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Who makes more than the average salary? What is the cheapest book? That's the kind of question that requires a comparison with a subquery that contains a set function. In other words, you need a condition with this general format:

... WHERE <value> <comparison-operator> (SELECT <set function> ...)

For example:

SELECT ... FROM  ... WHERE ... = (SELECT MIN(price) FROM book);

This is one of the cases where it's much easier to use a subquery than a join.

If you try to do something more complex, you will probably run into some restrictions because set functions within subqueries are hard to implement. We'll give just one example of a restriction (from SQL-92) if the set function's argument is an "outer reference" Column (i.e.: the name of a Column in the outer enclosing query), then that must be the only <Column reference> within that argument and the set function has to appear either in a select list or within a subquery that belongs to a HAVING clause. For example, this complex query uses illegal syntax:

SELECT * FROM Table_1 WHERE 0 = (
  SELECT SUM(Table_1.column_1+Table_2.column_1) FROM Table_2);

It would be even more illegal if the set function appeared in the WHERE clause, or if there was a third level of subquery nesting. Nobody can remember such a complex rule, and there are more like it, so the cautious programmer simply avoids trying anything fancy when using set functions within subqueries.

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