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A good rule of thumb -- if you need a distinct list of values in a single column or two, DISTINCT is the way to go. But if your results "don't look right" or you see some duplicate rows and can't figure out why, do NOT just add DISTINCT to your SELECT to "fix it" !! Step back, look at your joins, and re-write your query properly. Even worse, I've seen people simply add DISTINCT to all their SELECT's right from the start, to preemptively "avoid duplicates". Not good. (Side note: If you are a DISTINCT abuser, try adding meaningful primary keys to your tables).
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