Sub queries limit

See here: http://technet.microsoft.com/en-us/library/ms189575.aspx

I would propose the same answer as Matt for completeness but would also quote this section fron the link:

A subquery can be nested inside the WHERE or HAVING clause of an outer SELECT, INSERT, UPDATE, or DELETE statement, or inside another subquery. Up to 32 levels of nesting is possible, although the limit varies based on available memory and the complexity of other expressions in the query.

That way individuals get their answer here, in one click, and have the option of researching further if desired.

Even if you can have up to 32 levels of nested subqueries, I would advice you to not use 32 levels of subqueries.

• The code will be hard to debug if you run into any problem.
• SQL will probably have difficulties in optimization etc

As a rule, I use temporary tables rather than using more than 3 levels of subqueries. Ofcourse there are no rules without exceptions, I always test my solutions to find the most optimized query.

Interesting coincidence - 32 is also the max limit on nested stored procedure calls. I discovered that when I wrote some code to determine the natural unique key in a raw set of data (http://jessesql.blogspot.com/2009/02/combination-of-columns-with-unique.html)