“Some Restrictions Apply”
We already mentioned that adding a GROUP BY clause places certain restrictions on constructing your request. Let’s review those restrictions to make sure you don’t fall into common traps.
When you add a GROUP BY clause, you’re asking your database system to form unique groups of rows from those returned by the tables defined in your FROM clause and filtered by your WHERE clause. You can use as many aggregate expressions as you like in your SELECT clause, and these expressions can use any of the columns in the table defined by the FROM and WHERE clauses. As we pointed out in an earlier example, it probably does not make sense to reference a column in an aggregate expression and also include that ...