Chapter 7, Problem 15RQ

Program Plan Intro

Subquery:

Any SQL query can be written in the sub query approach.

• The SQL subquery returns one or more column as the result.
  • Most SQL systems return a pair wise joining of one and only one column in an inner query with one column of an outer query.
  • The data displayed form the table referenced in outer query only.

Correlated subquery: It is a subquery that depends on the outer query. Here, the “WHERE” clause of the correlated subquery uses the information of the outer query.

Processing order of correlated sub queries:

• The inner query is computed for each outer row in the processing order of correlated sub query.
  • First, the outer sub query will get executed and it returns the result.
  • Based on the result of the outer query, the inner subquery will be evaluated (i.e. the subquery will use the value of the outer query).