Explain mechanism of nested queries?

SQL gives a mechanism for nesting subqueries. A subquery is a select from where expression that is nested within one more query. A common use of sub queries is to perform tests for set membership, make set comparisons, and verify set cardinality.

a) Set membership

(select customer-name

from depositor)

b) Set comparison

select distinct T.branch-name

from branch as T,branch as S

where T.assets > S.assets and S.branch-city='Brooklyn'

a) Test for Empty Relations

Select customer-name

from borrower

where exits (select *

from depositor

where depositor.customername=borrower.customer-name)

a) Test for the Absence of Duplicate Tuples