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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
Write short notes on relational constraints? Relational Constraints are: 1. NOT NULL 2. Unique 3. Primary key 4. Foreign key 5. Table check
What is the need for triggers? Triggers are useful mechanisms for alerting humans or for starting particular tasks automatically when particular conditions are met.
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Nested Queries Let us see some more complex queries in this section. Sub-queries: Some of the basic concerns of sub-queries are: A sub-query is a SELECT statemen
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Describe substring comparison in SQL. For the relation Person (name, address), write a SQL query that retrieves the names of people whose name starts along with ‘A' and address co
write-set(T): Each data items that are written by a transaction T
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