Hi,

Can anybody explain me with example "What is the difference between the Normal subquery and Correlated Subquery".




A.D.Ramkumar

__________________
Ramkumar A D

Many queries can be evaluated by executing the subquery once and substituting the resulting value or values into the WHERE clause of the outer query. In queries that include a correlated subquery (also known as a repeating subquery), the subquery depends on the outer query for its values. This means that the subquery is executed repeatedly, once for each row that might be selected by the outer query.

This query finds the names of all authors who earn 100 percent of the shared royalty (royaltyper) on a book.

USE pubs
SELECT au_lname, au_fname
FROM authors
WHERE 100 IN
   (SELECT royaltyper
   FROM titleauthor
   WHERE titleauthor.au_ID = authors.au_id)

Here is the result set:

au_lname au_fname
---------------------------------------- --------------------
White Johnson
Green Marjorie
Carson Cheryl
Straight Dean
Locksley Charlene
Blotchet-Halls Reginald
del Castillo Innes
Panteley Sylvia
Ringer Albert

A correlated subquery is a SELECT statement nested inside another T-SQL statement, which contains a reference to one or more columns in the outer query. Therefore, the correlated subquery can be said to be dependent on the outer query. This is the main difference between a correlated subquery and just a plain subquery. A plain subquery is not dependent on the outer query, can be run independently of the outer query, and will return a result set. A correlated subquery, since it is dependent on the outer query will return a syntax errors if it is run by itself.

A correlated subquery will be executed many times while processing the SQL statement that contains the correlated subquery. The correlated subquery will be run once for each candidate row selected by the outer query. The outer query columns, referenced in the correlated subquery, are replaced with values from the candidate row prior to each execution. Depending on the results of the execution of the correlated subquery, it will determine if the row of the outer query is returned in the final result set.