Craig Ringer wrote:
> seiliki wrote:
>> I expect the SELECT to return two rows. Would some kind
>> soul explain for me why it gives only one row?
>
> Without having read the post in detail I'll make a guess: Because NULL =
> NULL results in NULL, not true, and the outer (or any other) join
> condition only accepts rows where the join condition is true.
>
> This is a FAQ. It probably needs to go in the PostgreSQL FAQ.
>
> The usual response is: Rethink your use of NULL values. Null really
> means "unknown" and if you're comparing for equality you probably don't
> really want NULLs to be present. If you absolutely must perform
> comparisons where NULL should be considered equal to NULL use `IS
> DISTINCT FROM` ... but as everybody here says, use of that often
> suggests design problems in your queries and schema.
Perhaps you should have read the post in detail. There is no NULL=NULL
comparison here.
If you add column y.c2 to the SQL that produces two rows; you will see
that y.c2 is NULL; which is not meet where condition of y.c2=9 in first
SQL so therefore row is not included in results.