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SQL Left join, how to include empty rows

Posted on 2003-11-13
Medium Priority
Last Modified: 2012-06-27
I have got the following 2 tables:




I would like to have a query, that returns for a certain transfk ALL users with either the amount or a NULL value. For example: I have got 3 users, for transfk = 14 only 2 users have got an amount:

userid  name    amount  transfk
------  -------     ------      --------
1       Ronald         3       14
2       John            4       14
3       Austin        NULL    14

What query do I have to perform. To make it a bit clearer, the tables contain the following values:

userfk   transfk   amount
-------  --------  --------
1        14        3
2        14        4
2        17        1

userid   name
-------  ----------
1        Ronald
2        John
3        Austin

Any suggestions?
Question by:dutchdevelopment
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LVL 23

Expert Comment

ID: 9738522

This should dot it:

SELECT userid, name, amount
FROM users
     LEFT JOIN amounts ON userid = userfk
WHERE transfk IS NULL or transfk = 14


Author Comment

ID: 9738583
Unfortunately this won't work in all cases.

It'll work in this case because both users 1 and 2 have an entry for transfk = 14 and user 3 doesn't have an entry at all.

But when you do the same for transfk = 17 you will get no entry for user 1 (Ronald):

userid  name    amount  transfk
------  -------     ------      --------
2       John            1        17
3       Austin        NULL    17

Any clues?
LVL 12

Accepted Solution

koutny earned 750 total points
ID: 9738624
SELECT userid, name, amount
FROM users
     LEFT JOIN amounts ON (userid = userfk AND transfk = 14)
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Author Comment

ID: 9738640
Thanks all.

I have used the answer of koutny as a starter with the following added:

SELECT userid, name, amount
FROM users
LEFT JOIN amounts ON (users.userid = amounts.userfk AND amounts.transfk = 17 OR amounts.transfk IS NULL)
LVL 59

Expert Comment

by:Julian Hansen
ID: 9738836
This might do it (modify fields you need to see)

select * from users left join
SELECT userid, name, amount, transfk
FROM users
     LEFT JOIN amounts ON userid = userfk
WHERE transfk IS NULL or transfk = 14
) A on A.name = users.name

LVL 12

Expert Comment

ID: 9738907
Hi, I don't think that any addition or modificiation is needed.
It is the property of the LEFT OUTER JOIN that it will return ALL records from the table on the left side on the join (i.e. users) and it will fill in the remaining columns from the table on the right side of the join if the the ON (...) condition is true, otherwise it will fill those columns with null values.
LVL 59

Expert Comment

by:Julian Hansen
ID: 9739489
You are right - delayed post on my part. I opened this thread before your response and obviously replied after you. We have such slow lines here that tomorrow and next week mean the same thing ;)

Your post is much better - simplier.

Author Comment

ID: 9740918
It is necessary to add the modification (at least in MS Access 2000).
I think that the addition of amounts.transfk = 17 has undone the "return ALL records"-effect of a LEFT OUTER JOIN.

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