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
