Why won't combined AND statement work

Question

I want to exclude records based on two criteria's: status = 'M' and the status reason = 52.
The table contains records with status = 'M' and status reason other than 52.
Also the table contains records with status other than 'M' and status reason = 52 and I want to include those records.
I first tried using the statement below reasoning that placing the two criteria's in parenthesis would combine them and only exclude records with M 52
Where (nvl(Status_CD,'A') <> 'M' AND nvl(STATUS_RESN_CD,0) <> 52 )
However the query removed all records with Status_CD 'M' and all records with Status_Resn_CD 52. 
Using the Where Not statement worked.
Where Not (External_Status_CD = 'M' and EXT_STAT_RESN_CD = 52 ) 
I’m curious why the first statement didn’t work?
Can anyone explain why the first query didn't combine the 2 statements but a Where Not did?
Thank you - Mark

Answer 1

HI, Mark,
markjames9 wrote:
I want to exclude records based on two criteria's: status = 'M' and the status reason = 52.
The table contains records with status = 'M' and status reason other than 52.
Also the table contains records with status other than 'M' and status reason = 52 and I want to include those records.
I first tried using the statement below reasoning that placing the two criteria's in parenthesis would combine them and only exclude records with M 52
Where (nvl(Status_CD,'A') <> 'M' AND nvl(STATUS_RESN_CD,0) <> 52 )
However the query removed all records with Status_CD 'M' and all records with Status_Resn_CD 52.
Using the Where Not statement worked.
Where Not (External_Status_CD = 'M' and EXT_STAT_RESN_CD = 52 ) 
I’m curious why the first statement didn’t work?
Can anyone explain why the first query didn't combine the 2 statements but a Where Not did?
Thank you - Mark
Another way of looking at it:
The condition (x AND y) will be TRUE when both x AND y are TRUE.
If
sub-condition x is    nvl (Status_CD, 'A') <> 'M'   and
sub-condition y is  nvl (STATUS_RESN_CD, 0) <> 52
then any row with status_cd = 'M' fails sub-condition x, therefore the compound condition is FALSE, regardless of status_resn_cd.
Likewise, and row with status_resn_cd=52 fails sub-condition y, so the compound condition is FALSE regardless of status_cd.
You used status_cd and status_resn_cd in one example; but
EXTERNAL_status_cd and EXT_stat_resn_cd in the other.  That's quite confusing.  Also, you'll confuse everyone (including yourself) if status is abbreviated as stat in some places but not in others.  Either way is okay, but be consistent.  The same goes for abbreviating external as ext; pick one or the other, and use the same word in all places.
Don't forget to check for NULLs.  If  external_status_cd and ext_stat_resn_cd can be NULL, you should say
Where Not (    NVL (External_Status_CD, 'A') = 'M'
          and  NVL (EXT_STAT_RESN_CD,    0)  = 52
depending on your requirements.