How does mysql define DISTINCT()

Posted by goran on Stack Overflow See other posts from Stack Overflow or by goran
Published on 2010-03-19T09:59:40Z Indexed on 2010/03/19 10:01 UTC
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AFAIK SQL defines two uses of DISTINCT keywords - SELECT DISTINCT field... and SELECT COUNT(DISTINCT field) ... However in one of web applications that I administer I've noticed performance issues on queries like

SELECT DISTINCT(field1), field2, field3 ...

DISTINCT() on a single column makes no sense and I am almost sure it is interpreted as

SELECT DISTINCT field1, field2, field3 ...

but how can I prove this?

I've searched mysql site for a reference on this particular syntax but could not find any. Does anyone have a link to definition of DISTINCT() in mysql or knows about other authoritative source on this?

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