Recently I ran into the difference between UNION and UNION ALL. Pity you don't mention that.

I encountered that a query result of (say) 300 records UNION'd with a query result of zero records could end up in an overall result set of less than 300 records!

After some googling and stuff I found out that the function UNION in fact also performs a GROUP BY on all fields. In other words: There will no longer be duplicate records in the end result set. If the duplicates are the result of the UNION or not doesn't matter.

A query result of (say) 300 records UNION ALL'd with a query result of zero records gets you a result set of exactly (say) 300 records!