Chapter - 8

MySQL - REVISION TOUR

1. Database: A database is an organised collection of data.

2. DBMS : Software used to manage database is called Data Base

Management System.

3. Relational Database: A database in which the data is stored in the form

of relations (also called tables) is called a Relational Database. In other

words a Relational Database is a collection of one or more tables.

3. RDBMS: A DBMS used to manage Relational Databases is called an

RDBMS (Relational Data Base Management System). Some popular

RDBMS software available are: Oracle, MySQL.

4. Benefits of using a DBMS are:

a. Redundancy can be controlled

b. Inconsistence can be avoided

c. Data can be shared

d. Security restrictions can be applied.

5. MySQL: It is an Open Source RDBMS Software. It is available free of

cost.

6. Relation/Table: A table refers to a two dimensional representation of

data arranged in columns and rows.

7. Primary Key: The group of one or more columns used to uniquely

identify each row of a relation is called its Primary Key.

8. Candidate Key: A Candidate Key is the one that is capable of becoming

Primary key i.e., a field or attribute that has unique value for each row in

the relation.

9. Alternate Key: A candidate key that is not primary key is called

alternate key.

10. Foreign Key: A non-key attribute, whose values are derived from the

primary key of some other table.

11. Degree : Number of columns in the table.

12. Cardinality: Number of rows in a table is called its cardinality.

13. Tuple: A row of a relation.

14. Attribute: A column of relation.

Working with MySQL

SQL (Structured Query Language): It is the language used to

manipulate and manage databases and tables within them using an RDBMS.

1. DDL (Data Definition Language): All the commands which are used to

create, destroy, or restructure databases and tables come under this

category. Examples - CREATE, DROP, ALTER.

2. DML (Data Manipulation Language): All the commands which are

used to manipulate data within tables come under this category. Examples -

INSERT, UPDATE, DELETE.

3. DCL (Data Control Language): All the commands which are used to

control the access to databases and tables fall under this category.

Examples - GRANT, REVOKE.

1. Command: SELECT DATABASE()

Purpose: Shows the name of the current database

2. Command: SHOW TABLES

Purpose: Shows a list of tables present in the current database.

3. Command: ALTER TABLE

Purpose: Modifies the structure of a table

Syntax: (i) ALTER TABLE <tablename>

ADD <columnname> <datatype>;

(ii) ALTER TABLE <tablename>

DROP <columnname>;

(iii) ALTER TABLE <tablename>

MODIFY <column> <new_definition>;

4. Command : USE

Syntax : USE <databasename>;

Purpose : Open the specified database for use.

5. Command : SHOW TABLES

Purpose : Show a list of tables present in the current database.

6. Eliminating duplicate data – DISTINCT keyword eliminates

duplicate rows form the result of a SELECT command.

(i) Display name of all department (non-redundant)

mysql>Select DISTINCT dept from emp;

7. Command: DESCRIBE

Purpose: Shows the structure of a table.

Example : DESC emp;

Example : DESCRIBE emp;

8. Command: UPDATE

Purpose: Updates/Modifies data in a table

(i) Update emp

SET basic = 16000;

(ii) Update emp

SET basic = 16000

Where emono = 5623;

9. Command: DELETE

Purpose: Deletes data from a table

(i) DELETE FROM EMP ;

(II) DELETE FROM EMP

WHERE EMPNO = 4125;

10 . Performing simple calculations

(i) To calculate 2*8*5

mysql> SELECT 2*8*5;

(ii) To calculate 10% DA of basic

mysql > SELECT basic*10/100 from emp;

11. Using Column Aliases

Example : Select empno As ‘Employee Number’ From emp;

12. Condition based on a Range – The MySQL BETWEEN Condition is

used to retrieve values within a range in a SELECT, UPDATE, or DELETE

statement.

Syntax :

expression BETWEEN value1 AND value2;

Range includes both values.

Example :

(i) Select * from employee

Where basic BETWEEN 15000 AND 20000;

(ii) SELECT rollno, name FROM student

WHERE rollno BETWEEN 10 AND 20;

11. Condition based on a List – The IN operator used to select values

that match any value in a list of Specified values.

Example :

(i) Select ename, basic from emp

Where dept IN ( ‘Accounts’, ‘Sales’, Purchase’);

(ii) SELECT rollno, name FROM student WHERE rollno IN (10, 20, 60);

The NOT IN operator finds rows that do not match in the list

(i) Select ename, basic from emp

Where dept NOT IN ( ‘Accounts’, ‘Sales’, Purchase’);

12. Condition based on Pattern Matches – Keyword LIKE is used for

pattern matching of string data using wildcard characters % and _ .

Percent ( % ) – Matches any substring.

Underscore ( _ ) – to match on a single character.

Example :

(i) Select ename, basic, dept from employee

Where ename LIKE ‘A%’;

(ii) Select ename, basic, dept from employee

Where ename LIKE ‘%S’;

Q- (To display name of those employee whose name are 4 characters long)

(iii) Select ename, basic from employee

Where ename LIKE ‘_ _ _ _’;

(iv) Select ename, basic from employee

Where ename LIKE ‘_J%’;

Q- To Display name of those employees where second last character is “r”

(v) Select ename, basic from employee

Where ename LIKE ‘%r_’;

(vi) Select * from contact

Where mobile LIKE ‘8%’;

13. Searching for NULL

Keyword IS NULL is used to select rows in which the specified column

is NULL.

(i) Select empno, ename, design

From emp

Where Design IS NULL;

(ii) Select empno, ename, design

From emp

Where Design IS NOT NULL;

(Relational operator can’t be used with NULL)

14. Sorting Results – ORDER BY Clause - Used to arrange the selected

rows in ascending or in descending order.

Example

1. To arrange/sort the list of employees in the alphabetical order of their

names.

SELECT * FROM emp;

ORDER BY ename;

2. SELECT * FROM student

WHERE marks>50

ORDER BY name;

Functions in MySQL

Numeric Functions:

1. POWER(x,y) OR POW (x,y) - Returns the value of x raised to the power of y.

Example :

(i) mysql> SELECT POWER (3,2) "RAISED";

RAISED

9

2. ROUND(x) Rounds the argument x to the nearest INTEGER.

Example

(i) mysql> SELECT ROUND(180.55);

ROUND(180.55)

181

3. ROUND(x,d) Rounds the argument x to d decimal places.

mysql> SELECT ROUND (15.193,1)

ROUND (15.193,1)

15.2

4. TRUNCATE(x,d) Truncates the argument x to d decimal places.

mysql> select truncate(150.292,1);

truncate(150.292,1)

150.2

String Functions: LENGTH(str) Returns the length of a column or a string in bytes.

2. CONCAT(str1,str2,...) Returns the string that results from

concatenating the arguments. May have one or more arguments.

3. INSTR(str,substr) Returns the position of the first occurrence of substring <substr> in the string <str>.

4. LOWER(STR) OR LCASE(STR) - Returns the argument <str> in

lowercase. i.e., It changes all the characters of the passed string to

lowercase.

5. UPPER(STR) OR UCASE(STR) - Returns the argument <str> in

uppercase. i.e., It changes all the characters of the passed string to

uppercase.

6. LEFT (STR,N) - Returns the first <n> characters from the string <str> 7. RIGHT(STR,N) - Returns the last <n> characters from the string <str>

8. LTRIM(str) Removes leading spaces, i.e., removes spaces from the left side of the string <str>.

9. RTRIM(str) Removes trailing spaces, i.e., removes spaces from the right side of the string <str>.

10. TRIM(str) Removes both leading and trailing spaces from the string str.

With Leading Space

11. SUBSTRING(str,m,n) or SUBSTR(str, m, n) or MID(str,m,n) –

12. ASCII(str) Returns the ASCII value of the first character of the string

<str>. Returns 0 if <str> is the empty string. Returns NULL if <str> is

NULL.

Date and Time Functions 1. CURDATE() - Returns the current date in YYYY-MM-DD format.

2. NOW() - Returns the current date and time in 'YYYY-MM-DD HH:MM:SS'.

3. SYSDATE() - Returns the current date and time in 'YYYY-MM-DD HH:MM:SS'. 4. DATE(expr) - Extracts the date part of a date or datetime expression.

5. MONTH(date) - Returns the numeric month from the date passed.

6. YEAR(date) - Returns the year for date passed.

7. DAYNAME (date) - It returns the name of the weekday for the specified date.

8. DAYOFMONTH(date) - Returns the day of the month in the range 0 to 31. 9. DAYOFWEEK (date) - Returns the day of week in number as 1 for Sunday, 2 for Monday and so on.

10. DAYOFYEAR (date) - Return the day of the year for the given date in

numeric format in the range 1 to 366.

11. Difference between NOW() and SYSDATE()

NOW()- function returns a constant date and time at which the statement started executing.

Sleep(argument) - pauses for the number of seconds given by the

argument.

Sysadate () - to get exact time at which the statement executes.

Q1. If a database “Employee” exists, which MySql command helps you to

start working in that database?

Q2. Sahil created a table in MySql. Later on, he found that there should

have been another column in the table. Which command should he use

to add another column to the table?

Q3. Pooja a student of class XI , created a table “Book” . Price is a column

of this table. To find the details of books whose prices have not been

entered she wrote the following query :

Select * from Books where Price = NULL;

Help Pooja to run the query by removing the errors from the query and

rewriting it.

Q4. Rama is not able to change a value in a column to NULL. What

constraints did she specify when she created the table?

Q5. Distinguish between a Primary key and candidate key with the help of

suitable example of each.

Q6. The LastName column of a table “Directory” is given below :

Last Name

Batra

Sehgal

Bhatia

Sharma

Mehta

Based on this information, find the output of the following quries :

(i) SELECT lastname FROM Directory WHERE lastname like ‘_a%’ ;

(ii) SELECT lastname FROM Directory WHERE lastname not like ‘%a’;

Q7. A table “Stock” in a database has 5 columns and contains 17 records.

What is the degree and cardinality of this table?

Ans5. Candidate key is a column or a group of columns that is

capable of becoming the primary key. A table can have multiple

candidates key but it can have only one primary key.

For example , a table STUDENT contains columns AdmNo, RollNo,

Address, PhoneNo. In the table AdmNo and RollNo (both are

unique for every row in the table.) are candidate keys. Out of

these, any one can be selected as the primary key of the table.

Q8. Explain the purpose of DDL and DML commands used in SQl.

Also give two examples of each.

Ans: DDL (Data Definition Language) – DDL commands are

used to create , destroy and to restructure the database objects.

For example , CREATE, ALTER

DML- DML commands are used to insert , delete, and change data

in the tables.

For Example SELECT, DELETE.

Q9. Write the output of the following SQL queries :

(i) SELECT ROUND(6.5675, 2);

(ii) SELECT TRUNCATE(5.3456,1);

(iii) SELECT DAYOFMONTH(‘2009-08-25);

(iv) SELECT MID(‘Class 12, 2, 3);

Q10. Write an SQL query to create the ‘Menu’ with the following structure.

Field Type Constraint

ItemCode Varchar(5) Primary Key ItemName Varchar(20)

Cateogry Varchar(20)

Price Decimal(5,2)

Q11. While creating a table ‘Customer’ Simrita forgot to set the primary key

for the table. Give the statement which she should write now to set the

column ‘CustID’ as the primary key of the table?

Ans: ALTER TABLE Customer

ADD PRIMARY KEY (CustId);

Q12. Can a table have multiple primary keys? Can it have multiple foreign

keys?

Q13. In a Student table, out of RollNumber, Name, Address which column

can be set ad Primary key and why?

Q14. A table ‘Customers’ in a database has 5 columns and no rows in it.

What is its cardinality ? What will be its cardinality if 4 rows are added in

the table ?

Q15. What is the purpose of DROP TABLE command in SQL? How is it

different form DELETE command?

Q16. Write an SQL query to create the table ‘TEAMS’ with the following

structure:

Field Type Constraint

TeamCode Varchar(5) Primary Key

TeamName Varchar(20)

TeamLeader Varchar(20)

NoOfMembers Integer

Team_Symbol Char(1) Not Null

Ans :

Q

17. Mr. Satish wants to remove a column AC_Type from Account table.

What MySQL command should he use?

Ans: ALTER TABLE Account DROP COLUMN AC_Type;

OR

ALTER TABLE Account DROP AC_Type;

Q18. Name the commands :

(i) To describe the structure of table

(ii) To delete the table physically.

Q19. Explain the IN clause with example.

Ans: The IN clause is used to specify conditions based on a list. The IN

clause selects values that matches any value present in the list of values.

For example.

SELECT * FROM STUDENT

WHERE GRADE IN (‘A’,’B’,’C’);

Q20. Write the resulting output of the following :

(i) SELECT ROUND(1023.432,1);

(ii) SELECT LENGTH(‘RAMESH SHARMA’);

(iii) SELECT UPPER(‘kv neemuch’);

(iv) SELECT MOD(ROUND(120.60,1),5);

Q21. Write the SQL command to create the table voter including its

constraints.

Column Name Data type Size Constraint Description

V_ID INT 8 Primary key

Vname VARCHAR 25 NOT NULL

Age INT 3 CHECK >17 Age should

not less than

equal to 17

Address VARCHAR 30

Phone VARCHAR 10

Ans: Age INT(3) CHECK Age > 17,

Q22. What is meant by cardinality of a table ? Give an example.

Q23. Ms. Sneha is working in a software company .She works under MySQL

database. She forgot to know the tables which she worked in last week.

What MySQL statement she should use to know the tables?

Q24. A table “Vehicle” in a database has degree 4 and cardinality 7. What is

the number of rows and columns in it ?

Q25. Write the SQL command to create the table EMPLOYEE including its

constraints.

Name of Column Type Constraint

ID INT(4) PRIMARY KEY

First_Name VARCHAR(15) NOT NULL

Last_Name VARCHAR(15) NOT NULL

Salary INT(10) DEFAULT 1000

Ans:

Q26.

Q27.

Q28.

Q29.

30