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Chapter 4: SQL
• Basic Structure
• Set Operations
• Aggregate Functions
• Null Values
• Nested Subqueries
• Derived Relations
• Views
• Modification of the Database
• Joined Relations
• Data Definition Language
• Embedded SQL
Database Systems Concepts 4.1 Silberschatz, Korth and Sudarshan c 1997
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Basic Structure
• SQL is based on set and relational operations with certain
modifications and enhancements
• A typical SQL query has the form:
select A1, A2, ..., An
from r1, r2, ..., rm
where P
– Ai s represent attributes
– ri s represent relations
– P is a predicate.
• This query is equivalent to the relational algebra expression:
ΠA1, A2, ..., An
(σP (r1 × r2 × ... × rm))
• The result of an SQL query is a relation.
Database Systems Concepts 4.2 Silberschatz, Korth and Sudarshan c 1997
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The select Clause
• The select clause corresponds to the projection operation of
the relational algebra. It is used to list the attributes desired in
the result of a query.
• Find the names of all branches in the loan relation
select branch-name
from loan
In the “pure” relational algebra syntax, this query would be:
Πbranch-name (loan)
• An asterisk in the select clause denotes “all attributes”
select ∗
from loan
Database Systems Concepts 4.3 Silberschatz, Korth and Sudarshan c 1997
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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 relation, and remove
duplicates
select distinct branch-name
from loan
• The keyword all specifies that duplicates not be removed.
select all branch-name
from loan
Database Systems Concepts 4.4 Silberschatz, Korth and Sudarshan c 1997
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The select Clause (Cont.)
• The select clause can contain arithmetic expressions involving
the operators, +, −, ∗, and /, and operating on constants or
attributes of tuples.
• The query:
select branch-name, loan-number, amount ∗ 100
from loan
would return a relation which is the same as the loan relation,
except that the attribute amount is multiplied by 100
Database Systems Concepts 4.5 Silberschatz, Korth and Sudarshan c 1997
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The where Clause
• The where clause corresponds to the selection predicate of
the relational algebra. It consists of a predicate involving
attributes of the relations that appear in the from clause.
• Find all loan numbers 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
• SQL uses the logical connectives and, or, and not. It allows
the use of arithmetic expressions as operands to the
comparison operators.
Database Systems Concepts 4.6 Silberschatz, Korth and Sudarshan c 1997
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The where Clause (Cont.)
• SQL includes a between comparison operator in order to
simplify where clauses that specify that a value be less than or
equal to some value and greater than or equal to some other
value.
• 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
Database Systems Concepts 4.7 Silberschatz, Korth and Sudarshan c 1997
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The from Clause
• The from clause corresponds to the Cartesian product
operation of the relational algebra. It lists the relations to be
scanned in the evaluation of the expression.
• Find the Cartesian product borrower × loan
select ∗
from borrower, loan
• Find the name and loan number of all customers having a loan
at the Perryridge branch.
select distinct customer-name, borrower.loan-number
from borrower, loan
where borrower.loan-number = loan.loan-number and
branch-name = “Perryridge”
Database Systems Concepts 4.8 Silberschatz, Korth and Sudarshan c 1997
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The Rename Operation
• The SQL mechanism for renaming relations and attributes is
accomplished through the as clause:
old-name as new-name
• Find the name and loan number of all customers having a loan
at the Perryridge branch; replace the column name
loan-number with the name loan-id.
select distinct customer-name, borrower.loan-number as loan-id
from borrower, loan
where borrower.loan-number = loan.loan-number and
branch-name = “Perryridge”
Database Systems Concepts 4.9 Silberschatz, Korth and Sudarshan c 1997
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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 for all
customers having a loan at some branch.
select distinct customer-name, T.loan-number
from borrower as T, loan as S
where T.loan-number = S.loan-number
• 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”
Database Systems Concepts 4.10 Silberschatz, Korth and Sudarshan c 1997
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String Operations
• SQL includes a string-matching operator for comparisons on
character strings. Patterns 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 “\”
Database Systems Concepts 4.11 Silberschatz, Korth and Sudarshan c 1997
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Ordering the Display of Tuples
• List in alphabetic order the names of all customers having a
loan at 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.
• SQL must perform a sort to fulfill an order by request. Since
sorting a large number of tuples may be costly, it is desirable to
sort only when necessary.
Database Systems Concepts 4.12 Silberschatz, Korth and Sudarshan c 1997
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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. If there are c1 copies of tuple t1 in r1, and t1 satisfies
selection σθ, then there are c1 copies of t1 in σθ(r1).
2. 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. If there are c1 copies of tuple t1 in r1 and c2 copies of tuple
t2 in r2, there are c1 × c2 copies of the tuple t1.t2 in r1 × r2.
Database Systems Concepts 4.13 Silberschatz, Korth and Sudarshan c 1997
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Duplicates (Cont.)
• Suppose relations r1 with schema (A, B) and r2 with schema
(C) are the following multisets:
r1 = {(1, a), (2, a)} r2 = {(2), (3), (3)}
• Then ΠB(r1) would be {(a), (a)}, while ΠB(r1) × 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:
ΠA1, A2, ..., An
(σP (r1 × r2 × ... × rm))
Database Systems Concepts 4.14 Silberschatz, Korth and Sudarshan c 1997
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Set Operations
• The set operations union, intersect, and except operate on
relations and correspond to the relational algebra operations ∪,
∩, and −.
• 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
Database Systems Concepts 4.15 Silberschatz, Korth and Sudarshan c 1997
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Set Operations
• Find all customers who have a loan, an account, or both:
(select customer-name from depositor)
union
(select customer-name from borrower)
• Find all customers who have both a loan and an account.
(select customer-name from depositor)
intersect
(select customer-name from borrower)
• Find all customers who have an account but no loan.
(select customer-name from depositor)
except
(select customer-name from borrower)
Database Systems Concepts 4.16 Silberschatz, Korth and Sudarshan c 1997
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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
Database Systems Concepts 4.17 Silberschatz, Korth and Sudarshan c 1997
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Aggregate Functions (Cont.)
• Find the average account balance at the Perryridge branch.
select avg (balance)
from account
where branch-name = “Perryridge”
• Find the number of tuples in the customer relation.
select count (*)
from customer
• Find the number of depositors in the bank
select count (distinct customer-name)
from depositor
Database Systems Concepts 4.18 Silberschatz, Korth and Sudarshan c 1997
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Aggregate Functions – Group By
• 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
Note: Attributes in select clause outside of aggregate
functions must appear in group by list.
Database Systems Concepts 4.19 Silberschatz, Korth and Sudarshan c 1997
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Aggregate Functions – Having Clause
• Find the names of all branches where the average account
balance is more than $1,200
select branch-name, avg (balance)
from account
group by branch-name
having avg (balance) > 1200
Note: predicates in the having clause are applied after the
formation of groups
Database Systems Concepts 4.20 Silberschatz, Korth and Sudarshan c 1997
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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 result of any arithmetic expression involving null is null.
• Roughly speaking, all comparisons involving null return false.
More precisely,
– Any comparison with null returns unknown
– (true or unknown) = true, (false or unknown) = unknown
(unknown or unknown) = unknown,
(true and unknown) = unknown, (false and unknown) = false,
(unknown and unknown) = unknown
– Result of where clause predicate is treated as false if it
evaluates to unknown
– “P is unknown” evaluates to true if predicate P evaluates
to unknown
Database Systems Concepts 4.21 Silberschatz, Korth and Sudarshan c 1997
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Null Values (Cont.)
• Find all loan numbers which appear in the loan relation with
null values for amount.
select loan-number
from loan
where amount is null
• 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.
Database Systems Concepts 4.22 Silberschatz, Korth and Sudarshan c 1997
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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.
Database Systems Concepts 4.23 Silberschatz, Korth and Sudarshan c 1997
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Set Membership
• F in r ⇔ ∃ t ∈ r (t = F)
0
(5 in 4 ) = true
5
0
(5 in 4 ) = false
6
0
(5 not in 4 ) = true
6
Database Systems Concepts 4.24 Silberschatz, Korth and Sudarshan c 1997
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Example Query
• Find all customers who have both an account and a loan at
bank.
select distinct customer-name
from borrower
where customer-name in (select customer-name
from depositor)
• 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)
Database Systems Concepts 4.25 Silberschatz, Korth and Sudarshan c 1997
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Example Query
• Find all customers who have both an account and a loan at the
Perryridge branch.
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)
Database Systems Concepts 4.26 Silberschatz, Korth and Sudarshan c 1997
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Set Comparison
• Find 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”
Database Systems Concepts 4.27 Silberschatz, Korth and Sudarshan c 1997
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The Some Clause
• F <comp> some r ⇔ ∃ t(t ∈ r∧ [F <comp> t])
Where <comp> can be: <, ≤, >, ≥, =, =
0
(5 < some 5 ) = true
6 (read: 5 < some tuple in the relation)
0
(5 < some 5 ) = false
0
(5 = some 5 ) = true
0
(5 = some 5 ) = true (since 0 = 5)
• (= some) ≡ in
• However, (= some) ≡ not in
Database Systems Concepts 4.28 Silberschatz, Korth and Sudarshan c 1997
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Example Query
• Find all branches that have greater assets than some branch
located in Brooklyn.
select branch-name
from branch
where assets > some
(select assets
from branch
where branch-city = “Brooklyn”)
Database Systems Concepts 4.29 Silberschatz, Korth and Sudarshan c 1997
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The All Clause
• F <comp> all r ⇔ ∀ t (t ∈ r ∧ [F <comp> t])
0
(5 < all 5 ) = false
6
6
(5 < all 10 ) = true
4
(5 = all 5 ) = false
4
(5 = all 6 ) = true (since 5 = 4 and 5 = 6)
• (= all) ≡ not in
• However, (= all) ≡ in
Database Systems Concepts 4.30 Silberschatz, Korth and Sudarshan c 1997
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Example Query
• 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”)
Database Systems Concepts 4.31 Silberschatz, Korth and Sudarshan c 1997
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Test for Empty Relations
• The exists construct returns the value true if the argument
subquery is nonempty.
• exists r ⇔ r = ∅
• not exists r ⇔ r = ∅
Database Systems Concepts 4.32 Silberschatz, Korth and Sudarshan c 1997
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Example Query
• 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 = ∅ ⇔ X ⊆ Y
Database Systems Concepts 4.33 Silberschatz, Korth and Sudarshan c 1997
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Test for Absence of Duplicate Tuples
• The unique construct tests whether a subquery has any
duplicate tuples in its result.
• Find all customers who have only 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”)
Database Systems Concepts 4.34 Silberschatz, Korth and Sudarshan c 1997
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Example Query
• Find all customers who have at least two accounts at the
Perryridge branch.
select distinct T.customer-name
from depositor 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”)
Database Systems Concepts 4.35 Silberschatz, Korth and Sudarshan c 1997
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Derived Relations
• 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 result (branch-name, avg-balance)
where avg-balance > 1200
Note that we do not need to use the having clause, since we
compute in the from clause the temporary relation result, and
the attributes of result can be used directly in the where
clause.
Database Systems Concepts 4.36 Silberschatz, Korth and Sudarshan c 1997
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Views
• Provide a mechanism to hide certain data from the view of
certain users. To create a view we use the command:
create view v as <query expression>
where:
– <query expression> is any legal expression
– the view name is represented by v
Database Systems Concepts 4.37 Silberschatz, Korth and Sudarshan c 1997
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Example Queries
• A view consisting of branches and their customers
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)
• Find all customers of the Perryridge branch
select customer-name
from all-customer
where branch-name = “Perryridge”
Database Systems Concepts 4.38 Silberschatz, Korth and Sudarshan c 1997
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Modification of the Database – Deletion
• Delete all account records at the Perryridge branch
delete from account
where branch-name = “Perryridge”
• Delete all accounts at every branch located in Needham.
delete from account
where branch-name in (select branch-name
from branch
where branch-city = “Needham”)
delete from depositor
where account-number in (select account-number
from branch, account
where branch-city = “Needham”
and branch.branch-name = account.branch-name)
Database Systems Concepts 4.39 Silberschatz, Korth and Sudarshan c 1997
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Example Query
• Delete the records 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)
Database Systems Concepts 4.40 Silberschatz, Korth and Sudarshan c 1997
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Modification of the Database – Insertion
• Add a new tuple to account
insert into account
values (“Perryridge”, A-9732, 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 (“Perryridge”, A-777, null)
Database Systems Concepts 4.41 Silberschatz, Korth and Sudarshan c 1997
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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 branch-name, loan-number, 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
Database Systems Concepts 4.42 Silberschatz, Korth and Sudarshan c 1997
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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 (Exercise
4.11)
Database Systems Concepts 4.43 Silberschatz, Korth and Sudarshan c 1997
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Update of a View
• Create a view of all loan data in the loan relation, hiding the
amount attribute
create view branch-loan as
select branch-name, loan-number
from loan
• Add a new tuple to branch-loan
insert into branch-loan
values (“Perryridge”, “L-307”)
This insertion must be represented by the insertion of the tuple
(“Perryridge”, “L-307”, null)
into the loan relation.
• Updates on more complex views are difficult or impossible to
translate, and hence are disallowed.
Database Systems Concepts 4.44 Silberschatz, Korth and Sudarshan c 1997
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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.
Join Types
inner join
left outer join
right outer join
full outer join
Join Conditions
natural
on <predicate>
using (A1, A2, . . . , An)
Database Systems Concepts 4.45 Silberschatz, Korth and Sudarshan c 1997
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Joined Relations – Datasets for Examples
• Relation loan
branch-name loan-number amount
Downtown L-170 3000
Redwood L-230 4000
Perryridge L-260 1700
• Relation borrower
customer-name loan-number
Jones L-170
Smith L-230
Hayes L-155
Database Systems Concepts 4.46 Silberschatz, Korth and Sudarshan c 1997
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Joined Relations – Examples
• loan inner join borrower on
loan.loan-number = borrower.loan-number
branch-name loan-number amount customer-name loan-number
Downtown L-170 3000 Jones L-170
Redwood L-230 4000 Smith L-230
• loan left outer join borrower on
loan.loan-number=borrower.loan-number
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
Database Systems Concepts 4.47 Silberschatz, Korth and Sudarshan c 1997
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Joined Relations – Examples
• loan natural inner join borrower
branch-name loan-number amount customer-name
Downtown L-170 3000 Jones
Redwood L-230 4000 Smith
• loan natural right outer join borrower
branch-name loan-number amount customer-name
Downtown L-170 3000 Jones
Redwood L-230 4000 Smith
null L-155 null Hayes
Database Systems Concepts 4.48 Silberschatz, Korth and Sudarshan c 1997
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Joined Relations – Examples
• loan full outer join borrower using (loan-number)
branch-name loan-number amount customer-name
Downtown L-170 3000 Jones
Redwood L-230 4000 Smith
Perryridge L-260 1700 null
null L-155 null Hayes
• 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 Systems Concepts 4.49 Silberschatz, Korth and Sudarshan c 1997
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Data Definition Language (DDL)
Allows the specification of not only a set of relations but also
information about each relation, including:
• The schema for each relation.
• The domain of values associated with each attribute.
• Integrity constraints.
• The set of indices to be maintained for each relation.
• Security and authorization information for each relation.
• The physical storage structure of each relation on disk.
Database Systems Concepts 4.50 Silberschatz, Korth and Sudarshan c 1997
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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.
Database Systems Concepts 4.51 Silberschatz, Korth and Sudarshan c 1997
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Domain Types in SQL (Cont.)
• 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.
• date. Dates, containing a (4 digit) year, month and date.
• time. Time of day, in hours, minutes and seconds.
– Null values are allowed in all the domain types. Declaring an
attribute to be not null prohibits null values for that attribute.
– create domain construct in SQL-92 creates user-defined
domain types
create domain person-name char(20) not null
Database Systems Concepts 4.52 Silberschatz, Korth and Sudarshan c 1997
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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 values in the domain of attribute Ai
• Example:
create table branch
(branch-name char(15) not null,
branch-city char(30),
assets integer)
Database Systems Concepts 4.53 Silberschatz, Korth and Sudarshan c 1997
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Integrity Constraints In Create Table
• not null
• primary key (A1, . . . , An)
• check (P), where P is a predicate
Example: Declare branch-name as the primary key for branch and
ensure that the values of assets are non-negative.
create table branch
(branch-name char(15) not null,
branch-city char(30),
assets integer,
primary key (branch-name),
check (assets >= 0))
• primary key declaration on an attribute automatically ensures
not null in SQL-92
Database Systems Concepts 4.54 Silberschatz, Korth and Sudarshan c 1997
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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. All tuples in the relation are assigned null as
the value for the new attribute. The form of the alter table
command is
alter table r add A D
where A is the name of the attribute be added to relation r and
and D is the domain of A.
• 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.
Database Systems Concepts 4.55 Silberschatz, Korth and Sudarshan c 1997
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Embedded SQL
• The SQL standard defines embeddings of SQL in a variety of
programming languages such as such as Pascal, PL/I, Fortran,
C, and Cobol.
• A language in which SQL queries are embedded is referred to
as a host language, and the SQL structures permitted in the
host language comprise embedded SQL.
• The basic form of these languages follows that of the System
R embedding of SQL into PL/I.
• EXEC SQL statement is used to identify embedded SQL
requests to the preprocessor
EXEC SQL <embedded SQL statement > END EXEC
Database Systems Concepts 4.56 Silberschatz, Korth and Sudarshan c 1997
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Example Query
From within a host language, find the names and account numbers
of customers with more than the variable amount dollars in some
account.
• Specify the query in SQL and declare a cursor for it
EXEC SQL
declare c cursor for
select customer-name, account-number
from depositor, account
where depositor.account-number = account.account-number
and account.balance > :amount
END-EXEC
Database Systems Concepts 4.57 Silberschatz, Korth and Sudarshan c 1997
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Embedded SQL (Cont.)
• The open statement causes the query to be evaluated
EXEC SQL open c END-EXEC
• The fetch statement causes the values of one tuple in the
query result to be placed in host language variables.
EXEC SQL fetch c into :cn :an END-EXEC
Repeated calls to fetch get successive tuples in the query
result; a variable in the SQL communication area indicates
when end-of-file is reached.
• The close statement causes the database system to delete
the temporary relation that holds the result of the query.
EXEC SQL close c END-EXEC
Database Systems Concepts 4.58 Silberschatz, Korth and Sudarshan c 1997
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Dynamic SQL
• Allows programs to construct and submit SQL queries at run
time.
• Example of the use of dynamic SQL from within a C program.
char * sqlprog = “update account set balance = balance ∗1.05
where account-number = ?”
EXEC SQL prepare dynprog from :sqlprog;
char account[10] = “A-101”;
EXEC SQL execute dynprog using :account;
• The dynamic SQL program contains a ?, which is a place
holder for a value that is provided when the SQL program is
executed.
Database Systems Concepts 4.59 Silberschatz, Korth and Sudarshan c 1997
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Other SQL Features
• Fourth-generation languages – special language to assist
application programmers in creating templates on the screen
for a user interface, and in formatting data for report
generation; available in most commercial database products
• SQL sessions – provide the abstraction of a client and a server
(possibly remote)
– client connects to an SQL server, establishing a session
– executes a series of statements
– disconnects the session
– can commit or rollback the work carried out in the session
• An SQL environment contains several components, including a
user identifier, and a schema, which identifies which of several
schemas a session is using.
Database Systems Concepts 4.60 Silberschatz, Korth and Sudarshan c 1997