A sub-query is a query embedded in another query. The sub query can be embedded in another SELECT, INSERT, UPDATE, or DELETE statement, or inside another sub query.
Properties:-
The SELECT query of a subquery is always enclosed in parentheses
View created by using a subquery cannot be updated.
The ntext, text, and image data types cannot be used in the select list of sub queries
If a table appears only in a subquery and not in the outer query, then columns from that table cannot be included in the output