The document discusses the concept of subqueries, which are queries nested within other queries. It explains scalar subqueries, which return a single row and column, and correlated subqueries, which depend on the outer query's data. The document also includes examples demonstrating the use of EXISTS and NOT EXISTS operators in these subqueries.

1. Sub -Query

2. Subquery is query within a query.
A SELECT statement can be nested inside another query
to form a subquery.
The query which contains the subquery is called outer
query.

3. Scalar Sub-query
A scalar subquery returns single row, single column result.
Q)Find the name of the student who has opted for the course RDBMS
Sol: select sname from student where sid=(select sid from
course where cname=‘RDBMS’);

4. Correlated subquery
the processing of subquery requires data from the outer query
Sub-query always depends on the results of outer sub-query.
Exists operator is used to find out subquery contains values
returned by outer query.
Not exists operator is used to find out subquery must not
contains values returned by outer query

5. ex
Select orderid from orders where
Exists(
select * from product
where orders.productid = product.productid
and product.productname=‘Mobile’);