sql by sayantini

7/27/2019 SQL by Sayantini

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SQL


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BACKGROUND

The language originally called sequel in the early1970s , has evolved since then, and its name haschanged to SQL (Structured Query Language).

SQL has several parts# Data Definition language (DDL)# Interactive data-manipulation language (DML)

# Embedded DML

# View definition

# Authorization# Integrity

# Transaction control


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BANKING ENTERPRISE

The enterprise used in the examples in this chapter is thebanking enterprise with the following schemas :

Branch-schema =(branch-name, branch-city, assets)Customer-schema =(customer-name, customer-street, customer-city)

Loan-schema=(branch-name, loan-number ,amount)

Borrower-schema=(customer-name,loan-number)

Account-schema=(account-number,branch-name, balance)

Depositor-schema=(customer-name, account-number)


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BASICSTRUCTUREThe basic structure of an SQL expression consists of three clauses : select , from

and where

# Theselect clause corresponds toprojection operation of the relational algebra.

# The from clause corresponds to cartesian product operation of the relational

algebra.

# The whereclause corresponds to the selection predicate of the relationalalgebra.

A typical SQL query has the form

select A1 ,A2..,An

from r1,r2,rmwhere P

Here each Ai = attribute , ri = relation and P = predicate

Equivalent relational algebra expression

A1,A2,.An( p (r1*r2*.*rm))


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SELECT Clause

It is used to list the attributes desired in the result of

a query

Example:Find the names of all branches in the loanrelation

selectbranch-name

from loan

The result will list each branch-name once for every tuple in

which it appears in the loan relation.in those cases where we

want to eliminate duplicates ,we insert the keyword distinct

afterselect.


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WHERE ClauseThe where clause consists of a predicate involving attributes of the

relations that appear in the from clause

Consider the query Find all the loan numbers for loans

made at the Perryridge branch with loan amounts

greater than $1200.

selectloan-number

from loan

where branch-name=Perryridge and amount >1200

SQL uses the logical and ,orand not rather than the mathematical

symbols , and in the where clause.SQL also uses comparisonoperators like < ,>, = ,= and to compare strings and

arithmetic expressions as well as data types.


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SQL includes abetween comparison operator to simplify

whereclauses that specify that a value be less than or equal tosome value and greater than or equal to some other value. If we

wish to find the loan numbers of those loans with loan amounts

between $90000 and $100000 we can use the between

comparison to writeselect loan-number

from loan

where amountbetween 90000 and 100000


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FROM ClauseIt lists the relations to be scanned in the evaluation of the expression.

Consider the queryFor all customers who have a loan from thebank ,find their names and loan numbers.

selectdistinct customer-name , borrower.loan-number

fromborrower , loan

whereborrower.loan-number=loan.loan-number

Let us consider another queryFind the names and loan numbers of

all customers who have a loan at the Perryridge branch

select distinct customer-name,borrower.loan-number

fromborrower , loan

where borrower.loan-number=loan.loan-numberand

branch-name=Perryridge


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The RENAME Operation

SQL provides a mechanism for renaming both relations and

attributes. It uses the as clause, taking the form :

old-name as new-nameThe as clause can appear in both the select and from clauses.

Consider the queryFind the names of all branches that have assets greater than

atleast one branch located in Brooklyn.

select distinct T.branch-name

from branch as T , branch as S

where T.assets > S.assets and S.branch-city =Brooklyn

Observe we could not use branch.assets since it would not be clear

which reference to branch is intended.


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String OperationThe most commonly used operations on strings is pattern matching

using the operator like.We describe patterns using two special

characters :

Percent(%) :The % character matches any substring

Underscore( _ ): The _character matches any character.

Patterns are case sensitive.

Consider the query Find the names of all customers whose street

address includes the substring `Main`.

select customer-namefrom customer

where customer-street like%Main%


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Ordering the Display of Tuples

The order by clause causes the tuples in the result of a query to

appear in sorted order.To list in alphabetic order all customers

who have a loan at the Perryridge bank , we write

select distinct customer-name

from borrower , loanwhere borrower.loan-number =loan.loan-number

and branch-name = Perryridge

order by customer-nameBy default, the order by clause lists items in ascending order.

To specify the sort order , we may specify desc for descending

order or asc for ascending order.


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Suppose that we wish to list the entire loan relation in

descending order of amount and if several loans have the

same amount, we order them in ascending order by loan

number.

We express this query as

select *from loan

orderby amount desc, loan-numberasc


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SET Operations

The operations union ,intersect and except operate on relations.The relations participating in the operations must be compatible

that is ,they must have the same set of attributes.We will

construct queries involving the union, intersect and except

operations of two sets : the set of all customers who have an

account at the bank, which can be derived

select customer-name

from depositor

And the set of customers who have a loan at thebank,which can be derived by


from borrower


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UNION OperationLet us consider the query Find all customers having a loan,

an account , or both at the bank.

( select customer-name

from depositor)

union

( select customer-name

fromborrower)The union operation automatically eliminates duplicates.if we

want to retain all duplicates we must write union all in place of

union.


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INTERSECT OperationLet us consider the query Find all customers who have

both a loan and a account at the bank

(select distinct customer-name

from depositor)

intersect

(select distinct customer-name

from borrower)The intersect operation automatically eliminates duplicates.

Thus in the previous example if the customer say Jones has

several accounts and loans at the bank, then Jones will appear

only once in the result.

If we want to retain duplicates, we must write intersect all in

place ofintersect .


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EXCEPT Operation

Let us consider the query Find all customers who have anaccount but no loan at the bank

(select customer-name

from depositor)

except(select customer-name

from borrower)

The except operation also automatically eliminates

duplicates.if we want to retain all duplicates we must writeexcept all in place of except in the previous example.


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AGGREGATE Functions

Aggregate functions are functions that take a collection of values

as input and return a single value.SQL offers five built in

aggregate functions:

Average :avg

Minimum:min

Maximum:max

Total:sum

Count:count

The input to sum and avg must be a collection of numbers, but

other operators can operate on collections of nonnumeric data

types such as strings.


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Let us consider the query Find the average account balance at

the Perryridge branch

select avg (balance)

from account

where branch-name= Perryridge

The result of this query is a relation with a single attribute,

containing a single row with a numerical value corresponding to

the average balance at the Perryridge branch.

We can apply the aggregate functions to a group of set of tuples,

we specify this using the group by clause .the attribute or

attributes given in the group by clause are used to form groups.

Tuples with the same value on all attributes in the group by

clause are placed in one group.


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Let us consider the query Find the number of depositors for each branch

select branch-name, count (distinct customer-name)

from depositor,account

where depositor.account-number=account.account-number

group by branch-name

At times it is useful to state a condition that applies to groups rather than to tuples.toexpress such query we use the having clause.For example we might be interested in

only those branches where the average account balance is more than $1200.

select branch-name, avg (balance)

from account

group by branch-name

having avg(balance) >1200


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Nested SubqueriesA subquery is a select-from-where expression that is nested with

another query.

The in connective tests for set membership, where the set is a

collection of values produced by a select clause.

Consider the query Find the customers who have both a loan and

an account at the bank

select distinct customer-name

from borrower

where customer-name in (select customer-name

from depositor)


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The not in connective tests for the absence of set membership.

Consider the query Find all customers who have aloan at the

bank but do not have an account select distinct customer-name

from borrower

where customer-name not in (select customer-name

from depositor)

The phrase greater than at least one is represented in SQL by

>some.Consider the query Find the names of all branches that have

assets greater than those of at least one branch located in

Brooklyn


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select branch-name

from branch

where assets > some (select assetsfrombranch

where branch-city=Brooklyn)

The construct >allcorresponds to the phrase greater than all.Consider the query Find the names of all branches that have assets greater

than that of each branch in Brooklyn .

select branch-name

frombranch

where assets >all (select assets

from branch

where branch-cit = Brookl n


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The exists construct returns true if the argument subquery is nonempty.

Consider the query Find all customers who have an account and a loan

at the bank


fromborrower

where exists (select *

from depositor

where depositor.customer-name = borrower.customer-name)

The nonexistence of tuples in a subquery is tested by using not exists.

Consider the query Find all customers who have an account at all the

branches located in Brooklyn


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select distinct S.customer-name

from depositoras S

where not exists ((select branch-name

frombranch

where branch-city = Brooklyn)except

(select R.branch-name

from depositoras T, account as Rwhere T.account-number = R.account-number

and S.customer-name=T.customer-name))


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VIEWSTo define a view we must give the view a name,and must state the query that

computes the view.The form of the create view command is

create view v as Consider the view consisting of branch names and the names of customers who

either have an account or a loan at that branch.here we want the view to be called

all-customer

create view all-customeras

(select branch-name, customer-name

from depositor , account

where depositor.account-number =account.account-number)

union

( select branch-name, customer-name

fromborrower , loan

where borrower.loan-number=loan.loan-number)


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UPDATE OF A VIEW

Consider the following view definition

create view branch-loan as

select branch-name ,loan-number

from loan

We can insert into this view as follows

insert into branch-loan

values(Perryridge , L-307)

This insertion is represented by an insertion into the relation

loan, since loan is the actual relation from which the view

branch-loan is constructed.Thus we have null value for amount.

Thus the insert results in the insertion of the tuple

Perr rid e L-307 null

O l i


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JOINED Relations

Branch name Loan number amount

Downtown L-170 3000

Redwood L-230 4000

Perryridge L-260 1700

Customer-name Loan-number

Jones L-170

Smith L-230

Hayes L-155

loan borrower

We illustrate join operations using the relations loan and borrower

Branch-name Loan-number amount Customer-name Loan-number

Downtown L-170 3000 Jones L-170

Redwood L-230 4000 Smith L-230

Result of loan inner join borroweron

loan.loan-number =borrower.loan-number


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Branch-name Loan-number amount Customer-name Loan-number

Downtown L-170 3000 Jones L-170

Redwood L-230 4000 Smith L-230

Perryridge L-260 1700 null null

Result of loan left outer join borroweron

loan.loan-number=borrower.loan-number

Branch-name Loan-number amount Customer-name

Downtown L-170 3000 Jones

Redwood L-230 4000 Smith

Result of loan natural inner join borrower


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null L-155 null Hayes

Result ofnatural right outer join borrower




Perryridge L-260 1700 null

null L-155 null Hayes

Result of loan full outer join borrowerusing (loan-number)

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