A sub-query is a query which can be nested inside a main query like Select, Update, Insert or Delete statements. This can be used when expression is allowed.
Properties of sub query can be defined as:
- A sub query should not have order by clause
- A sub query should be placed in the right hand side of the comparison operator of the main query
- A sub query should be enclosed in parenthesis because it needs to be executed first before the main query
- More than one sub query can be included