Structured Query Language

– A query is a user request to retrieve data or information with a certain condition.

– SQL is a query language that allows user to specify the conditions.

– SQL works with database programs like MS Access, DB2, MS SQL Server, Oracle, etc.

What is SQL?

The Data Definition Language (DDL) part of SQL permits database tables to be created or deleted. pecify links between tables, and impose constraints between database tables.

The most important DDL statements in SQL are: 

•CREATE TABLE - creates a new database table •ALTER TABLE - alters (changes) a database table •DROP TABLE - deletes a database table •CREATE INDEX - creates an index (search key) •DROP INDEX - deletes an index

Data Definition Language (DDL)

The Data Manipulation Language (DML) part of SQL permits database users to insert, delete and update data in a database.

The most important DML statements in SQL are: 

•INSERT INTO – insert new records into the database table •UPDATE – changes the content of the database table •DELETE – deletes records from the database table

Data Manipulation Language (DML)

Basic structure of an SQL queryBasic structure of an SQL query

General Structure SELECT, ALL / DISTINCT, *,AS, FROM, WHERE

Comparison IN, BETWEEN, LIKE "% _"Grouping GROUP BY, HAVING,

COUNT( ), SUM( ), AVG( ), MAX( ), MIN( )Logical Operators AND, OR, NOTDisplay Order ORDER BY, ASC / DESC

◦ The query will select rows from the source tablename and output the result in table form.

– expr1, expr2,… are Expressions– col1, col2 are their corresponding column names in the

output table.– DISTINCT will eliminate duplication in the output

while ALL will keep all duplicated rows.– condition can be :

• an inequality, or• a string comparison• using logical operators AND, OR, NOT.

SELECTSELECT [[ALL / DISTINCTALL / DISTINCT] ] expr1expr1 [ [ASAS col1col1], ], expr2expr2 [ [ASAS col2col2] ] ;;

FROMFROM tablenametablename WHEREWHERE conditioncondition

fieldfield typetype widthwidth contentscontentsid numeric 4 student id numbername character 25 namedob date 8 date of birthsex character 1 sex: M / Fclass character 2 classemark numeric 2 entrance Mark city character 20 city

Student Table

eg. 1 List all the student records.SELECT * FROM student

id name dob sex Dept emark City9801 Anitha 06/04/90 F Mechanical 70 Chennai9802 Bindhu 01/10/90 F Electrical 92 Bangalore9803 Vimal 03/16/90 M Mechanical 91 Chennai9804 Kannan 07/09/90 F Mechanical 84 Mumbai9805 Ram 10/20/90 M Electrical 88 Chennai

: : : : : :

Result

eg. 2 List the names,dept and emark of Electrical students:

SELECT name,dept, emark, dept FROM student ;

WHERE dept=“Electrical"

name dept emarkBindhu Electrical 92Ram Electrical 88

Result

eg. 3 List the names and ages of Electrical girls.

Electrical Girls ?Electrical Girls ?

Condition for “Electrical Girls":Condition for “Electrical Girls":

1)1) class = class = " Electrical "" Electrical "

2)2) sex = sex = "F""F"

3)3) Both ( AND operator)Both ( AND operator)

eg. 3 List the names and ages of Electrical girls.

What is "age“?What is "age“?

Functions:Functions:

# days :# days : DATE( ) – dobDATE( ) – dob

# years :# years : (DATE( ) – dob) / 365(DATE( ) – dob) / 365

1 d.p.1 d.p. :: ROUND(__ , 0)ROUND(__ , 0)

eg. 3 List the names and ages of Electrical girls.

SELECT name, ROUND((DATE( )-dob)/365,0) AS age FROM student WHERE class=“Electrical" AND sex="F"

name ageBindhu 21.0

: :

Result

Comparison

expr IN ( value1, value2, value3)expr BETWEEN value1 AND value2expr LIKE "%_"

eg. 4 List the students who were born on Wednesday or Saturdays.

SELECT name, dept, CDOW(dob) AS bdate FROM student WHERE DOW(dob) IN (4,7)

name dept bdateBindhu Electrical WednesdayRam Electrical Saturday

Result

eg. 5 List the students whose names start with “R".SELECT name, dept FROM student ;

WHERE name LIKE “R%"

name deptRam Electrical

: :Result

GroupingSELECT ...... FROM ...... WHERE condition ;GROUP BY groupexpr [HAVING requirement]

Group functions: COUNT( ), SUM( ), AVG( ), MAX( ), MIN( )

• groupexpr specifies the related rows to be grouped as one entry. Usually it is a column.

• WHERE condition specifies the condition of individual rows before the rows are group. HAVING requirement specifies the condition involving the whole group.

dept cntMechanical 3Electrical 2

eg. 6 List the number of students of each dept.SELECT dept, COUNT(*) FROM student

GROUP BY dept

Result

Group By deptGroup By dept

AVG( )AVG( )

AVG( )AVG( )

ElectricalElectrical

MechanicalMechanical

MechanicalMechanical

MechanicalMechanical

MechanicalMechanical

MechanicalMechanical

MechanicalMechanical

StudentStudent

dept

ElectricalElectrical

ElectricalElectrical

ElectricalElectrical

eg. 7 List the average entrance test score of each dept.

SELECT dept, AVG(etest) FROM student GROUP BY dept

dept avg_etestElectrical 90.00

Mechanical 81.66Result

Grouping - Having

eg. 8 List the average Entrance test score of the boys in each class. The list should not contain dept with less than 3 boys.

SELECT AVG(mtest), class FROM student ;WHERE sex="M" GROUP BY dept ;HAVING COUNT(*) >= 3

Display Order

SELECT ...... FROM ...... WHERE ...... SELECT ...... FROM ...... WHERE ...... GROUP BY ..... ;GROUP BY ..... ;ORDER BY ORDER BY colnamecolname ASC / DESC ASC / DESC

Natural Join

A Natural Join is a join operation that joins two tables by their common column. This operation is similar to the setting relation of two tables.

SELECT a.comcol, a.col1, b.col2, expr1, expr2 ;FROM table1 a, table2 b ;WHERE a.comcol = b.comcol

MusicMusic

idid

98019801

typetype

StudentStudent

98019801

idid namename DEPTDEPT

98019801

ProductProduct

idid namename DEPTDEPT typetype

Same idSame id

JoinJoin

Natural Join

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