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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
explain cascading rollback
What is the use of integrity constraints? Integrity constraints make sure that changes made to the database by authorized users do not result in a loss of data consistency. The
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