ch3[1].ppt

Database System Concepts, 5th Ed.
©Silberschatz, Korth and Sudarshan
See www.db-book.com for conditions on re-use
Chapter 3: SQL

3.2
Database System Concepts, 5th Ed., June 2006
Chapter 3: SQL
 Data Definition
 Basic Query Structure
 Set Operations
 Aggregate Functions
 Null Values
 Nested Subqueries
 Complex Queries
 Views
 Modification of the Database
 Joined Relations**

3.3
History
 IBM Sequel language developed as part of System R project at the
IBM San Jose Research Laboratory
 Renamed Structured Query Language (SQL)
 ANSI and ISO standard SQL:
 SQL-86
 SQL-89
 SQL-92
 SQL:1999 (language name became Y2K compliant!)
 SQL:2003
 Commercial systems offer most, if not all, SQL-92 features, plus
varying feature sets from later standards and special proprietary
features.
 Not all examples here may work on your particular system.

3.4
Data Definition Language
 The schema for each relation, including attribute types.
 Integrity constraints
 Authorization information for each relation.
 Non-standard SQL extensions also allow specification of
 The set of indices to be maintained for each relations.
 The physical storage structure of each relation on disk.
Allows the specification of:

3.5
Create Table Construct
 An SQL relation is defined using the create table command:
create table r (A1 D1, A2 D2, ..., An Dn,
(integrity-constraint1),
...,
(integrity-constraintk))
 r is the name of the relation
 each Ai is an attribute name in the schema of relation r
 Di is the data type of attribute Ai
 Example:
create table branch
(branch_name char(15),
branch_city char(30),
assets integer)

3.6
Domain Types in SQL
 char(n). Fixed length character string, with user-specified length n.
 varchar(n). Variable length character strings, with user-specified maximum
length n.
 int. Integer (a finite subset of the integers that is machine-dependent).
 smallint. Small integer (a machine-dependent subset of the integer
domain type).
 numeric(p,d). Fixed point number, with user-specified precision of p digits,
with n digits to the right of decimal point.
 real, double precision. Floating point and double-precision floating point
numbers, with machine-dependent precision.
 float(n). Floating point number, with user-specified precision of at least n
digits.
 More are covered in Chapter 4.

3.7
Integrity Constraints on Tables
 not null
 primary key (A1, ..., An )
Example: Declare branch_name as the primary key for branch
.
create table branch
(branch_name char(15),
branch_city char(30) not null,
assets integer,
primary key (branch_name))
primary key declaration on an attribute automatically ensures
not null in SQL-92 onwards, needs to be explicitly stated in
SQL-89

3.8
Basic Insertion and Deletion of Tuples
 Newly created table is empty
 Add a new tuple to account
insert into account
values ('A-9732', 'Perryridge', 1200)
 Insertion fails if any integrity constraint is violated
 Delete all tuples from account
delete from account
Note: Will see later how to delete selected tuples

3.9
Drop and Alter Table Constructs
 The drop table command deletes all information about the dropped
relation from the database.
 The alter table command is used to add attributes to an existing
relation:
alter table r add A D
where A is the name of the attribute to be added to relation r and D
is the domain of A.
 All tuples in the relation are assigned null as the value for the
new attribute.
 The alter table command can also be used to drop attributes of a
relation:
alter table r drop A
where A is the name of an attribute of relation r
 Dropping of attributes not supported by many databases

3.10
Basic Query Structure
 A typical SQL query has the form:
select A1, A2, ..., An
from r1, r2, ..., rm
where P
 Ai represents an attribute
 Ri represents a relation
 P is a predicate.
 This query is equivalent to the relational algebra expression.
 The result of an SQL query is a relation.
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3.11
The select Clause
 The select clause list the attributes desired in the result of a query
 corresponds to the projection operation of the relational algebra
 Example: find the names of all branches in the loan relation:
select branch_name
from loan
 In the relational algebra, the query would be:
branch_name (loan)
 NOTE: SQL names are case insensitive (i.e., you may use upper- or
lower-case letters.)
 E.g. Branch_Name ≡ BRANCH_NAME ≡ branch_name
 Some people use upper case wherever we use bold font.

3.12
The select Clause (Cont.)
 SQL allows duplicates in relations as well as in query results.
 To force the elimination of duplicates, insert the keyword distinct after
select.
 Find the names of all branches in the loan relations, and remove
duplicates
select distinct branch_name
from loan
 The keyword all specifies that duplicates not be removed.
select all branch_name
from loan

3.13
The select Clause (Cont.)
 An asterisk in the select clause denotes “all attributes”
select *
from loan
 The select clause can contain arithmetic expressions involving the
operation, +, –, , and /, and operating on constants or attributes of
tuples.
 E.g.:
select loan_number, branch_name, amount  100
from loan

3.14
The where Clause
 The where clause specifies conditions that the result must satisfy
 Corresponds to the selection predicate of the relational algebra.
 To find all loan number for loans made at the Perryridge branch with
loan amounts greater than $1200.
select loan_number
from loan
where branch_name = 'Perryridge' and amount > 1200
 Comparison results can be combined using the logical connectives and,
or, and not.

3.15
The from Clause
 The from clause lists the relations involved in the query
 Corresponds to the Cartesian product operation of the relational algebra.
 Find the Cartesian product borrower X loan
select 
from borrower, loan
 Find the name, loan number and loan amount of all customers
having a loan at the Perryridge branch.
select customer_name, borrower.loan_number, amount
from borrower, loan
where borrower.loan_number = loan.loan_number and
branch_name = 'Perryridge'

3.16
The Rename Operation
 SQL allows renaming relations and attributes using the as clause:
old-name as new-name
 E.g. Find the name, loan number and loan amount of all customers;
rename the column name loan_number as loan_id.
select customer_name, borrower.loan_number as loan_id, amount
from borrower, loan
where borrower.loan_number = loan.loan_number

3.17
Tuple Variables
 Tuple variables are defined in the from clause via the use of the as
clause.
 Find the customer names and their loan numbers and amount for all
customers having a loan at some branch.
 Find the names of all branches that have greater assets than
some 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'
Keyword as is optional and may be omitted
borrower as T ≡ borrower T
 Some database such as Oracle require as to be omitted
select customer_name, T.loan_number, S.amount
from borrower as T, loan as S
where T.loan_number = S.loan_number

3.18
String Operations
 SQL includes a string-matching operator for comparisons on character
strings. The operator “like” uses patterns that are described using two
special characters:
 percent (%). The % character matches any substring.
 underscore (_). The _ character matches any character.
 Find the names of all customers whose street includes the substring
“Main”.
select customer_name
from customer
where customer_street like '% Main%'
 Match the name “Main%”
like 'Main%' escape ''
 SQL supports a variety of string operations such as
 concatenation (using “||”)
 converting from upper to lower case (and vice versa)
 finding string length, extracting substrings, etc.

3.19
Ordering the Display of Tuples
 List in alphabetic order the names of all customers having a loan in
Perryridge branch
select distinct customer_name
from borrower, loan
where borrower loan_number = loan.loan_number and
branch_name = 'Perryridge'
order by customer_name
 We may specify desc for descending order or asc for ascending
order, for each attribute; ascending order is the default.
 Example: order by customer_name desc

3.20
Duplicates
 In relations with duplicates, SQL can define how many copies of
tuples appear in the result.
 Multiset versions of some of the relational algebra operators – given
multiset relations r1 and r2:
1.  (r1): If there are c1 copies of tuple t1 in r1, and t1 satisfies
selections ,, then there are c1 copies of t1 in  (r1).
2. A (r ): For each copy of tuple t1 in r1, there is a copy of tuple
A (t1) in A (r1) where A (t1) denotes the projection of the single
tuple t1.
3. r1 x r2 : If there are c1 copies of tuple t1 in r1 and c2 copies of tuple
t2 in r2, there are c1 x c2 copies of the tuple t1. t2 in r1 x r2

3.21
Duplicates (Cont.)
 Example: Suppose multiset relations r1 (A, B) and r2 (C) are as
follows:
r1 = {(1, a) (2,a)} r2 = {(2), (3), (3)}
 Then B(r1) would be {(a), (a)}, while B(r1) x r2 would be
{(a,2), (a,2), (a,3), (a,3), (a,3), (a,3)}
 SQL duplicate semantics:
select A1,, A2, ..., An
from r1, r2, ..., rm
where P
is equivalent to the multiset version of the expression:
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3.22
Set Operations
 The set operations union, intersect, and except operate on relations
and correspond to the relational algebra operations 
 Each of the above operations automatically eliminates duplicates; to
retain all duplicates use the corresponding multiset versions union all,
intersect all and except all.
Suppose a tuple occurs m times in r and n times in s, then, it occurs:
 m + n times in r union all s
 min(m,n) times in r intersect all s
 max(0, m – n) times in r except all s

3.23
Set Operations
 Find all customers who have a loan, an account, or both:
(select customer_name from depositor)
except
(select customer_name from borrower)
intersect
 Find all customers who have an account but no loan.
union
 Find all customers who have both a loan and an account.

3.24
Aggregate Functions
 These functions operate on the multiset of values of a column of
a relation, and return a value
avg: average value
min: minimum value
max: maximum value
sum: sum of values
count: number of values

3.25
Aggregate Functions (Cont.)
 Find the average account balance at the Perryridge branch.
 Find the number of depositors in the bank.
 Find the number of tuples in the customer relation.
select avg (balance)
from account
where branch_name = 'Perryridge'
select count (*)
from customer
select count (distinct customer_name)
from depositor

3.26
Aggregate Functions – Group By
 Find the number of depositors for each branch.
Note: Attributes in select clause outside of aggregate functions must
appear in group by list
select branch_name, count (distinct customer_name)
from depositor, account
where depositor.account_number = account.account_number
group by branch_name

3.27
Aggregate Functions – Having Clause
 Find the names of all branches where the average account balance is
more than $1,200.
Note: predicates in the having clause are applied after the
formation of groups whereas predicates in the where
clause are applied before forming groups
select branch_name, avg (balance)
from account
having avg (balance) > 1200

3.28
Nested Subqueries
 SQL provides a mechanism for the nesting of subqueries.
 A subquery is a select-from-where expression that is nested within
another query.
 A common use of subqueries is to perform tests for set membership, set
comparisons, and set cardinality.

3.29
“In” Construct
 Find all customers who have both an account and a loan at the bank.
 Find all customers who have a loan at the bank but do not have
an account at the bank
from borrower
where customer_name not in (select customer_name
from depositor )
from borrower
where customer_name in (select customer_name
from depositor )

3.30
Example Query
 Find all customers who have both an account and a loan at the
Perryridge branch
 Note: Above query can be written in a much simpler manner. The
formulation above is simply to illustrate SQL features.
from borrower, loan
where borrower.loan_number = loan.loan_number and
branch_name = 'Perryridge' and
(branch_name, customer_name ) in
(select branch_name, customer_name
where depositor.account_number =
account.account_number )

3.31
“Some” Construct
 Find all branches that have greater assets than some branch located
in Brooklyn.
 Same query using > some clause
select branch_name
from branch
where assets > some
(select assets
from branch
where branch_city = 'Brooklyn')
select distinct T.branch_name
from branch as T, branch as S
where T.assets > S.assets and
S.branch_city = 'Brooklyn'

3.32
“All” Construct
 Find the names of all branches that have greater assets than all
branches located in Brooklyn.
select branch_name
from branch
where assets > all
(select assets
from branch

3.33
“Exists” Construct
 Find all customers who have an account at all branches located in
Brooklyn.
select distinct S.customer_name
from depositor as S
where not exists (
(select branch_name
from branch
except
(select R.branch_name
from depositor as T, account as R
where T.account_number = R.account_number and
S.customer_name = T.customer_name ))
 Note that X – Y = Ø  X Y
 Note: Cannot write this query using = all and its variants

3.34
Absence of Duplicate Tuples
 The unique construct tests whether a subquery has any duplicate
tuples in its result.
 Find all customers who have at most one account at the Perryridge
branch.
select T.customer_name
from depositor as T
where unique (
select R.customer_name
from account, depositor as R
where T.customer_name = R.customer_name and
R.account_number = account.account_number and
account.branch_name = 'Perryridge')

3.35
Example Query
 Find all customers who have at least two accounts at the Perryridge
branch.
select distinct T.customer_name
from depositor as T
where not unique (
select R.customer_name
from account, depositor as R
where T.customer_name = R.customer_name and
R.account_number = account.account_number and
account.branch_name = 'Perryridge')
 Variable from outer level is known as a correlation variable

3.36
Modification of the Database – Deletion
 Delete all account tuples at the Perryridge branch
delete from account
 Delete all accounts at every branch located in the city ‘Needham’.
delete from account
where branch_name in (select branch_name
from branch
where branch_city = 'Needham')

3.37
Example Query
 Delete the record of all accounts with balances below the average at
the bank.
delete from account
where balance < (select avg (balance )
from account )
 Problem: as we delete tuples from deposit, the average balance
changes
 Solution used in SQL:
1. First, compute avg balance and find all tuples to delete
2. Next, delete all tuples found above (without recomputing avg or
retesting the tuples)

3.38
Modification of the Database – Insertion
 Add a new tuple to account
insert into account
values ('A-9732', 'Perryridge', 1200)
or equivalently
insert into account (branch_name, balance, account_number)
values ('Perryridge', 1200, 'A-9732')
 Add a new tuple to account with balance set to null
insert into account
values ('A-777','Perryridge', null )

3.39
Modification of the Database – Insertion
 Provide as a gift for all loan customers of the Perryridge branch, a $200
savings account. Let the loan number serve as the account number for the
new savings account
insert into account
select loan_number, branch_name, 200
from loan
insert into depositor
select customer_name, loan_number
from loan, borrower
and loan.account_number = borrower.account_number
 The select from where statement is evaluated fully before any of its
results are inserted into the relation
 Motivation: insert into table1 select * from table1

3.40
Modification of the Database – Updates
 Increase all accounts with balances over $10,000 by 6%, all other
accounts receive 5%.
 Write two update statements:
update account
set balance = balance  1.06
where balance > 10000
update account
set balance = balance  1.05
where balance  10000
 The order is important
 Can be done better using the case statement (next slide)

3.41
Case Statement for Conditional Updates
 Same query as before: Increase all accounts with balances over
$10,000 by 6%, all other accounts receive 5%.
update account
set balance = case
when balance <= 10000 then balance *1.05
else balance * 1.06
end

More Features

3.43
Joined Relations**
 Join operations take two relations and return as a result another
relation.
 These additional operations are typically used as subquery
expressions in the from clause
 Join condition – defines which tuples in the two relations match, and
what attributes are present in the result of the join.
 Join type – defines how tuples in each relation that do not match any
tuple in the other relation (based on the join condition) are treated.

3.44
Joined Relations – Datasets for Examples
 Relation loan
 Relation borrower
 Note: borrower information missing for L-260 and loan
information missing for L-155

3.45
Joined Relations – Examples
 loan inner join borrower on
loan.loan_number = borrower.loan_number
 loan left outer join borrower on
loan.loan_number = borrower.loan_number

3.46
 loan natural inner join borrower
 loan natural right outer join borrower
 Find all customers who have either an account or a loan (but not both) at the bank.
from (depositor natural full outer join borrower )
where account_number is null or loan_number is null

3.47
 Natural join can get into trouble if two relations have an attribute with
same name that should not affect the join condition
 e.g. an attribute such as remarks may be present in many tables
 Solution:
 loan full outer join borrower using (loan_number)

3.48
Derived Relations
 SQL allows a subquery expression to be used in the from clause
 Find the average account balance of those branches where the average
account balance is greater than $1200.
select branch_name, avg_balance
from (select branch_name, avg (balance)
from account
group by branch_name )
as branch_avg ( branch_name, avg_balance )
where avg_balance > 1200
Note that we do not need to use the having clause, since we compute
the temporary (view) relation branch_avg in the from clause, and the
attributes of branch_avg can be used directly in the where clause.

3.49
View Definition
 A relation that is not of the conceptual model but is made visible to
a user as a “virtual relation” is called a view.
 A view is defined using the create view statement which has the
form
create view v as < query expression >
where <query expression> is any legal SQL expression. The view
name is represented by v.
 Once a view is defined, the view name can be used to refer to the
virtual relation that the view generates.

3.50
Example Queries
 A view consisting of branches and their customers
 Find all customers of the Perryridge branch
create view all_customer as
where depositor.account_number =
account.account_number )
union
from borrower, loan
where borrower.loan_number = loan.loan_number )
from all_customer

3.51
Uses of Views
 Hiding some information from some users
 Consider a user who needs to know a customer’s name, loan number
and branch name, but has no need to see the loan amount.
 Define a view
(create view cust_loan_data as
select customer_name, borrower.loan_number, branch_name
from borrower, loan
where borrower.loan_number = loan.loan_number )
 Grant the user permission to read cust_loan_data, but not borrower or
loan
 Predefined queries to make writing of other queries easier
 Common example: Aggregate queries used for statistical analysis of
data

3.52
Processing of Views
 When a view is created
 the query expression is stored in the database along with the view
name
 the expression is substituted into any query using the view
 Views definitions containing views
 One view may be used in the expression defining another view
 A view relation v1 is said to depend directly on a view relation v2 if
v2 is used in the expression defining v1
 A view relation v1 is said to depend on view relation v2 if either v1
depends directly to v2 or there is a path of dependencies from
v1 to v2
 A view relation v is said to be recursive if it depends on itself.

3.53
View Expansion
 A way to define the meaning of views defined in terms of other views.
 Let view v1 be defined by an expression e1 that may itself contain uses
of view relations.
 View expansion of an expression repeats the following replacement
step:
repeat
Find any view relation vi in e1
Replace the view relation vi by the expression defining vi
until no more view relations are present in e1
 As long as the view definitions are not recursive, this loop will
terminate

3.54
With Clause
 The with clause provides a way of defining a temporary view whose
definition is available only to the query in which the with clause
occurs.
 Find all accounts with the maximum balance
with max_balance (value) as
select max (balance)
from account
select account_number
from account, max_balance
where account.balance = max_balance.value

3.55
Complex Queries using With Clause
 Find all branches where the total account deposit is greater than the
average of the total account deposits at all branches.
with branch_total (branch_name, value) as
select branch_name, sum (balance)
from account
with branch_total_avg (value) as
select avg (value)
from branch_total
select branch_name
from branch_total, branch_total_avg
where branch_total.value >= branch_total_avg.value
 Note: the exact syntax supported by your database may vary slightly.
 E.g. Oracle syntax is of the form
with branch_total as ( select .. ),
branch_total_avg as ( select .. )
select …

3.56
Update of a View
 Create a view of all loan data in the loan relation, hiding the amount
attribute
create view loan_branch as
select loan_number, branch_name
from loan
 Add a new tuple to loan_branch
insert into loan_branch
values ('L-37‘, 'Perryridge‘)
This insertion must be represented by the insertion of the tuple
('L-37', 'Perryridge', null )
into the loan relation

3.57
Updates Through Views (Cont.)
 Some updates through views are impossible to translate into
updates on the database relations
 create view v as
select loan_number, branch_name, amount
from loan
where branch_name = ‘Perryridge’
insert into v values ( 'L-99','Downtown', '23')
 Others cannot be translated uniquely
 insert into all_customer values ('Perryridge', 'John')
 Have to choose loan or account, and
create a new loan/account number!
 Most SQL implementations allow updates only on simple views
(without aggregates) defined on a single relation

3.58
Null Values
 It is possible for tuples to have a null value, denoted by null, for some
of their attributes
 null signifies an unknown value or that a value does not exist.
 The predicate is null can be used to check for null values.
 Example: Find all loan number which appear in the loan relation
with null values for amount.
select loan_number
from loan
where amount is null
 The result of any arithmetic expression involving null is null
 Example: 5 + null returns null
 However, aggregate functions simply ignore nulls
 More on next slide

3.59
Null Values and Three Valued Logic
 Any comparison with null returns unknown
 Example: 5 < null or null <> null or null = null
 Three-valued logic using the truth value unknown:
 OR: (unknown or true) = true,
(unknown or false) = unknown
(unknown or unknown) = unknown
 AND: (true and unknown) = unknown,
(false and unknown) = false,
(unknown and unknown) = unknown
 NOT: (not unknown) = unknown
 “P is unknown” evaluates to true if predicate P evaluates to
unknown
 Result of where clause predicate is treated as false if it evaluates to
unknown

3.60
Null Values and Aggregates
 Total all loan amounts
select sum (amount )
from loan
 Above statement ignores null amounts
 Result is null if there is no non-null amount
 All aggregate operations except count(*) ignore tuples with null
values on the aggregated attributes.

End of Chapter 3

3.62
The where Clause (Cont.)
 SQL includes a between comparison operator
 Example: Find the loan number of those loans with loan amounts between
$90,000 and $100,000 (that is,  $90,000 and  $100,000)
select loan_number
from loan
where amount between 90000 and 100000

3.63
Figure 3.1: Database Schema
branch (branch_name, branch_city, assets)
customer (customer_name, customer_street, customer_city)
loan (loan_number, branch_name, amount)
borrower (customer_name, loan_number)
account (account_number, branch_name, balance)
depositor (customer_name, account_number)

3.64
Definition of Some Clause
0
5
0
5
(5  some ) = true (since 0  5)
) = true
(5 = some
 (= some)  in
 However, ( some) is not equivalent to not in

3.65
Definition of all Clause
0
5
6
(5 < all ) = false
6
10
4
) = true
5
4
6
(5  all ) = true (since 5  4 and 5  6)
(5 < all
) = false
(5 = all
 ( all)  not in
 However, (= all) is not equivalent to in

3.66
Test for Empty Relations
 The exists construct returns the value true if the argument subquery is
nonempty.
 exists r  r  Ø
 not exists r  r = Ø

3.67
Figure 3.3: Tuples inserted into loan and
borrower

3.68
Figure 3.4:
The loan and borrower relations

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