A sub-query is a query that can be nested inside the main query like Select, Update, Insert, or Delete statements. This can be used when the expression is allowed. Properties of sub query you can follow points
- sub query should be placed on the right-hand side of the comparison operator of the main query
- 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
- sub query should not have order by clause