Database queries

Chapter 6:
Introduction to SQL
Modern Database Management
10th Edition
Jeffrey A. Hoffer, Mary B. Prescott,
Fred R. McFadden

Introduction
 Also pronounced as “Sequel”
 A de-facto standard for relational DBMS
 Standard accepted by bodies like ANSI
and ISO

Introduction
 First commercial DBMS that support SQL
in 1979 was Oracle
 Another form of query language is
Query-by-Example (QBE) supported by
PC-based DBMSs

History
 Relational data model introduced
by Codd in 1970
 A relational DBMS project started
by IBM was system R that
included sequel as manipulation
language

History
 First commercial DBMS launched
was Oracle in 1979
 Other early DBMSs DB2, INGRES
 ANSI and ISO defined first standard
of SQL in 1986 known as SQL-86
that was further extended to SQL-
89

History
 Later two more standards known as
SQL-92 and SQL-99 were defined
 Almost all of the current DBMSs
support SQL-92 and many support
SQL-99 partially or completely
 SQL today is supported in all sort of
machines

Benefits of Standard SQL
 Reduced training cost
 Application portability
 Application longevity
 Reduced dependence on a single
vendor
 Cross-system communication

SQL
 SQL is used for any type of interaction
with the database through DBMS
 Right from creating database, tables,
views, users
 Insertion, updation, deletion of data in
the database

SQL and DBMSs
 Implementation of SQL always includes
the flavor of the particular DBMS, like in
names of data types
 Proprietary SQLs exist, like T-SQL and
PL-SQL

MS SQL Server
 The DBMS for our course is
Microsoft’s SQL Server 2000
desktop edition
 Two main tools Query Analyzer
and Enterprise Manager; both can
be used
 For SQL practice we will use QA

Terminology of SS
 Instance of SQL Server
 Instance contains different objects
like databases, security, DTS etc.
 Then databases object of SS contains
multiple databases, each database
contains multiple objects like tables,
views, users, roles etc.

Working in SS
 After installing SS, we will create
database using create command
 After this we will create tables and
other objects within this database

DDL
 Create
 Alter
 Drop

Create Command
 Creating database
CREATE DATABASE db_name
Create database exam
 Next step is to create tables
 Two approaches:
 Through SQL Create command
 Through Enterprise Manager

Create Table Command
 Create table command is used to:
 Create a table
 Define attributes of the table with data types
 Define different constraints on attributes, like
primary and foreign keys, check constraint,
not null, default value etc.

Format of Create Table
 CREATE TABLE
[ database_name.[ owner ] . | owner. ] table_name
( { < column_definition >
| column_name AS computed_column_expression
| < table_constraint >
}
| [ { PRIMARY KEY | UNIQUE } [ ,...n ] ]
)

Chevrons Explained
 < column_definition > ::= { column_name data_type }
[ DEFAULT constant_expression ]
[ < column_constraint > ] [ ...n ]

Chevrons Explained
 < column_constraint > ::= [ CONSTRAINT
constraint_name ]
{ [ NULL | NOT NULL ]
| [ { PRIMARY KEY | UNIQUE }
]
| [ [ FOREIGN KEY ]
REFERENCES ref_table [ ( ref_column ) ]
[ ON DELETE { CASCADE | NO ACTION } ]
[ ON UPDATE { CASCADE | NO ACTION } ]
]
| CHECK( logical_expression )
}

CREATE TABLE Program (
prName char(4),
totSem tinyint,
prCredits smallint)
Create Table Command

CREATE TABLE Student
(stId char(5),
stName char(25),
stFName char(25),
stAdres text,
stPhone char(10),
prName char(4)
curSem smallint,
cgpa real)
Create Table
Command

CREATE TABLE Student (
stId char(5) constraint ST_PK primary key
constraint ST_CK check (stId like
‘S[0-9][0-9][0-9][0-9]'),
stName char(25) not null,
stFName char(25),
stAdres text,
stPhone char(10),
prName char(4),
curSem smallint default 1,
cgpa real)
Create Table with Constraint

stId char(5) constraint ST_PK primary key
stId char(5) primary key

CREATE TABLE Student
(stId char(5) primary key
check (stId like 'S[0-9][0-9][0-9][0-9]'),
stName char(25) not null,
stFName char(25),
stAdres text,
stPhone char(10),
prName char(4),
curSem tinyint default 1,
cgpa real)

 CREATE TABLE semester (
semName char(5) primary key,
stDate smalldatetime,
endDate smalldatetime,
constraint ck_date_val check
(datediff(days, stDate, endDate) between
100 and 120))

Alter Table Statement
 Purpose is to make changes in
the definition of a table already
created through Create statement
 Can add, drop attributes or
constraints, activate or deactivate
constraints; the format is

 ALTER TABLE table
{ [ ALTER COLUMN column_name
{ new_data_type [ ( precision [ , scale ] ) ]
[ NULL | NOT NULL ]
]
| ADD
{ [ < column_definition > ]
| column_name AS computed_column_expression
} [ ,...n ]
| DROP
{ [ CONSTRAINT ] constraint_name
| COLUMN column } [ ,...n ]
| { CHECK | NOCHECK } CONSTRAINT
{ ALL | constraint_name [ ,...n ] }
}

ALTER TABLE Student
add constraint fk_st_pr
foreign key (prName) references
Program (prName)
ALTER TABLE program
add constraint ck_totSem
check (totSem < 9)
Alter Table Command

Removing or Changing Attribute
 ALTER TABLE student
ALTER COLUMN stFName char(20)
 Alter table student
drop column curSem
 Alter table student
drop constraint ck_st_pr

Removing Tables
 TRUNCATE TABLE table_name
 Truncate table class
 Delete can also be used
 DROP TABLE table_name

Data Manipulation
Language
 Insert
 Select
 Update

Insert Statement
 To insert records into tables
 INSERT [ INTO] table
{ [ ( column_list ) ]
{ VALUES
( { DEFAULT | NULL | expression } [ ,...n] )
}
}
| DEFAULT VALUES

Insert
 COURSE (crCode, crName, crCredits,
prName)
INSERT INTO course VALUES (‘CS-211',
‘Operating Systems’, 4, ‘MCS’)
INSERT INTO course (crCode, crName)
VALUES (‘CS-316’, Database Systems’)
INSERT INTO course (‘MG-103’, ‘Intro to
Management’, NULL, NULL)

stId stName prName cgpa
S1020 Sohail Dar MCS 2.8
S1038 Shoaib Ali BCS 2.78
S1015 Tahira Ejaz MCS 3.2
S1034 Sadia Zia BIT
S1018 Arif Zia BIT 3.0
STUDENT
prName totSem prCrdts
BCS 8 134
BIT 8 132
MBA 4 65
MCS 4 64
PROGRAM
crCode crTitle crCrdts prName
CS-616 Intro to Database Systems 4 MCS
MG-314 Money & Capital Market 3 BIT
CS-516 Data Structures and Algos 4 MCS
MG-105 Introduction to Accounting 3 MBA
stId crCode semName mTerm sMrks fMrks totMrks grade gPoint
S1020 CS-616 F04 25 12.5 35 72.5 B- 2.7
S1018 MG-314 F04 26.5 10.5 39 76 B 3.1
S1015 CS-616 F04 30 10 40 80 B+ 3.5
S1015 CS-516 F04 32 12 42 86 A- 4.1
COURSE
ENROLL

Select Statement
 Maximum used command in DML
 Used not only to select certain rows but also
the columns
 Also used for different forms of product, that
is, different joins

Select
 Selecting rows from one or more
tables
 SELECT {*|col_name[,….n]}
FROM table_name

Select
Q: Get the data about students
SELECT * FROM student

Select
Q: Give the names of the students
with the program name
SELECT stName, prName
FROM student

Attribute Alias
 Another Name “Urf” for an attribute
SELECT {*|col_name [[AS] alias] [, …n]}
FROM tab_name
SELECT stName as ‘Student Name’ , prName
‘Program’ FROM Student

Expression in Select
 In the column list we can also give the
expression; value of the expression is
computed and displayed

Expression Example
Q: Display the total sessional marks of each
student obtained in each subject
Select stId, crCode, mTerm + sMrks
‘Total out of 50’ from enroll

Expression Example
Q: List the names of the students and the
program they are enrolled in, in the format
“std studies in prg”
SELECT stName + ‘ studies in ‘ + prName
FROM student

Select Distinct
 The DISTINCT keyword is used to return
only distinct (different) values

 Just add a DISTINCT keyword to
the SELECT statement
 SELECT DISTINCT
column_name(s) FROM
table_name
Select Distinct

Q: Get the program names in which
students are enrolled
SELECT prName FROM Student
Select Distinct

Q: Get the program names in which
students are enrolled
SELECT prName FROM Student
SELECT DISTINCT prName FROM
Student
Select Distinct

Placing the Checks on
Rows
 Limit rows or to select certain rows, like,
 Programs of certain length, credits;
students with particular names, programs,
age, places etc.

The WHERE Clause
 We used names to limit columns, but rows
cannot be named due to the dynamicity
 We limit the rows using conditions

The WHERE Clause
Conditions are defined on the values of
one or more attributes from one or more
tables and placed in the WHERE clause

Where: Format
 SELECT [ALL|DISTINCT]
{*|culumn_list [alias][,…..n]} FROM
table_name
[WHERE <search_condition>]

< search_condition > ::=
{ [ NOT ] < predicate > | ( < search_condition > ) }
[ { AND | OR } [ NOT ] { < predicate > |
( < search_condition > ) } ]
} [ ,...n ]
< predicate > ::=
{ expression { = | < > | ! = | > | > = | ! > | < | < = | ! < }
expression
| string_expression [ NOT ] LIKE string_expression
| expression [ NOT ] BETWEEN expression AND
expression
| expression IS [ NOT ] NULL
| expression [ NOT ] IN ( subquery | expression [ ,...n ] )
| expression { = | < > | ! = | > | > = | ! > | < | < = | ! < }
{ ALL | SOME | ANY} ( subquery )
| EXISTS ( subquery )
}

Where Example
Q: Display all courses of the MCS program
Select crCode, crName, prName from
course
where prName = ‘MCS’

Not Operator
Inverses the predicate’s value
Q: List the course names offered to
programs other than MCS
SELECT crCode, crName, prName
FROM course
WHERE not (prName = ‘MCS’)

SELECT crCode, crName, prName
FROM course
WHERE (prName != ‘MCS’)

The BETWEEN Operator
Checks the value in a range
Q: List the IDs of the students with course
codes having marks between 70 and 80
SELECT stId, crCode, totMrks
From ENROLL
WHERE totMrks between 70 and 80

The IN Operator
 Checks in a list of values
Q: Give the course names of MCS and BCS
programs
SELECT crName, prName
From course
Where prName in (‘MCS’, ‘BCS’)

SELECT crName, prName
From course
Where (prName = ‘MCS’) OR (prName =
‘BCS’)

Like Operator
Q: Display the names and credits of CS
programs
SELECT crName, crCrdts, prName FROM
course
WHERE prName like '%CS'

“Order By” Clause
 Sorts the rows in a particular order
SELECT select_list
FROM table_source
[ WHERE search_condition ]
[ ORDER BY order_expression [ ASC | DESC ]
[,……n]
]

Order By Example
Q: Display the students’ data in the
ascending order of names
SELECT * from STUDENT
ORDER BY stName

Practice Query
Display the name and cgpa of students for
all those students who are in second or
above semester in descending order of
names

Functions in SQL
 Built-in functions are pre-written programs
to perform specific tasks
 Accept certain arguments and return the
result

Categories of Functions
 Depending on the arguments and the return
value, categorized
 Mathematical (ABS, ROUND, SIN, SQRT)
 String (LOWER, UPPER, SUBSTRING, LEN)
 Date (DATEDIFF, DATEPART, GETDATE())
 System (USER, DATALENGTH, HOST_NAME)
 Conversion (CAST, CONVERT)

Using Functions
SELECT upper(stName),
lower(stFName), stAdres,
len(convert(char, stAdres)),
FROM student

Aggregate Functions
 Operate on a set of rows and return a
single value, like, AVG, SUM, MAX, MIN,
STDEV
 Attribute list cannot contain other
attributes if an aggregate function is being
used

Aggregate Function Example
 SELECT avg(cgpa) as 'Average CGPA',
max(cgpa) as 'Maximum CGPA' from student
 Output is……

Aggregate Function Example
SELECT avg(cgpa) as 'Average
CGPA', max(cgpa) as 'Maximum
CGPA' from student
SELECT convert( decimal(5,2),
avg(cgpa)) as 'Average CGPA',
max(cgpa) as 'Maximum CGPA' from
student

Group By Clause
 SELECT stName, avg(cgpa) as
'Average CGPA', max(cgpa) as
'Maximum CGPA' from student
 SELECT prName, max(cgpa) as ‘Max
CGPA', min(cgpa) as ‘Min CGPA'
FROM student GROUP BY prName

HAVING Clause
 We can restrict groups by using having
clause; groups satisfying having condition
will be selected

HAVING Clause
SELECT select_list
[ INTO new_table ]
FROM table_source
[ WHERE search_condition ]
[ GROUP BY group_by_expression ]
[ HAVING search_condition ]
[ ORDER BY order_expression [ ASC |
DESC ] ]

HAVING Example
SELECT prName, min(cgpa), max(cgpa)
FROM student
GROUP BY prName
HAVING max(cgpa) > 3

Where and having can be combined

Accessing Multiple
Tables
 Cartesian Product
 Inner join
 Outer Join
 Full outer join
 Semi Join
 Natural Join

Cartesian Product
 No specific command; Select is used
 Simply give the names of the tables
involved; Cartesian product will be
produced

Cartesian Product
 Produces m x n rows
 Select * from program, course

Cartesian Product
 Certain columns can be selected, same
column name needs to be qualified
 Similarly can be applied to more than one
tables, and even can be applied on the
same table
SELECT * from Student, class, program

© Virtual University of
Pakistan
Inner join
 Only those rows from both tables are
merged that have same value for the
common attribute; equijoin
 Implemented by different methods

Pakistan
Inner join
 Common attributes need not to have same
name, but must have same domain
 Applied generally between tables having
referential integrity between them

Pakistan

Pakistan
Inner Join
 SELECT *
FROM course INNER JOIN program
ON course.prName = program.prName
 Select * FROM
Course c inner join program p
ON c.prName = p.prName

Pakistan
Inner Join
 Can also be performed using the where
clause, like
SELECT * FROM course, program
WHERE
course.prName =
program.prName

Pakistan
Outer Join
 Inner join plus the missing rows from one
or more tables
 Left, Right and Full Outer Joins

Pakistan
Outer Joins
 Right Outer Join: Inner join plus
rows from the non-matching rows
from right table
 Left outer join performs the same
thing but missing rows of the left
side table

Pakistan
Outer Joins
 A Left Outer Join B = B Right Outer Join
A
 Missing values are replaced with NULLs
 Full Outer Join: Inner join plus the non-
matching rows from both tables

Pakistan
Outer Join Examples
 Select * from course c LEFT
OUTER JOIN program p on
c.prName = p.prName
 Select * from program p RIGHT
OUTER JOIN course c on c.prName
= p.prName

Pakistan
Outer Join Examples
Select * from program p LEFT
OUTER JOIN course c on p.prName =
c.prName
Select * from course c RIGHT
OUTER JOIN program p on c.prName
= p.prName

Full Outer Join
SELECT * FROM program p FULL OUTER
JOIN
course c
ON p.prName = c.prName

Pakistan
Subqueries
 A query within a query
 Useful when condition has to be
applied against an unknown value
 Get the names of those students
who have more cgpa than that of
maximum of BCS students

Pakistan
Subqueries
 Can be used anywhere where an
expression is allowed
 Given in parentheses
 Generally in where
 Can be nested

Pakistan
Subqueries
 Careful about operator; depends whether
subquery returns single or multiple values
SELECT * from student where cgpa
> (select max(cgpa) from student where
prName = 'BCS‘)

Pakistan
Subqueries
SELECT * from student
WHERE cgpa =
ANY (select max(cgpa)
FROM student
GROUP BY prName)

Database queries

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