Subqueries in Oracle with example

Generally a query is an operation that retrieves data from one or more tables or views. If you define both query and subquery specifically then query is the top-level SELECT statement and the subquery is the nested within another SQL statement. So in simple terms we can say subquery is the query within a query.

A subquery answer multiple parts of the question. For example whenever you asked to find the student name of a department CSE then a simple query

SELECT name from student where dept_name='CSE';

is enough. But if you asked for find the student name who are in Arju's department. Then what you will do? First you have to determine the department where Arju belong to. This part is subquery. And then you have to determine the name of the students with Arju's department. Like,

SELECT name from student where dept_name in(SELECT dept_name from student where name='ARJU');

Here, the later part SELECT dept_name from student where name='ARJU' is subquery and SELECT name from student part is identified as query.

A subquery can contain another subquery. Oracle Database imposes no limit on the number of subquery levels in the FROM clause of the top-level query. You can nest up to 255 levels of subqueries in the WHERE clause.
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