On Sun, 2005-08-28 at 16:22 +0800, Tang Tim Hei wrote:
> The following commands are little different from the previous one.
> (1) select A.* from test.currency A, test.price_list B where
> A.curr_cd=B.curr_cd and A.curr_cd='USD'
> (2) select A.* from test.currency A, test.price_list B, test.country C where
> A.curr_cd=B.curr_cd and A.curr_cd='USD'
>
> For command (1), it is ok. The result is what I expect.
> However, for command (2), it has problem. I added the "test.country C" to it,
> here I actually just write a table name to it and no more other purpose.
I do not understand what you mean by that.
the added table name means an additional cartesian join
> However, the result maybe totally different.
that is because of the added cartesian join
> If the table "country" is not empty, the result is just the same as in
> command (1)
it will only be the same if the table contains EXACTLY 1 row
> but if "country" is empty, there are no result row.
a cartesian join to 0 rows results in 0 rows
if you are talking about something else, please
show us a concrete simple example.
gnari
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