* What is a subquery?
* Multiple Subqueries
* Nested subquery
* Using subquery in DML commands
* Correlated subquery
* EXISTS, NOT EXISTS, ANY and ALL operators
What Is A Subquery
A subquery is a query within another query. The outer query is called as main query and
inner query is called as subquery.
In this chapter, we will see how subqueries are
used to retrieve the required data and also how to use subqueries in DML and
DDL commands.
The general syntax of subquery will be as
follows:
Query
(subquery)
The following is a simple example of a subquery.
select name, qual from faculty
where
fcode in
(
select fcode from course_faculty
where ccode = 'ora')
NAME QUAL
------------------------------
------------------------------
George Koch MS Computer Science
Jason Couchman OCP DBA
Kevin Loney MS Electronics
In the above example we will take the details of
faculty members who can handle course ora. COURSE_FACULTY table contains information
about which faculty members can take course ora. So we first use a subquery to
get the list of the codes of the faculty members who can handle Oracle course. Then we send the list of faculty codes to
outer query, which will then display the details of those faculty members.
Subquery is always executed first and the result
is passed to the main query. Main query
is executed by taking the result from subquery.
IN operator plays a very important role in subqueries
as generally subqueries generate a list of values and main query is to compare
a single value against the list of values supplied by subquery.
In the above example, subquery supplies the list
of faculty codes to main query. Then main query compares each faculty code of
the FACULTY table with the list supplied by subquery. If the faculty code
exists in the list then it will display the details of the faculty.
The following are a few other examples of
subqueries.
Get the details of students who have paid
today.
select * from students where
rollno in
(
select rollno from payments where trunc(dp) = trunc(sysdate));
Display the details of batches handled by
faculty name ‘Kevin Loney’.
select * from batches
where fcode =
( select fcode from faculty
where name = 'Kevin Loney');
The following query displays the details of
the faculty members who have not taken any batch in the last three months.
select * from faculty
where
fcode NOT IN
(select
fcode from batches
where months_between(sysdate,stdate)
<= 3)
For example, the following is invalid.
SQL> select * from subjects
2 where fulldur
3 between 25
4 and
(select max(fulldur) from subjects);
and (select max(fulldur) from
subjects)
*
ERROR at line 4:
ORA-00936: missing expression
Multiple Subqueries
It is possible for a main query to receive
values from more than one subquery. The following example displays the details
of batches that are taken by faculty with qualification MS or the course fee is
more than 5000.
select
from batches
where fcode in
(select fcode from faculty where
qual like '%MS%')
or ccode in
(
select ccode from courses where fee > 5000);
BCODE CCODE FCODE STDATE ENDDATE
TIMING
----- ----- ----- --------- --------- ---------
b1
ora gk 12-JAN-01 20-FEB-01 1
b3
c hs 20-JAN-01 27-FEB-01 3
b4
xml dh 02-MAR-01 30-MAR-01 3
b5
java hs 05-APR-01 10-MAY-01 1
b6
vbnet da 12-JUL-01 1
In the above query first the subquery – select
fcode from faculty where qual like ‘%MS%’ – is
executed. It then retrieves faculty codes where qualification of the faculty
contains MS. Then it executes second subquery and then returns course codes of
the courses where FEE is more than 5000. After both the subqueries are executed
then the main query is executed with the data that is passed by subqueries to
main query.
Nesting Subquery
It is also possible to nest subqueries. So far
we have seen examples where a single subquery is executed and sends values to
main query. It is also possible for a
subquery to depend on another subquery and that subquery on another and so on.
The following example displays the details of
the students who belong to batches that are taken by faculty with qualification
MS.
select rollno,name, bcode from students
where
bcode in
(
select bcode from batches
where fcode in ( select fcode
from faculty where qual like '%MS%')
);
ROLLNO
NAME BCODE
--------- ------------------------------ -----
1
George Micheal b1
2
Micheal Douglas b1
6
Chirs Evert b3
7
Ivan Lendal b3
8 George Micheal b4
9
Richard Marx b5
10
Tina Turner b5
11
Jody Foster b5
Note: Subqueries can be nested up to 16 levels. But
that limit is seldom reached. Moreover it is not recommended to use more than 3
levels of nesting considering the performance.
The following is another example where we
will take details of payments made by students of the batch that started on
12-jul-2001.
select * from payments
where rollno in
(select
rollno from students
where bcode in
(select bcode from batches
where stdate = '12-jul-01'
)
);
Comparing more than one value
A subquery can return multiple columns.
These multiple columns must be compared with multiple values. The following
query displays the details of the batches that have taken maximum duration
among the batches of the same course.
select * from batches
where (ccode, enddate-stdate) in
(select ccode,
max(enddate-stdate)
from batches
group by ccode);
BCODE
CCODE FCODE STDATE ENDDATE TIMING
-----
----- ----- --------- --------- ---------
b2 asp
da 15-JAN-01 05-MAR-01 2
b3 c
hs 20-JAN-01 27-FEB-01 3
b5 java
hs 05-APR-01 10-MAY-01 1
b1 ora
gk 12-JAN-01 20-FEB-01 1
b4 xml
dh 02-MAR-01 30-MAR-01 3
First subquery returns the course and maximum
duration for that course from BATCHES table using MAX function and GROUP BY
clause. Then the values are sent to main query where they are compared with
CCODE and duration of each batch. If in
a row of BATCHES table the CCODE and the duration are equivalent to CCODE and
maximum duration of any of rows returned by subquery then the row of BATCHES table
is selected.
Now let us see
few more examples of subqueries.
Get the details of course that has highest
course fee.
select ccode,name,fee from courses
where fee =
(select max(fee) from courses);
CCODE NAME FEE
----- ------------------------------ ---------
cs C
Sharp 7000
Get the details of students who have made a
payment in the last month but no in the current month.
select * from students
where rollno not in
(select rollno from payments
where
to_char(dp,'mmyyyy') = to_char(sysdate,'mmyyyy')
)
and rollno in
( select
rollno from payments
where
to_char(dp,'mmyyyy')=to_char(add_months(sysdate,-1),'mmyyyy')
);
First we take roll numbers of students who have
made payment in the current month and roll numbers of students who have made
payment in the previous month. Then the
outer query selects students who are not
part of the first list (who made payment in current month) and part of
second list ( who made payment in previous month).
Subqueries in DML and DDL commands
Subqueries can also be used with DML commands.
WHERE clause of UPDATE and DELETE can always contain a subquery. The following
UPDATE command increases the FEE of the course if more than 5 batches have
started for that course.
update courses set fee = fee * 1.1
where
ccode in
( select
ccode from batches
group
by ccode
having
count(*) > 5);
In the above example, subquery returns course
codes for which more than 5 batches have started and then UPDATE will update
only those courses.
The following DELETE command uses subquery in
WHERE clause to find out batches for which there are no students.
delete from batches
where
bcode not in ( select bcode from students);
The examples above are the cases where we used
subquery in WHERE clause and not precisely in DML command. The following UPDATE
command updates FCODE of batch b7 to the faculty code of the batch b1.
update batches set fcode =
(select
fcode from batches where bcode = 'b1')
where bcode = 'b7';
Subquery returns a single value that is to be
copied to FCODE of UPDATE.
Similarly the following INSERT inserts rows into
a table called COMP_BATCH. The data is
taken from BATCHES table.
The following subquery creates a new table from
an existing table.
create table new_batches
as select bcode, ccode,fcode, stdate ,timing
from batches
where stdate > sysdate;
The subquery is used to retrieve the data using
which the new table is created. The structure of the new table will be same as
the structure of the query. In the above query as query selects
BCODE,CCODE,FCODE, STDATE , and TIMING columns the table is also created with
same columns.
The following is the structure of the new table.
SQL> desc new_batches
Name Null? Type
--------------------------------------
-------- ------------------
BCODE
VARCHAR2(5)
CCODE
VARCHAR2(5)
FCODE VARCHAR2(5)
STDATE NOT NULL DATE
TIMING
NUMBER(1)
SQL> desc batches
Name Null? Type
---------------------------------------
-------- ------------------
BCODE NOT NULL
VARCHAR2(5)
CCODE
VARCHAR2(5)
FCODE
VARCHAR2(5)
STDATE NOT NULL DATE
ENDDATE DATE
TIMING
NUMBER(1)
If you observe the above two structures, you
would notice that only NOT NULL constraint of STDATE of BATCHES table is taken
to NEW_BATCHES table.
select * from new_batches
Will display the details of batches where STDATE
of the batch is after SYSDATE.
If you want to insert details again of new batches after some time
you can issue the following INSERT command.
insert into new_batches
select
bcode, ccode, fcode, stdate , timing from batches
where stdate > sysdate;
1.
Note: The order and type of columns in the query and NEW_BATCHES should be
same. Otherwise you may have to list out columns in the query according to the
requirement.
Renaming a column using subquery
The following procedure will illustrate how to
use subquery with DDL to rename a column in a table. Renaming a column is not
permitted in Oracle. So to rename a
column, follow the given procedure. However, it is to be noted that this
procedure is lengthy and not very refined. But you can consider in case of
desperate need.
Assume we created a table called PRODUCTS as
follows.
create table products
( id number(5) primary key ,
name
varchar2(30),
qty number(4)
check ( qty >= 0 ),
pric
number(5)
);
But the column PRIC is misspelt. It should have
been PRICE. Now let us see how to rename
the column.
First create a new table called newproducts
using a subquery. Give an alias to column PRIC so that the alias becomes the
column name in the new table.
create table newproducts
as select
id, name, qty, pric price from
products;
see the structure of the new table using DESC
command.
desc newproducts
name Null?
Type
------------------------------------------------
-------- -------------
ID
NUMBER(5)
NAME
VARCHAR2(30)
QTY NUMBER(4)
PRICE
NUMBER(5)
As you can see in the output of DESC command, no
constraint are defined in new table -
NEWPRODUCTS. So we need to define all
constraints again on this table using ALTER TABLE command. But before that let us drop original table
and rename NEWPRODUCTS to PRODUCTS.
SQL> drop table products;
Table dropped.
2.
Note: If the table being dropped has any dependent tables with rows then
you have to drop those rows also. For example, if you have SALES table
referring to PRODUCTS table then first SALES table is to be emptied before rows
in PRODUCTS table can be deleted.
SQL> rename newproducts to products;
Table renamed.
Now we have to define all constraints that we
had in PRODUCTS table. This step is
required as constraints of PRODUCTS table are copied to NEWPRODUCTS table.
alter table products
add (
constraint products_pk primary key(id));
alter table products
add (
constraint products_qty_chk check(qty
>= 0));
Now see the structure of the new table using
DESCRIBE command and constraints using USER_CONSTRAINTS view.
What’s new in Oracle8i?
Orale8i has introduced to new possibilities
related to subqueries.
Subquery in VALUES clause
Since Oracle8i it is possible to use a subquery
in VALUES clause of INSERT command. Prior to Oracle8i it was possible to use
subquery with INSERT command but not in VALUES clause of INSERT command.
The following INSERT command gets the next
ROLLNO for a new student by using a subquery.
insert into
students values ( (select
max(rollno) + 1 from students),
'b7', 'Robert Lafore', 'm', sysdate, null,
null);
ORDER BY is permitted in subquery – TOPn
analysis
Oracle8i has allowed the ORDER BY clause to be
used with subquery. The following query will use ORDER BY clause to get courses
in descending order. Then main query will take the data sent by the subquery
and selects only first two rows. As the result, the query will display the
details of course with first two highest course fee.
ROWNUM pseudo column contains the row number for
the retrieved rows. The query uses ROWNUM and takes only those rows that have
row number less than 3.
select ccode,name,fee from
(select
* from courses order by fee desc)
where rownum < 3;
CCODE NAME FEE
----- ------------------------------ ---------
cs C
Sharp 7000
vbnet VB.NET 5500
Correlated Subquery
If there is any correlation between main query
and subquery then subquery is called as correlated subquery.
A correlated subquery is a subquery that
receives some input from main query and sends result back to main query. Unlike normal subquery, a correlated subquery
receives value from main query. It uses
the value (generally in condition) and sends the results of the query back to
main query.
Though most of the requirements can be
accomplished with normal subqueries, some requirements do need correlated
subquery. For example, we have to display the details of the batches where
duration of the batch is more than the average duration of all the batches of
that course.
Before we write the required query, let us look
at the data present in BATCHES table.
select * from batches;
BCODE CCODE FCODE STDATE ENDDATE
TIMING
----- ----- ----- --------- --------- ---------
b1
ora gk 12-JAN-01 20-FEB-01 1
b2
asp da 15-JAN-01 05-MAR-01 2
b3
c hs 20-JAN-01 27-FEB-01 3
b4
xml dh 02-MAR-01 30-MAR-01 3
b5
java hs 05-APR-01 10-MAY-01 1
b6
vbnet da 12-JUL-01
31-AUG-01 1
b7
ora gk 15-AUG-01 04-OCT-01 2
b9
ora kl 05-MAY-01 10-JUN-01 3
b10
c kl 15-JUN-01 20-JUL-01 2
b11
vbnet da 15-JUN-01
30-JUL-01 2
Now let us write correlated query to get details
of batches where duration (ENDDATE-STDATE) is more than the average duration of
the course.
select * from batches b1
where
enddate - stdate >=
(select
avg(enddate - stdate)
from
batches
where ccode = b1.ccode);
BCODE CCODE FCODE STDATE ENDDATE
TIMING
----- ----- ----- --------- --------- ---------
b2
asp da 15-JAN-01 05-MAR-01 2
b3
c hs 20-JAN-01 27-FEB-01 3
b4
xml dh 02-MAR-01 30-MAR-01 3
b5
java hs 05-APR-01 10-MAY-01 1
b6
vbnet da 12-JUL-01
31-AUG-01 1
b7
ora gk 15-AUG-01 04-OCT-01 2
In correlated subquery, subquery is executed
once for each row of the main query. This is required as subquery uses a value
sent from main query, which may change from row to row in main query.
A correlated subquery is identified by the use
of a column of main query in the subquery. In the above example, for each row
of the BATCHES table the course code (CCODE) is passed to subquery and then
subquery finds out the average duration of all batches of that course. The
condition ccode = b1.ccode is used to select batches that belong to the
course to which the batch in the main query belongs.
When subquery returns the average duration then
main query checks whether duration of the batch is greater than or equal to the
average duration sent by subquery. If the condition is satisfied then row in
the main query is selected otherwise it is not selected. The process continues
with the next row in the main query and so on until all rows of the main query
are processed.
Differences between normal and correlated
subquery
The following are the differences between a
correlated subquery and ordinary subquery.
|
Subquery
|
Correlated subquery
|
|
Executed only for once before main-query
|
Executed once for each row of main-query.
|
|
Sends a value to main-query.
|
Receives value(s) from main query and sends
value(s) to main-query.
|
Table 1: Normal subquery Vs. Correlated subquery
The following is another example of correlated
subquery where we get third highest course fee.
select name, fee from courses c1
where 2=
( select count(*)
from
courses
where
fee > c1.fee);
NAME FEE
------------------------------ ---------
ASP.NET 5000
In the above example, subquery returns the
number of courses where FEE is more than the fee of the row in the main query.
If the count returned by subquery is equal to 2 then it means above that course
fee there are two more. That means that course fee becomes the third highest.
The above query is pretty useful. Especially
prior to Oracle8i. As Oracle8i allowed ORDER BY clause in subquery, what was
achieved with this correlated subquery can to some extent be achieved with
ORDER BY clause in subquery.
The following two queries, will display the
details of courses with two lowest fees.
The first query uses ORDER BY clause in subquery
to retrieve courses in the ascending order of FEE. Then main query retrieves
only first two rows of the subquery.
select * from ( select ccode, name, fee from
courses order by fee)
where
rownum < 3;
CCODE NAME FEE
----- ------------------------------ ---------
c C
programming 3500
xml XML
Programming 4000
The second query uses a correlated subquery to
get number of rows where FEE is less than the FEE of the row in main query. If the count returned by subquery is greater
than or equal to 1 then row of the main query is retrieved.
select ccode,name, fee from courses c1
where 1
>= ( select count(*)
from courses
where fee < c1.fee);
CCODE NAME FEE
----- ------------------------------ ---------
c C
programming 3500
xml XML
Programming 4000
EXISTS and NOT EXISTS operators
These two operators are exclusively used in
correlated subquery. EXISTS checks
whether any row is returned by subquery and condition will be true if subquery
returns any rows. Whereas, NOT EXISTS returns true if subquery doesn’t retrieve
any row.
EXISTS is different from other operators like
IN,ANY etc., because it doesn’t compare values of columns, instead, it checks whether any row is retrieved from subquery or not. If any row is retrieved from subquery the
EXISTS returns true otherwise it returns false.
The following query displays details of courses
for which at least one batch started in this month.
select * from courses
where
exists
( select * from batches
where
courses.ccode= ccode
and to_char(stdate,'mmyy') =
to_char(sysdate,'mmyy')
);
When using EXISTS operator, what you select in
the inner-query does NOT matter. What
does matter is whether any row is retrieved by inner query or not.
The following example displays the details of
courses for which no batch has yes started.
select ccode,name,fee from courses
where not exists
(
select * from batches
where
ccode = courses.ccode);
CCODE NAME FEE
----- ------------------------------ ---------
cs C
Sharp 7000
It is possible to replace the above NOT EXISTS
with NOT IN operator as shown below.
select ccode,name,fee from courses
where ccode not in ( select ccode from batches);
CCODE NAME FEE
----- ------------------------------ ---------
cs C
Sharp 7000
Let us see two other operators ALL and ANY.
ANY and ALL Operators
Both are used for comparing one value against a
set of values. ALL specifies that all the values given in the list should be
taken into account, whereas ANY specifies that the condition is satisfied when
any of the values satisfies the condition.
operator ANY
list
operator ALL list
The operator can be any one of the standard
relational operators (=, >=, >,
<, <=, !=) , and list is a series of values.
What if you want to display name of the courses
where FEE is more than FEE of any course with DURATION 25? The following query
will do just that.
select name from courses
where fee
> any ( select fee from courses where duration = 25);
NAME
------------------------------
VB.NET
ASP.NET
C Sharp
ANY operator specifies if FEE is more than any
value in the list supplied by subquery then the condition is true. The same
query can also be written as follows using MIN function.
select name from courses
where fee
> ( select min(fee) from courses where duration = 25);
NAME
------------------------------
VB.NET
ASP.NET
C Sharp
The following list illustrates the result of ANY
and ALL operator.
|
Rate
|
ANY/ALL Operator
|
Result
|
|
10
10
10
10
|
Rate > ANY (15,20)
Rate > ANY (5,15)
Rate > ALL (10,20)
Rate > ALL (5,7)
|
False
True
False
True
|
Summary
Subquery is a query placed within another query.
A subquery may return either one or multiple rows. Understanding how to use subqueries is very
important. So, make sure you are comfortable with subqueries. Because
subqueries, joining, and grouping the data are the areas which you must master.
Subqueries can be used with DML and even DDL
commands. Oracle8i has allowed ORDER BY
clause in subqueries. This allows top n analysis. Oracle8i has also allowed usage
of subquery in VALUES clause of INSERT command.
When a subquery takes data from main query it is called as correlated subquery. And
operators EXISTS and NOT EXISTS are used exclusively with correlated subquery.
