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Don Burleson Blog 







Non-correlated subquery tips

Oracle Database Tips by Donald BurlesonJanuary 1, 2015

Question:  Can you give me some tips on when to write non-correlated subqueries?  Are non-correlated subqueries faster than a correlated subquery?

Answer:  In non-correlated subqueries the subquery should be executed before the outer query and the non-correlated subquery is executed only once. The data from the outer query and the subquery are independent and one execution of the subquery will work for all the rows from the outer query.

Remember, any SQL statement is "declarative and any given query can be written in many ways. They will all have the same results, but with vastly different execution speeds!  For more details on non-correlated sub queries, see the book "Advanced Oracle SQL Tuning  The Definitive Reference".

In this example of a non-correlated subquery, we locate all authors who have not yet published a book.  Note that the subquery is non-correlated because the subquery makes no reference to anything in the outer query.

   author_key not in
    (select author_key from book_author);

This type of NOT IN subquery is called a non-correlated subquery because the subquery does not make any references to the outside query.  We can sometimes re-write a non-correlated subquery into a correlated.  Here is a non-correlated subquery:

   key IN
   -- noncorrelated subquery

Here is the correlated subquery equivalent. Note the reference in the inner query to the column value in the outer query:

   key IN
   -- correlated subquery
      tablename.key = inner_table.key

Also see my notes on Oracle correlated subqueries.  
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