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How to use if exists in a select statement

Posted on 2008-10-02
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Last Modified: 2010-04-21
I need to figure out a way to indicate when a user has a monitor, a box or both. Here's the query I'm currently using:
select u.user_id,
(SELECT top 1 lm.link_monitor_id
FROM monitor_links lm
WHERE lm.user_id = u.user_id) as monitor,
(SELECT top 1 lb.link_box_id
FROM box_links lb
WHERE lb.user_id = u.user_id) as box

With this query, I get the following results:
user_id monitor_link_id box_link_id
12345   M123                B123
98765   M234                NULL
87654   NULL                B678

I would lke to get the following:
12345   both
98765   monitor
87654   box

I was thinking of using IF EXISTS, but couldn't make it work. Any ideas? Thanks!
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Question by:horalia
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4 Comments
 
LVL 18

Expert Comment

by:David Robitaille
ID: 22629734

use a case
select u.user_id,
case (SELECT 1 FROM monitor_links lm
WHERE lm.user_id = u.user_id) +
(SELECT top 1 lb.link_box_id
when 3 then 'both'
when 2 then 'monitor'
when 1 then 'box'
else 0 then 'nothing' end   as x
FROM box_links lb
WHERE lb.user_id = u.user_id)

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0
 
LVL 35

Expert Comment

by:Terry Woods
ID: 22629760
I think this should do it:
select distinct u.user_id, "both" equipment
from user_table u inner join monitor_links lm on lm.user_id = u.user_id
                  inner join box_links lb on lb.user_id = u.user_id
union 
select distinct u.user_id, "monitor"
from user_table u inner join monitor_links lm on lm.user_id = u.user_id
                  left join box_links lb on lb.user_id = u.user_id
where lb.user_id is null
union
select distinct u.user_id, "box"
from user_table u left join monitor_links lm on lm.user_id = u.user_id
                  inner join box_links lb on lb.user_id = u.user_id
where lm.user_id is null

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0
 
LVL 11

Accepted Solution

by:
yuching earned 500 total points
ID: 22630215
try this

SELECT u.user_id,
   (CASE WHEN b.monitor IS NOT NULL AND c.box IS NOT NULL THEN 'both'
               WHEN b.monitor IS NOT NULL THEN 'monitor'
               WHEN c.box IS NOT NULL THEN 'box'
                ELSE 'no match'
     END) As columnB
FROM usertables u
LEFT OUTER JOIN  (
     SELECT lm.user_id, MAX(lm.link_monitor_id) As monitor
     FROM monitor_links lm
      GROUP BY lm.user_id
) b ON b.user_id = u.user_id
LEFT OUTER JOIN (
      SELECT lb.user_id,  lb.link_box_id AS box
      FROM box_links lb
      GROUP BY lb.user_id
) c ON c.user_id = u.user_id
0
 

Author Closing Comment

by:horalia
ID: 31502600
Thanks!
0

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