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 (9 row(s) affected)
Unlike most of the subqueries shown earlier, the subquery in this statement cannot be resolved independently of the main query. It needs a value for authors.au_id, but this value is a variable. It changes as Microsoft® SQL Server™ examines different rows of the authors table.
That is exactly how this query is evaluated: SQL Server considers each row of the authors table for inclusion in the results by substituting the value in each row into the inner query. For example, if SQL Server first examines the row for Cheryl Carson, the variable authors.au_id takes the value 238-95-7766, which SQL Server substitutes into the inner query.
USE pubs SELECT royaltyper FROM titleauthor WHERE au_id = '238-95-7766'
The result is 100, so the outer query evaluates to:
USE pubs SELECT au_lname, au_fname FROM authors WHERE 100 IN (100)
Because this is true, the row for Cheryl Carson is included in the results. Go through the same procedure with the row for Abraham Bennet; you'll see that this row is not included in the results.