I'll go one further and say that if you're reaching for DISTINCT and you have joins, you may have joined the data the wrong way. It's not a RULE, but it's ALWAY

I'll go one further and say that if you're reaching for DISTINCT and you have joins, you may have joined the data the wrong way. It's not a RULE, but it's ALWAYS a 'smell' when I see a query that uses DISTINCT to shove away duplicate matches. I always add a comment for the exceptions.