Sql

DATA BASE MANAGEMENT
SYSTEMS
Basic queiries in sql
By : Nagashayan Reddy
V th sem, Deptof ISE
SJC INSTITUTE OF
TECHNOLOGY
CHICKBALLAPUR-562101
Jai sri gurudev

1. Introduction
2. SELECT-FROM-WHERE Structure
3. Ambiguous Attribute Names,Aliasing,and Tuple
variables
4. Unspecified WHERE Clause and Use of Asterisk
5. Tables as sets in SQL
6. Substring Pattern Matching and Arithmetic
Operators
7. Ordering of Query Results

First Concept:
1. Introduction

Defination of Queries:
• Definition: Queries are the primary
mechanism for retrieving information from a
database and consist of questions presented
to the database in a predefined format. Many
database management systems use the
Structured Query Language (SQL) standard
query format.

1. SQL has one basic statement for retrieving
information from database: SELECT statement
2. The SELECT statement has no relationship to the
SELECT operation of relational algebra
3. SQL table is not a set of tuples,because set
doesnot allow two identical members.
4. We will call SQL table as a multiset(sometimes
called a bag) of tuples.

Second Cocept:
2. SELECT-FROM-
WHERE
STRUCTURE

• ANY QUERIES IN SQL WILL BE IN THE FOLLOWING
FORM:-
SELECT [DISTINCT] <ATTRIBUTE LIST>
FROM <TABLE LIST>
WHERE<CONDITION>;
• Such a query intuitively corresponds to a relational
algebra expression, involving selections,projections,
& cross products.

• CONCEPTUAL EVALUATION STATERGY OF SELECTOPERATION:
1. Computethe cross product of the tables in the from_list
2. Deletethose rows in thecross-product thatfailthe qualification
conditions
3. Deleteallcoloumns thatdo not appear in the select-list
4. If distinct isspecified,eliminateduplicate rows

BAGEPALLY_UNION
NAME ADDRESS
FAMILY
INCOME
NAGA
SHAYANA
REDDY
NETHAJI
CIRCLE,
BAGEPALLY
100000
MAHESHA NATIONAL
COLLEGE,
BAGEPALLY
100000
VENKATEH
NAIK
PENUMALE
,BAGEPALY 150000
MEHR TAJ 21ST
WARD,
BAGEPALLY
250000000
0000
SHRAVANI GULUR,
BAGEPALLY
230000
SUSHMA
REDDY
JILAKARAP
ALLY,
235000
BNO
1
2
3
4
5
6
UNION_INFO
BNO AGGREGATES(%)
1. 65
2. 58
3. 64
4. 68
5. 66
6. 69

• TIPS FOR WRITING QUIERIES
• 1. As soon as Query is given first we have
• to make a note that what are the tables
involving in this
2. After getting an idea about tables we
have to search for a link attribute
3. we have to write a relational algebra
first
4. Then converting a same to quierie

• Quierie : Get the names of all the students who
have the greater than 200000 family income
Relational algebra:
π NAME[σ FAMILY INCOME>200000 (bagepally_union) ]
SQL QUIERIE
SELECT NAME
FROM BAGEPALLY_UNION
WHERE FAMILY INCOME>200000

Concept 3:-
Ambigious Attribute
Names,Aliasing, and tuple
Variables

• IN SQL AMBIGUITY WILL ARRISES MAINLY IN
TWO DIFFERENT CASES:-
1. Using same name for two attributes that are
there in two different relations
2. Quieries that referring to a same relation twice

•
• QUERY 1:
Retrieve the names of the students who have
above 60% aggreate:
Relational algebra:
π NAME[ σagregate>60 (bagepally_union ⋈bno=bno union_info) ]

• SQL QUIERIE:
Select name
From bagepally_union as b,
union_info as u
Where b.bno=u.bno

•
 1. Aliases are the alternative relation
 names

 2. Aliases can also called as tuple
 variable

• Concept Four:
• Unspecified Where clause
• and use of asterisk
•

1. If WHERE condition is missing in Quierie
all the tuples that are specified in the FROM
will retrieved
2. It will become problamatic if two or more rela
-tions are specified it will make to retrieve
all the possibilities of the cartesian product.

• To Rerieve all the attribute values of the
selected tuples,we do not have to list the
attribute names explicitly in SQL
• We just specify an asterisk(*) which means that
all the attributes
• Querie:
select *
from bagepally_union

• CONCEPT 4:
SUBSTRING PATTERN MATCHING AND ARITHMETIC
OPERATORS

• SQL provides the facility to the programmers to
have pattern matching features in the data base
• It also gives the flexibility in using arithmetic
operations for data base
• The first feature allows comparision conditions on
only parts of a character string, by using the LIKE
comparision operation
• This LIKE operation can be used for string pattern
matching
• Partial keys in the quieries arer specified using two
reserve characters:% ,_

• % can be used as follows
select address
where name like‘%mehr taj%’
• _ can be used as follows
selcect address
where family income like ‘2_______’

• ORDERING OF QUERY RESULTS
1. SQL allows the user to order the tuples in the
resulot of a query by the values of one or more
attributes,using ORDER BY clause. This can be
illustrated as follows
Select name,address
From bagepally_union as b,
union_info as u
Where b.bno=u.bno
order by address,name

Sql

More Related Content

What's hot (19)

Viewers also liked (16)

Similar to Sql (20)

Recently uploaded (20)

Sql