Sub Queries

=>    A Query embedded in Another Query is called a Sub Query.
=>    There are 2 Queries:
        a) Parent Query     or    Outer Query    or    Main Query
        b) Child Query     or    Inner Query    or    Main Query
=>    When it is executed 1st Inner Query is executed then Outer Query.
=>    The result of inner query acts as input for Outer Query.
=>    Outer Query can be Insert / Update / Delete / Select.
    Inner Query must always be Select.


Types of Sub Queries:

    1. Single Row Sub Query
    2. Multi Row Sub Query
    3. Nested Sub Query
    4. Multi Column Sub Query
    5. Co-related Sub Query