mysql count unique row values

Question

TABLE quotation

id  clientid
1   25
2   25
3   25
4   25
5   26

How can I query how many different clients exist in TABLE quotation? I don't want duplicate entries to be counted more than once.

I need the answer to be 2, in 1 row, because the only non-duplicated entries are (25, 26).

Answer 1

select count(distinct clientid) from quotation

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