RE: Classic trap : using SQL while not fully competent

["Wilt, Charles" <WiltC@xxxxxxxxxx>]

David,

In your original query, the WHERE clause looks at the entire joined record; after the join has been
accomplished.

However, you said yourself that the tcrtcd field was from the right hand file and there wasn't a
record in the right.  Thus no wonder the DB couldn't find a record that met the where clause.

In other words, consider the results set from the following:
SELECT * FROM prdcap exception join adhtst on ad1cad=tadhno

The only records that will be returned have NULL for all fields on the right.  That's what an
exception join does.  Thus when you add a WHERE tcrtcd ='14', it is impossible for any rows to match.

HTH,

Charles Wilt 
Software Engineer
CINTAS Corporation - IT 92B
513.701.1307

wiltc@xxxxxxxxxx


> -----Original Message-----
> From: midrange-l-bounces@xxxxxxxxxxxx [mailto:midrange-l-
> bounces@xxxxxxxxxxxx] On Behalf Of David FOXWELL
> Sent: Thursday, April 03, 2008 5:46 AM
> To: Midrange Systems Technical Discussion
> Subject: Classic trap : using SQL while not fully competent
>
> I was going to cry for help :
>
> I know I have a record on the left that ain't on the right  so I did
>
> SELECT * FROM prdcap exception join adhtst on ad1cad=tadhno
> WHERE tcrtcd ='14'
>
> Which returned no record.
>
>
> But
>
> SELECT * FROM prdcap  WHERE P ad1cad not in
> ( select tadhno from adhtst where tcrtcd ='14')
>
> Found the record.
>
> After a while I thought, maybe its because tcrtcd is in the right hand
> table, so I tried
> SELECT * FROM prdcap exception join adhtst on ad1cad=tadhno
> and tcrtcd = '14'
>
> Which worked.
>
> Its the first time I put the where criteria in the JOIN. Can anyone give
> me the technical explication as to why the first example didn't work so I
> don't fall in again please ?
>
> Thanks.
>
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