStorage

datetimeFrom January 1, 1753 to December 31, 9999 with an accuracy of 3.33 milliseconds8 bytes

datetime2From January 1, 0001 and December 31, 9999 with an accuracy of 100 nanoseconds6-8 bytes

smalldatetimeFrom January 1, 1900 to June 6, 2079 with an accuracy of 1 minute4 bytes

dateStore a date only. From January 1, 0001 to December 31, 99993 bytes

timeStore a time only to an accuracy of 100 nanoseconds3-5 bytes

datetimeoffsetThe same as datetime2 with the addition of a time zone offset8-10 bytes

timestampStores a unique number that gets updated every time a row gets created or modified. The timestamp value is based upon an internal clock and does not correspond to real time. Each table may have only one timestamp variable

Other data types:Data typeDescription

sql_variantStores up to 8,000 bytes of data of various data types, except text, ntext, and timestamp

uniqueidentifierStores a globally unique identifier (GUID)

xmlStores XML formatted data. Maximum 2GB

cursorStores a reference to a cursor used for database operations

tableStores a result-set for later processing

SQL Quick Reference

HYPERLINK "http://www.w3schools.com/sql/sql_summary.asp"

SQL Quick Reference from W3Schools. Print it, and fold it in your pocket.

SQL SyntaxStatementSyntax

AND / ORSELECT column_name(s)FROM table_nameWHERE conditionAND|OR condition

ALTER TABLE (add column)ALTER TABLE table_name ADD column_name datatype

ALTER TABLE (drop column)ALTER TABLE table_name DROP COLUMN column_name

AS (alias for column)SELECT column_name AS column_aliasFROM table_name

AS (alias for table)SELECT column_nameFROM table_name AS table_alias

BETWEENSELECT column_name(s)FROM table_nameWHERE column_nameBETWEEN value1 AND value2

CREATE DATABASECREATE DATABASE database_name

CREATE INDEXCREATE INDEX index_nameON table_name (column_name)

CREATE TABLECREATE TABLE table_name(column_name1 data_type,column_name2 data_type,.......)

CREATE UNIQUE INDEXCREATE UNIQUE INDEX index_nameON table_name (column_name)

CREATE VIEWCREATE VIEW view_name ASSELECT column_name(s)FROM table_nameWHERE condition

DELETE FROMDELETE FROM table_name (Note: Deletes the entire table!!)

orDELETE FROM table_nameWHERE condition

DROP DATABASEDROP DATABASE database_name

DROP INDEXDROP INDEX table_name.index_name

DROP TABLEDROP TABLE table_name

GROUP BYSELECT column_name1,SUM(column_name2)FROM table_nameGROUP BY column_name1

HAVINGSELECT column_name1,SUM(column_name2)FROM table_nameGROUP BY column_name1HAVING SUM(column_name2) condition value

INSELECT column_name(s)FROM table_nameWHERE column_nameIN (value1,value2,..)

INSERT INTOINSERT INTO table_nameVALUES (value1, value2,....)

orINSERT INTO table_name(column_name1, column_name2,...)VALUES (value1, value2,....)

LIKESELECT column_name(s)FROM table_nameWHERE column_nameLIKE pattern

ORDER BYSELECT column_name(s)FROM table_nameORDER BY column_name [ASC|DESC]

SELECTSELECT column_name(s)FROM table_name

SELECT *SELECT *FROM table_name

SELECT DISTINCTSELECT DISTINCT column_name(s)FROM table_name

SELECT INTO(used to create backup copies of tables)SELECT *INTO new_table_nameFROM original_table_name

orSELECT column_name(s)INTO new_table_nameFROM original_table_name

TRUNCATE TABLE(deletes only the data inside the table)TRUNCATE TABLE table_name

UPDATEUPDATE table_nameSET column_name=new_value[, column_name=new_value]WHERE column_name=some_value

WHERESELECT column_name(s)FROM table_nameWHERE condition

Source : http://www.w3schools.com/sql/sql_quickref.asp

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