"terry" <94487...@qq.com> wrote:
> In one command, why can we update the same tuple for twice?
> TEST=# update t1 set a = t2.d from t2 where a=t2.c;
> UPDATE 1
It says it updated it once. Why do you think otherwise?
> And the result is not predicated!!
When you select multiple rows without specifying an order, the order
cannot be reliably predicted. If you want the update to pick a
particular row as the source for the update, tell it which one.
Something like:
update t1 set a = t2.d from t2
where a=t2.c
and not exists
(
select * from t2 x
where x.c = a
and x.d > t2.d -- or whatever logic you want here
)
;
-Kevin
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