Subquery Rules

A subquery is subject to the following restrictions:

• The select list of a subquery introduced with a comparison operator can include only one expression or column name (except that EXISTS and IN operate on SELECT * or a list, respectively).

• If the WHERE clause of an outer query includes a column name, it must be join-compatible with the column in the subquery select list.

• The ntext, text, and image data types cannot be used in the select list of subqueries.

• Because they must return a single value, subqueries introduced by an unmodified comparison operator (one not followed by the keyword ANY or ALL) cannot include GROUP BY and HAVING clauses.

• The DISTINCT keyword cannot be used with subqueries that include GROUP BY.

• The COMPUTE and INTO clauses cannot be specified.

• ORDER BY can only be specified when TOP is also specified.

• A view created by using a subquery cannot be updated.

• The select list of a subquery introduced with EXISTS, by convention, has an asterisk (*) instead of a single column name. The rules for a subquery introduced with EXISTS are the same as those for a standard select list, because a subquery introduced with EXISTS creates an existence test and returns TRUE or FALSE, instead of data.