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SQL | Subquery

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In SQL a Subquery can be simply defined as a query within another query. In other words we can say that a Subquery is a query that is embedded in WHERE clause of another SQL query. Important rules for Subqueries:

  • You can place the Subquery in a number of SQL clauses: WHERE clause, HAVING clause, FROM clause. Subqueries can be used with SELECT, UPDATE, INSERT, DELETE statements along with expression operator. It could be equality operator or comparison operator such as =, >, =, <= and Like operator.
  • A subquery is a query within another query. The outer query is called as main query and inner query is called as subquery.
  • The subquery generally executes first when the subquery doesn’t have any co-relation with the main query, when there is a co-relation the parser takes the decision on the fly on which query to execute on precedence and uses the output of the subquery accordingly.
  • Subquery must be enclosed in parentheses.
  • Subqueries are on the right side of the comparison operator.
  • ORDER BY command cannot be used in a Subquery. GROUPBY command can be used to perform same function as ORDER BY command.
  • Use single-row operators with singlerow Subqueries. Use multiple-row operators with multiple-row Subqueries.

Syntax: There is not any general syntax for Subqueries. However, Subqueries are seen to be used most frequently with SELECT statement as shown below:

SELECT column_name
FROM table_name
WHERE column_name expression operator 
    ( SELECT COLUMN_NAME  from TABLE_NAME   WHERE ... );

Sample Table:

DATABASE

NAMEROLL_NOLOCATIONPHONE_NUMBER
Ram101Chennai9988775566
Raj102Coimbatore8877665544
Sasi103Madurai7766553344
Ravi104Salem8989898989
Sumathi105Kanchipuram8989856868

STUDENT

NAMEROLL_NOSECTION
Ravi104A
Sumathi105B
Raj102A

Sample Queries

:

  • To display NAME, LOCATION, PHONE_NUMBER of the students from DATABASE table whose section is A
Select NAME, LOCATION, PHONE_NUMBER from DATABASE 
WHERE ROLL_NO IN
(SELECT ROLL_NO from STUDENT where SECTION=’A’); 
  • Explanation : First subquery executes “ SELECT ROLL_NO from STUDENT where SECTION=’A’ ” returns ROLL_NO from STUDENT table whose SECTION is ‘A’.Then outer-query executes it and return the NAME, LOCATION, PHONE_NUMBER from the DATABASE table of the student whose ROLL_NO is returned from inner subquery. Output:
NAMEROLL_NOLOCATIONPHONE_NUMBER
Ravi104Salem8989898989
Raj102Coimbatore8877665544
  • Insert Query Example:

Table1: Student1

NAMEROLL_NOLOCATIONPHONE_NUMBER 
Ram101chennai9988773344 
Raju102coimbatore9090909090 
Ravi103salem8989898989 

Table2: Student2

NAMEROLL_NOLOCATIONPHONE_NUMBER 
Raj111chennai8787878787 
Sai112mumbai6565656565 
Sri113coimbatore7878787878 
  • To insert Student2 into Student1 table:
INSERT INTO Student1  SELECT * FROM Student2;
  • Output:
NAMEROLL_NOLOCATIONPHONE_NUMBER 
Ram101chennai9988773344 
Raju102coimbatore9090909090 
Ravi103salem8989898989 
Raj111chennai8787878787 
Sai112mumbai6565656565 
Sri113coimbatore7878787878 
  • To delete students from Student2 table whose rollno is same as that in Student1 table and having location as chennai
DELETE FROM Student2 
WHERE ROLL_NO IN ( SELECT ROLL_NO 
                   FROM Student1 
                   WHERE LOCATION = ’chennai’);
  • Output:
1 row delete successfully.
  • Display Student2 table:
NAMEROLL_NOLOCATIONPHONE_NUMBER 
Sai112mumbai6565656565 
Sri113coimbatore7878787878 
  • To update name of the students to geeks in Student2 table whose location is same as Raju,Ravi in Student1 table
UPDATE Student2 
SET NAME=’geeks’ 
WHERE LOCATION IN ( SELECT LOCATION 
                    FROM Student1 
                    WHERE NAME IN (‘Raju’,’Ravi’));
  • Output:
1 row updated successfully.
  • Display Student2 table:
NAMEROLL_NOLOCATIONPHONE_NUMBER 
Sai112mumbai6565656565 
geeks113coimbatore7878787878 


Last Updated : 24 Jul, 2022
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