RE: SQL help with EXISTS

Vern:

OK please explain:

The following returns the 6 rows that I want and works perfectly.

select * from miscchg A
where (a.AMSCHGC in ('S','B') or a.AMSCHGA in ('S','B'))
and
exists
(select ASHPNBR, ABLDNGD, AMSCHRG from MISCCHG b
where a.ASHPNBR = b.ashpnbr and a.ABLDNGD = b.ABLDNGD
and
ashPNBR = 'BN460619' and ABLDNGD = '20120817'
group by ASHPNBR,ABLDNGD, AMSCHRG )
order by ASHPNBR,ABLDNGD,APLSQNB,AMSCHRG


I removed the GROUP BY, you are correct, same result.

If I remove the entire EXISTS clause, millions of rows are returned.

How can what you and others are stating be accurate if the result set with the EXISTS returns 6 rows, and without EXISTS returns millions?

Are you suggesting that the EXISTS "does nothing to change the final result" ? But yet it does change the final result.

What am I missing?

Thanks






-----Original Message-----
From: midrange-l-bounces@xxxxxxxxxxxx [mailto:midrange-l-bounces@xxxxxxxxxxxx] On Behalf Of Vernon Hamberg
Sent: Thursday, December 06, 2012 2:01 PM
To: Midrange Systems Technical Discussion
Subject: Re: SQL help with EXISTS

To add to Charles' reply - your SELECT inside the parentheses - after
the EXISTS - has records. Therefore, records EXIST in the result set of
THAT query. So the EXISTS predicate is unconditionally TRUE in this case
- you get the result that is the outer SELECT with no further conditioning.

Your EXISTS does nothing to change the final result, because it is like
testing for 1 = 1; it is always true.

You need to refer, inside the EXISTS, to a column in the outer, or main,
SELECT. And, as I said earlier, it is basically nonsense to have an
EXISTS test over the same table you are querying.

And, because the result of the internal query are discarded, you have no
need for grouping, without the HAVING clause with a condition on some
aggregate function such as COUNT(*).

HTH
Vern

On 12/6/2012 11:54 AM, Charles Wilt wrote:

No, he's saying it's not functioning the way you think it is.

Charles


On Thu, Dec 6, 2012 at 12:52 PM, Stone, Joel <Joel.Stone@xxxxxxxxxx> wrote:

So - you are stating that the SQL I presented CANNOT function? But it
does. It returns a set based on the EXISTS .

I don't understand.

-----Original Message-----
From: midrange-l-bounces@xxxxxxxxxxxx [mailto:
midrange-l-bounces@xxxxxxxxxxxx] On Behalf Of CRPence
Sent: Thursday, December 06, 2012 11:50 AM
To: midrange-l@xxxxxxxxxxxx
Subject: Re: SQL help with EXISTS

On 06 Dec 2012 11:06, Stone, Joel wrote:

I don't think that is correct. It seems to do the subselect (return
a set of rows) and THEN join the result to the outer select. A few
responses have stated what you said - that EXISTS simply returns TRUE
or FALSE. But I think that it must do this for each row in the outer
select in the case where a join is occurring?

Maybe I am wrong, I don't know SQL and it is elusive and powerful and
complex so I don't understand all the subtleties.

Dan Kimmel on Thursday, December 06, 2012 11:01 AM wrote:

The EXISTS says "are there any". You get the same results in the
outer select whether there's one or many results from the
subselect.

The documentation states explicitly that the "values returned by the
fullselect [of the EXISTS predicate] are ignored" and states clearly
that the only possible results are True and False. No type of matching
join can be done without some values against which to compare; that
would leave only a cartesian product join, and that certainly would be
of no value to the query.

IBM i 7.1 Information Center -> Database -> Reference -> SQL reference
-> Language elements -> Predicates
_i EXISTS predicate i_

http://publib.boulder.ibm.com/infocenter/iseries/v7r1m0/topic/db2/rbafzexists.htm
"The EXISTS predicate tests for the existence of certain rows.

>>-EXISTS--( fullselect )---------------------------------><

The fullselect may specify any number of columns, and

* The result is true only if the number of rows specified by the
fullselect is not zero.
* The result is false only if the number of rows specified by the
fullselect is zero.
* The result cannot be unknown.

The values returned by the fullselect are ignored.
..."

--
Regards, Chuck
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