A QUERY WITHIN THE QUERY is called sub-query. Here’s an example with or without sub-query that tells how to write quickly.
SQL Without Sub-query
Select employee_name from employee where dept_no in (100,200);
From the above query, you can understand that the values in the brackets act as a sub Query. Really simple. Also You May Like: Top Rules You Need to Write a Sub Query
SQL With Sub-query
Here sub query will supply a list of values.
where dept_no in (
Here, the inner SELECT supplies list of department numbers. For example 100, 200, 300, 400 etc. This way you can write any sub-query quickly.