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How can I show whether a JOIN is successful or not in my SELECT statement

Posted on 2008-10-22
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Last Modified: 2012-05-05
Hello

I have a simple LEFT JOIN SQL statement which I want to improve on so I don't have to do multiple SQL calls:

SELECT m.id, m.name
FROM modules m
LEFT JOIN file_allocations fa ON m.moduleid = fa.moduleid
WHERE fa.fileid=20

This is fine, it brings back one record because that's what's in the database, but at the moment there are 4 modules in the database. What I'd like to do is bring all 4 modules back from the database and mark each row stating whether it was a match or not, for example:

id: 1, name: news, exists: yes
id: 2, name: sport, exists: no
id: 3, name: weather, exists: no
id: 4, name: business, exists: no

In a nutshell, I'd like a way to fill the exists.

I hope I've explained that well enough.

Regards

Jonathan
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Question by:jwfranklin
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9 Comments
 
LVL 9

Expert Comment

by:jamesgu
ID: 22779300
SELECT m.id, m.name, case fa.moduleid is null then 'no' else 'yes' as exists
FROM modules m
LEFT JOIN file_allocations fa ON m.moduleid = fa.moduleid
WHERE fa.fileid=20
0
 
LVL 16

Expert Comment

by:brad2575
ID: 22779312
SELECT m.id, m.name, Case WHEN FA.Moduleid IS NULL Then 'NO' ELSE 'YES' END as ExistYN
FROM modules m
LEFT JOIN file_allocations fa ON m.moduleid = fa.moduleid
WHERE fa.fileid=20
0
 
LVL 16

Expert Comment

by:brad2575
ID: 22779325
using "exists" as a column alias will cause issues with SQL because it is a reserved word.  Either name it something else, or do
as [Exists]
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LVL 1

Author Comment

by:jwfranklin
ID: 22779405
I ran brad2575's SQL but that just brings back one row, I need it to bring back all the rows from the modules table with a No next to the ones it doesn't match on.
0
 
LVL 16

Expert Comment

by:brad2575
ID: 22779491
did you take off the where clause?
WHERE fa.fileid=20

try running just this one:
SELECT m.id, m.name, Case WHEN FA.Moduleid IS NULL Then 'NO' ELSE 'YES' END as ExistYN
FROM modules m
LEFT JOIN file_allocations fa ON m.moduleid = fa.moduleid
0
 
LVL 1

Author Comment

by:jwfranklin
ID: 22779536
I did and that did produce 4 results, but I need that where clause in there to help determine whether the ExistYN column should be YES or NO.
0
 
LVL 9

Expert Comment

by:jamesgu
ID: 22779576
SELECT m.id, m.name, case fa.moduleid is null then 'no' else 'yes' as [exists]
FROM modules m
LEFT JOIN file_allocations fa ON m.moduleid = fa.moduleid
WHERE fa.moduleid is null
--for all 'no' records


SELECT m.id, m.name, case fa.moduleid is null then 'no' else 'yes' as [exists]
FROM modules m
LEFT JOIN file_allocations fa ON m.moduleid = fa.moduleid
WHERE fa.moduleid is not null
--for all 'yes' records
0
 
LVL 8

Accepted Solution

by:
jorgesv13 earned 125 total points
ID: 22780057
Try this one:

SELECT m.id, m.name, Case WHEN FA.Moduleid IS NULL Then 'NO' ELSE 'YES' END as ExistYN
FROM modules m
LEFT JOIN file_allocations fa ON m.moduleid = fa.moduleid AND fa.fileid=20
0
 
LVL 1

Author Closing Comment

by:jwfranklin
ID: 31508868
Excellent, thanks for your solution worked straight away.
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