Le 20/10/2014 15:58, Tom Lane a écrit :
Laurent Martelli <laurent.martelli@xxxxxxxxxxxx> writes:
Do we agree that both queries are identical ?
No, they *aren't* identical. Go consult any SQL reference. Left join
conditions don't work the way you seem to be thinking: after the join,
the RHS column might be null, rather than equal to the LHS column.
Yes, I was wrong to assume that c.user_info=u.id because of the LEFT JOIN.
But since I only want rows where u.id IS NOT NULL, in any case I will
also have c.user_info IS NOT NULL.
Also, having a foreign key, if c.user_info is not null, it will have a
match in u. So in that case, either both c.user_info and c.id are null
in the result rows, or they are equal.
Regards,
Laurent
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