What is the difference between the SubQuery and Corelated SubQuery ?

Answer : The inner query is executed only once…

Subquery

  • The inner query is executed only once.
  • The inner query will get executed first and the output of the inner query used by the outer query.
  • The inner query is not dependent on outer query.
SELECT Employee_name, dept_no 
FROM Employee 
WHERE Employee_name IN (SELECT Employee_name FROM Customer);

Correlated SubQuery

  • The outer query will get executed first and for every row of outer query, inner query will get executed.
  • So the inner query will get executed as many times as number of rows in result of the outer query.
  • The outer query output can use the inner query output for comparison. This means inner query and outer query dependent on each other.
SELECT Employee_name,dept_id 
FROM Employee
WHERE Employee_name IN (SELECT Employee_name FROM dept WHERE Employee.dept_id=dept.dept_id);
Subquery and Corelated Subquery
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