SQL | SUB Queries

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.

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

NAME ROLL_NO LOCATION PHONE_NUMBER
Ram 101 Chennai 9988775566
Raj 102 Coimbatore 8877665544
Sasi 103 Madurai 7766553344
Ravi 104 Salem 8989898989
Sumathi 105 Kanchipuram 8989856868

STUDENT

NAME ROLL_NO SECTION
Ravi 104 A
Sumathi 105 B
Raj 102 A

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:

    NAME ROLL_NO LOCATION PHONE_NUMBER
    Ravi 104 Salem 8989898989
    Raj 102 Coimbatore 8877665544
  • Insert Query Example:

    Table1: Student1

    NAME ROLL_NO LOCATION PHONE_NUMBER

    Ram 101 chennai 9988773344
    Raju 102 coimbatore 9090909090
    Ravi 103 salem 8989898989

    Table2: Student2



    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;
    

    Output:

    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’);
    

    Output:

    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’));
    

    Output:

    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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