Cơ sở dữ liệu - Chapter 3: SQL
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 where branch_city = 'Brooklyn') 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
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Database System Concepts, 5th Ed.
©Silberschatz, Korth and Sudarshan
See www.dbbook.com for conditions on reuse
Chapter 3: SQL
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Chapter 3: SQL
n Data Definition
n Basic Query Structure
n Set Operations
n Aggregate Functions
n Null Values
n Nested Subqueries
n Complex Queries
n Views
n Modification of the Database
n Joined Relations**
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
History
n IBM Sequel language developed as part of System R project at the
IBM San Jose Research Laboratory
n Renamed Structured Query Language (SQL)
n ANSI and ISO standard SQL:
l SQL86
l SQL89
l SQL92
l SQL:1999 (language name became Y2K compliant!)
l SQL:2003
n Commercial systems offer most, if not all, SQL92 features, plus
varying feature sets from later standards and special proprietary
features.
l Not all examples here may work on your particular system.
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Data Definition Language
n The schema for each relation.
n The domain of values associated with each attribute.
n Integrity constraints
n The set of indices to be maintained for each relations.
n Security and authorization information for each relation.
n The physical storage structure of each relation on disk.
Allows the specification of not only a set of relations but also
information about each relation, including:
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Domain Types in SQL
n char(n). Fixed length character string, with userspecified length n.
n varchar(n). Variable length character strings, with userspecified maximum
length n.
n int. Integer (a finite subset of the integers that is machinedependent).
n smallint. Small integer (a machinedependent subset of the integer
domain type).
n numeric(p,d). Fixed point number, with userspecified precision of p digits,
with n digits to the right of decimal point.
n real, double precision. Floating point and doubleprecision floating point
numbers, with machinedependent precision.
n float(n). Floating point number, with userspecified precision of at least n
digits.
n More are covered in Chapter 4.
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Create Table Construct
n An SQL relation is defined using the create table command:
create table r (A1 D1, A2 D2, ..., An Dn,
(integrityconstraint1),
...,
(integrityconstraintk))
l r is the name of the relation
l each Ai is an attribute name in the schema of relation r
l Di is the data type of values in the domain of attribute Ai
n Example:
create table branch
(branch_name char(15) not null,
branch_city char(30),
assets integer)
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Integrity Constraints in Create Table
n not null
n 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),
assets integer,
primary key (branch_name))
primary key declaration on an attribute automatically ensures
not null in SQL92 onwards, needs to be explicitly stated in
SQL89
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Drop and Alter Table Constructs
n The drop table command deletes all information about the dropped
relation from the database.
n 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.
l All tuples in the relation are assigned null as the value for the
new attribute.
n 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
l Dropping of attributes not supported by many databases
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Basic Query Structure
n SQL is based on set and relational operations with certain
modifications and enhancements
n A typical SQL query has the form:
select A1, A2, ..., An
from r1, r2, ..., rm
where P
l Ai represents an attribute
l Ri represents a relation
l P is a predicate.
n This query is equivalent to the relational algebra expression.
n The result of an SQL query is a relation.
))(( 21,,, 21 mPAAA rrrn ×××∏ σ
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
The select Clause
n The select clause list the attributes desired in the result of a query
l corresponds to the projection operation of the relational algebra
n Example: find the names of all branches in the loan relation:
select branch_name
from loan
n In the relational algebra, the query would be:
∏branch_name (loan)
n NOTE: SQL names are case insensitive (i.e., you may use upper or
lowercase letters.)
l E.g. Branch_Name BRANCH_NAME branch_name
l Some people use upper case wherever we use bold font.
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
The select Clause (Cont.)
n SQL allows duplicates in relations as well as in query results.
n To force the elimination of duplicates, insert the keyword distinct after
select.
n Find the names of all branches in the loan relations, and remove
duplicates
select distinct branch_name
from loan
n The keyword all specifies that duplicates not be removed.
select all branch_name
from loan
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
The select Clause (Cont.)
n An asterisk in the select clause denotes “all attributes”
select *
from loan
n The select clause can contain arithmetic expressions involving the
operation, +, –, ∗, and /, and operating on constants or attributes of
tuples.
n The query:
select loan_number, branch_name, amount ∗ 100
from loan
would return a relation that is the same as the loan relation, except that
the value of the attribute amount is multiplied by 100.
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
The where Clause
n The where clause specifies conditions that the result must satisfy
l Corresponds to the selection predicate of the relational algebra.
n 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
n Comparison results can be combined using the logical connectives and,
or, and not.
n Comparisons can be applied to results of arithmetic expressions.
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
The where Clause (Cont.)
n SQL includes a between comparison operator
n 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
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
The from Clause
n The from clause lists the relations involved in the query
l Corresponds to the Cartesian product operation of the relational algebra.
n Find the Cartesian product borrower X loan
select ∗
from borrower, loan
n 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'
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
The Rename Operation
n The SQL allows renaming relations and attributes using the as clause:
oldname as newname
n 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
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Tuple Variables
n Tuple variables are defined in the from clause via the use of the as
clause.
n Find the customer names and their loan numbers for all customers
having a loan at some branch.
n 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'
nKeyword as is optional and may be omitted
borrower as T borrower T
select customer_name, T.loan_number, S.amount
from borrower as T, loan as S
where T.loan_number = S.loan_number
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
String Operations
n SQL includes a stringmatching operator for comparisons on character
strings. The operator “like” uses patterns that are described using two
special characters:
l percent (%). The % character matches any substring.
l underscore (_). The _ character matches any character.
n Find the names of all customers whose street includes the substring
“Main”.
select customer_name
from customer
where customer_street like '% Main%'
n Match the name “Main%”
like 'Main\%' escape '\'
n SQL supports a variety of string operations such as
l concatenation (using “||”)
l converting from upper to lower case (and vice versa)
l finding string length, extracting substrings, etc.
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Ordering the Display of Tuples
n 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
n We may specify desc for descending order or asc for ascending
order, for each attribute; ascending order is the default.
l Example: order by customer_name desc
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Duplicates
n In relations with duplicates, SQL can define how many copies of tuples
appear in the result.
n Multiset versions of some of the relational algebra operators – given
multiset relations r1 and r2:
1. σθ (r1): If there are c1 copies of tuple t1 in r1, and t1 satisfies
selections σθ,, then there are c1 copies of t1 in σθ (r1).
2. ΠA (r ): For each copy of tuple t1 in r1, there is a copy of tuple ΠA
(t1) in ΠA (r1) where ΠA (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
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Duplicates (Cont.)
n Example: Suppose multiset relations r1 (A, B) and r2 (C) are as
follows:
r1 = {(1, a) (2,a)} r2 = {(2), (3), (3)}
n Then ΠB(r1) would be {(a), (a)}, while ΠB(r1) x r2 would be
{(a,2), (a,2), (a,3), (a,3), (a,3), (a,3)}
n SQL duplicate semantics:
select A1,, A2, ..., An
from r1, r2, ..., rm
where P
is equivalent to the multiset version of the expression:
))(( 21,,, 21 mPAAA rrrn ×××∏ σ
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Set Operations
n The set operations union, intersect, and except operate on relations
and correspond to the relational algebra operations ∪, ∩, −.
n 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:
l m + n times in r union all s
l min(m,n) times in r intersect all s
l max(0, m – n) times in r except all s
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Set Operations
n Find all customers who have a loan, an account, or both:
(select customer_name from depositor)
except
(select customer_name from borrower)
(select customer_name from depositor)
intersect
(select customer_name from borrower)
n Find all customers who have an account but no loan.
(select customer_name from depositor)
union
(select customer_name from borrower)
n Find all customers who have both a loan and an account.
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Aggregate Functions
n 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
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Aggregate Functions (Cont.)
n Find the average account balance at the Perryridge branch.
n Find the number of depositors in the bank.
n 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
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Aggregate Functions – Group By
n 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
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Aggregate Functions – Having Clause
n 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
group by branch_name
having avg (balance) > 1200
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Null Values
n It is possible for tuples to have a null value, denoted by null, for some
of their attributes
n null signifies an unknown value or that a value does not exist.
n The predicate is null can be used to check for null values.
l 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
n The result of any arithmetic expression involving null is null
l Example: 5 + null returns null
n However, aggregate functions simply ignore nulls
l More on next slide
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Null Values and Three Valued Logic
n Any comparison with null returns unknown
l Example: 5 null or null = null
n Threevalued logic using the truth value unknown:
l OR: (unknown or true) = true,
(unknown or false) = unknown
(unknown or unknown) = unknown
l AND: (true and unknown) = unknown,
(false and unknown) = false,
(unknown and unknown) = unknown
l NOT: (not unknown) = unknown
l “P is unknown” evaluates to true if predicate P evaluates to
unknown
n Result of where clause predicate is treated as false if it evaluates to
unknown
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Null Values and Aggregates
n Total all loan amounts
select sum (amount )
from loan
l Above statement ignores null amounts
l Result is null if there is no nonnull amount
n All aggregate operations except count(*) ignore tuples with null
values on the aggregated attributes.
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Nested Subqueries
n SQL provides a mechanism for the nesting of subqueries.
n A subquery is a selectfromwhere expression that is nested within
another query.
n A common use of subqueries is to perform tests for set membership, set
comparisons, and set cardinality.
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Example Query
n Find all customers who have both an account and a loan at the bank.
n Find all customers who have a loan at the bank but do not have
an account at the bank
select distinct customer_name
from borrower
where customer_name not in (select customer_name
from depositor )
select distinct customer_name
from borrower
where customer_name in (select customer_name
from depositor )
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Example Query
n Find all customers who have both an account and a loan at the
Perryridge branch
n Note: Above query can be written in a much simpler manner. The
formulation above is simply to illustrate SQL features.
select distinct customer_name
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
from depositor, account
where depositor.account_number =
account.account_number )
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Set Comparison
n Find all branches that have greater assets than some branch located
in Brooklyn.
n 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'
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Definition of Some Clause
n F some r ⇔ ∃ t ∈ r such that (F t )
Where can be: , =, ≠
0
5
6
(5 < some ) = true
0
5
0
) = false
5
0
5(5 ≠ some ) = true (since 0 ≠ 5)
(read: 5 < some tuple in the relation)
(5 < some
) = true(5 = some
(= some) ≡ in
However, (≠ some) ≡ not in
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Example Query
n 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
where branch_city = 'Brooklyn')
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Definition of all Clause
n F all r ⇔ ∀ t ∈ r (F t)
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) ≡ in
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Test for Empty Relations
n The exists construct returns the value true if the argument subquery is
nonempty.
n exists r ⇔ r ≠ Ø
n not exists r ⇔ r = Ø
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Example Query
n 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
where branch_city = 'Brooklyn')
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 ))
n Note that X – Y = Ø ⇔ X ⊆ Y
n Note: Cannot write this query using = all and its variants
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Test for Absence of Duplicate Tuples
n The unique construct tests whether a subquery has any duplicate
tuples in its result.
n 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')
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Example Query
n 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')
n Variable from outer level is known as a correlation variable
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Derived Relations
n SQL allows a subquery expression to be used in the from clause
n 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.
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
With Clause
n 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.
n 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
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Complex Queries using With Clause
n 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
group by branch_name
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
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Views
n In some cases, it is not desirable for all users to see the entire logical
model (that is, all the actual relations stored in the database.)
n Consider a person who needs to know a customer’s name, loan number
and branch name, but has no need to see the loan amount. This person
should see a relation described, in SQL, by
(select customer_name, borrower.loan_number, branch_name
from borrower, loan
where borrower.loan_number = loan.loan_number )
n A view provides a mechanism to hide certain data from the view of
certain users.
n Any relation that is not of the conceptual model but is made visible to a
user as a “virtual relation” is called a view.
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
View Definition
n A view is defined using the create view statement which has the
form
create view v as
where is any legal SQL expression. The view
name is represented by v.
n Once a view is defined, the view name can be used to refer to the
virtual relation that the view generates.
n When a view is created, the query expression is stored in the
database; the expression is substituted into queries using the view.
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Example Queries
n A view consisting of branches and their customers
n Find all customers of the Perryridge branch
create view all_customer as
(select branch_name, customer_name
from depositor, account
where depositor.account_number =
account.account_number )
union
(select branch_name, customer_name
from borrower, loan
where borrower.loan_number = loan.loan_number )
select customer_name
from all_customer
where branch_name = 'Perryridge'
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Views Defined Using Other Views
n One view may be used in the expression defining another view
n A view relation v1 is said to depend directly on a view relation v2 if v2 is
used in the expression defining v1
n 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
n A view relation v is said to be recursive if it depends on itself.
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
View Expansion
n A way to define the meaning of views defined in terms of other views.
n Let view v1 be defined by an expression e1 that may itself contain uses
of view relations.
n 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
n As long as the view definitions are not recursive, this loop will
terminate
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Modification of the Database – Deletion
n Delete all account tuples at the Perryridge branch
delete from account
where branch_name = 'Perryridge'
n 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')
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Example Query
n Delete the record of all accounts with balances below the average at
the bank.
delete from account
where balance < (select avg (balance )
from account )
l Problem: as we delete tuples from deposit, the average balance
changes
l 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)
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Modification of the Database – Insertion
n Add a new tuple to account
insert into account
values ('A9732', 'Perryridge', 1200)
or equivalently
insert into account (branch_name, balance, account_number)
values ('Perryridge', 1200, 'A9732')
n Add a new tuple to account with balance set to null
insert into account
values ('A777','Perryridge', null )
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Modification of the Database – Insertion
n 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
where branch_name = 'Perryridge'
insert into depositor
select customer_name, loan_number
from loan, borrower
where branch_name = 'Perryridge'
and loan.account_number = borrower.account_number
n The select from where statement is evaluated fully before any of its
results are inserted into the relation (otherwise queries like
insert into table1 select * from table1
would cause problems)
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Modification of the Database – Updates
n Increase all accounts with balances over $10,000 by 6%, all other
accounts receive 5%.
l Write two update statements:
update account
set balance = balance ∗ 1.06
where balance > 10000
update account
set balance = balance ∗ 1.05
where balance ≤ 10000
l The order is important
l Can be done better using the case statement (next slide)
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Case Statement for Conditional Updates
n 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
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Update of a View
n 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
n Add a new tuple to branch_loan
insert into branch_loan
values ('L37‘, 'Perryridge‘)
This insertion must be represented by the insertion of the tuple
('L37', 'Perryridge', null )
into the loan relation
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Updates Through Views (Cont.)
n Some updates through views are impossible to translate into
updates on the database relations
l create view v as
select loan_number, branch_name, amount
from loan
where branch_name = ‘Perryridge’
insert into v values ( 'L99','Downtown', '23')
n Others cannot be translated uniquely
l insert into all_customer values ('Perryridge', 'John')
Have to choose loan or account, and
create a new loan/account number!
n Most SQL implementations allow updates only on simple views
(without aggregates) defined on a single relation
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Joined Relations**
n Join operations take two relations and return as a result another
relation.
n These additional operations are typically used as subquery
expressions in the from clause
n Join condition – defines which tuples in the two relations match, and
what attributes are present in the result of the join.
n 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.
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Joined Relations – Datasets for Examples
n Relation loan
n Relation borrower
n Note: borrower information missing for L260 and loan
information missing for L155
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Joined Relations – Examples
n loan inner join borrower on
loan.loan_number = borrower.loan_number
n loan left outer join borrower on
loan.loan_number = borrower.loan_number
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Joined Relations – Examples
n loan natural inner join borrower
n loan natural right outer join borrower
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Joined Relations – Examples
n loan full outer join borrower using (loan_number)
n Find all customers who have either an account or a loan (but not both)
at the bank.
select customer_name
from (depositor natural full outer join borrower )
where account_number is null or loan_number is null
Database System Concepts, 5th Ed.
©Silberschatz, Korth and Sudarshan
See www.dbbook.com for conditions on reuse
End of Chapter 3
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
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)
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Figure 3.3: Tuples inserted into loan and
borrower
©Silberschatz, Korth and Sudarshan3.Database System Concepts, 5th Ed., June 2006
Figure 3.4:
The loan and borrower relations
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