DISTINCTclause in a query can help us quickly remove duplicates from our results. Sometimes it can be beneficial to stop and ask why. Why do we need to use the clause, why are we receiving duplicates from our data?
I see this typically due to a
JOINbeing used where we don’t really want all of those results. This could be a ‘does something exist’ check such as if a customer has ever ordered before. The issue comes when there are multiple rows returned like a frequent customer in this example.
As an alternative to this, Andy shows how you can use the
EXISTS clause to find records matching some criterion.