Practical SQL Handbook, The: Using SQL Variants, Fourth Edition

GROUP BY and HAVING Must Return a Single Value

Comparison operator subqueries cannot include GROUP BY and HAVING clauses unless you know that they return a single value. For example, this query finds the books priced the same as the lowest-priced book in any category in which the minimum price is one-200th of the minimum advance. The subquery finds one qualifying row so there is no problem.

SQL

select title, type, price
from titles
where price =
   (select min(price)
   from titles
   group by type
   having min (price) < min(advance) /200  )
title                                         type            price
============================================= =============== =======
You Can Combat Computer Stress!               business        12.99
The Gourmet Microwave                         mod_cook        12.99
[2 rows]

If you modify the HAVING clause to ...