Subquery in SQL

A subquery is a query within a query i.e a nested query. It is placed inside a query and its result is used to further evaluate the outer query.

There are some rules that a subquery must follow in SQL. Some of these are −

The subquery should be placed within parenthesis.
The subquery can be used with different operators like <,>,<=,>=, IN,BETWEEN etc. Also operators like SELECT, INSERT, DELETE, UPDATE etc. be used.
The ORDER BY operator cannot be used in the subquery. However, it can be there in the main query.
A subquery cannot be written with a BETWEEN operator. But the subquery can contain the BETWEEN operator.
The subquery that returns more than one row cannot be used with all the operators. It can only be used with operators that accept multiple values like IN.

An example of subqueries in SQL is −

Student_Number	Student_Name	Student_Phone	Student_Marks
1	Andrew	6615927284	95
2	Sara	6583654865	65
3	Harry	4647567463	48
4	Megan	7554764844	88
5	John	3436446454	25

Select *
from student
where student_marks IN( select student_marks from student where student_marks>50)

This query will return details about all the students who have more than 50 marks i.e. Andrew, Sara and Megan.

Student_Number	Student_Name	Student_Phone	Student_Marks
1	Andrew	6615927284	95
2	Sara	6583654865	65
4	Megan	7554764844	88

Published on 03-Aug-2018 08:00:15