SQL Question

Posted on 2002-05-07
Last Modified: 2010-05-02
This is killing me because I'm certain I should know why this isn't working, but I can't seem to figure it out.  I have two queries in an Access database that I'm trying to join into 1 query that I can use as the data source for a report.  Here are the 2 queries:

Area  Machine   UnitOp  Mode  Occur  DT
----  -------   ------  ----  -----  -----
NULL  MachA     NULL    Leak  8      6.7
NULL  MachA     NULL    Can   4      0.28

Area  Machine   UT
----  -------   -------
NULL  MachA     331.497

The results should look like this:

Area  Machine   UnitOp  Mode  Occur  DT      UT
----  -------   ------  ----  -----  -----   -------
NULL  MachA     NULL    Leak  8      6.7     331.497
NULL  MachA     NULL    Can   4      0.28    331.497

However, when I run my query, either nothing from Query1 or Query2 shows, depending on which JOIN I use.  For example, originally my SQL looked like this:

SELECT Query1.*, Query2.UT
FROM Query1 LEFT JOIN Query2 ON (Query1.Machine = Query2.Machine) AND (Query1.Area = Query2.Area);

When I ran this, the data from Query1 showed up just fine, but the data from Query2 did not.  So, just to test, I switched to a RIGHT JOIN and the data from Query2 showed up just fine, but the data from Query1 didn't show up.  Does anyone see anything wrong with my queries or have a suggestion of how to better accomplish this?

Question by:beercat2
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LVL 143

Accepted Solution

Guy Hengel [angelIII / a3] earned 50 total points
ID: 6994104
The problem is the NULL value:

SELECT Query1.*, Query2.UT
FROM Query1 JOIN Query2 ON (Query1.Machine = Query2.Machine) AND (IIF(Query1.Area IS NULL, 999,Query1.Area) = IIF(Query2.Area IS NULL, 999, Query2.Area));

LVL 75

Expert Comment

by:Anthony Perkins
ID: 6994111
I could be wrong, but I don't believe you can join fields that are Null.  In other words if the value is Null than it will not be included.

LVL 143

Expert Comment

by:Guy Hengel [angelIII / a3]
ID: 6994117
to explain:
-> returns false !!!!
This is true for other database systems also.
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LVL 18

Expert Comment

ID: 6994122
i'm guessing the problem is here:
Query1.Area = Query2.Area

because you are doing where null = null which should probably be null is null so maybe modify query to be:

SELECT Query1.*, Query2.UT
FROM Query1 LEFT JOIN Query2 ON (Query1.Machine = Query2.Machine) AND ((Query1.Area is null AND Query2.Area is null) OR (Query1.Area = Query2.Area));

LVL 43

Expert Comment

ID: 6994124
Select Query1.Area,Query1.Machine,QUery1.UnitOp,Query1.Mode,Query1.Occur,Query1.DT,Query2.UT From Query1 Inner Join Query2 On Query1.Machine = Query2.Machine And Query1.Area = Query2.Area


Select Query1.Area,Query1.Machine,QUery1.UnitOp,Query1.Mode,Query1.Occur,Query1.DT,UT = (Select Query2.UT From Query2 Where Query2.Machine = Query1.Machine And Query2.Area = Query1.Area) From Query1
LVL 45

Expert Comment

ID: 6995509
SELECT Query1.*, Query2.UT
FROM Query1 LEFT JOIN Query2 ON (Query1.Machine = Query2.Machine)
Where Query1.Area Is Null and Query2.Area Is Null


SELECT Query1.*, Query2.UT
FROM Query1 LEFT JOIN Query2 ON (Query1.Machine = Query2.Machine) AND (Query1.Area = Query2.Area)
Where Query1.Area Is Not Null and Query2.Area Is Not Null

Author Comment

ID: 6996485
Wow!  Thanks for all the quick replies.  Since I'm certain all of these would have worked, I'm just going to award the points to the first answer.

Thanks for all of your expertise!
LVL 45

Expert Comment

ID: 6996542

Please verify answers before accepting them as valid.  Every suggested answer may NOT solve your problem.  Not all solutions are of equal value in all environments.  We experts only see the "tip of the iceberg" part of your problem and the easiest-to-produce answer might not be your best (production) solution.

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