SQL | Subquery
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, and its output is used to complete the query condition for the main or outer query.
- 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.
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 ... );
- 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.
NAME ROLL_NO LOCATION PHONE_NUMBER Ravi 104 Salem 8989898989 Raj 102 Coimbatore 8877665544
- Insert Query Example:
NAME ROLL_NO LOCATION PHONE_NUMBER Ram 101 chennai 9988773344 Raju 102 coimbatore 9090909090 Ravi 103 salem 8989898989
NAME ROLL_NO LOCATION PHONE_NUMBER Raj 111 chennai 8787878787 Sai 112 mumbai 6565656565 Sri 113 coimbatore 7878787878
To insert Student2 into Student1 table:
INSERT INTO Student1 SELECT * FROM Student2;
NAME ROLL_NO LOCATION PHONE_NUMBER Ram 101 chennai 9988773344 Raju 102 coimbatore 9090909090 Ravi 103 salem 8989898989 Raj 111 chennai 8787878787 Sai 112 mumbai 6565656565 Sri 113 coimbatore 7878787878
- 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’);
1 row delete successfully.
Display Student2 table:
NAME ROLL_NO LOCATION PHONE_NUMBER Sai 112 mumbai 6565656565 Sri 113 coimbatore 7878787878
- 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’));
1 row updated successfully.
Display Student2 table:
NAME ROLL_NO LOCATION PHONE_NUMBER Sai 112 mumbai 6565656565 geeks 113 coimbatore 7878787878
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