Properties of Sub-Query
- A sub-query must be enclosed in the parenthesis.
- A sub-query must be put in the right hand of the comparison operator, and
- A sub-query cannot contain an ORDER-BY clause.
- A query can contain more than one sub-query.
Types of Sub-query
- Single-row sub-query, where the sub-query returns only one row.
- Multiple-row sub-query, where the sub-query returns multiple rows,. and
- Multiple column sub-query, where the sub-query returns multiple columns