| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | "Josh Berkus" <josh(at)agliodbs(dot)com> |
| Cc: | jasiek(at)klaster(dot)net, PgSQL Performance ML <pgsql-performance(at)postgresql(dot)org> |
| Subject: | Re: EXISTS vs IN vs OUTER JOINS |
| Date: | 2002-12-20 00:02:13 |
| Message-ID: | 10090.1040342533@sss.pgh.pa.us |
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| Lists: | pgsql-performance |
"Josh Berkus" <josh(at)agliodbs(dot)com> writes:
> If I run the query:
> SELECT t1.*
> FROM table1 t1
> LEFT JOIN table2 t2 ON t1.xid = t2.xid
> WHERE t2.label IS NULL
> I will get rows in t1 for which there is no row in t2. This does not
> seem SQL-spec to me; shouldn't I get only rows from t1 where a row
> exists in t2 and t2.label IS NULL?
No; that would be the behavior of an inner join, but you did a left
join. The above will give you t1 rows for which there is no match in t2
(plus rows for which there is a match containing null in t2.label).
regards, tom lane
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