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better way to get a count

Posted on 2008-10-13
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Medium Priority
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Last Modified: 2010-04-21
hello
currently am getting a count based on a catid as in:
select a,b,c,d,
(select count(*)
from tableb b
where b.catid = a.catid) as count
from tablea a
where whatever

ok so then I have a situation where I need to say
if (select count(*)
from tableb b
where b.catid = a.catid)
> 0 then
(select count(*)
from tableb b
where b.catid = a.catid)
else
(select count(*)
from tablec c
where c.catid = a.catid)
end
as count

Q: this seems inefficient
what should I do in situations like this?

thanks
0
Comment
Question by:dgrafx
  • 4
  • 4
  • 4
  • +1
13 Comments
 
LVL 17

Expert Comment

by:HuyBD
ID: 22708254
try this
case when (select count(*)
from tableb b
where b.catid = a.catid)
> 0 then
(select count(*)
from tableb b
where b.catid = a.catid)
else
(select count(*)
from tablec c
where c.catid = a.catid)
end
as count

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0
 
LVL 17

Expert Comment

by:HuyBD
ID: 22708266
completed query
select a,b,c,d,
case when (select count(*)
from tableb b
where b.catid = a.catid)
> 0 then
(select count(*)
from tableb b
where b.catid = a.catid)
else
(select count(*)
from tablec c
where c.catid = a.catid)
end
as count
from tablea a

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0
 
LVL 25

Author Comment

by:dgrafx
ID: 22708276
ok - i typed wrong
what i'm using is
case when ...

so what i was concerned about was the overhead from counting at least twice
I keep thinking there might be a way to count once

any ideas on that?
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LVL 60

Expert Comment

by:Kevin Cross
ID: 22708286
Try this:
select a.a, a.b, a.c, a.d,
case isnull(b.countB, 0) when 0 then isnull(c.countC,0) 
else b.countB end as [count]
from tablea a 
LEFT JOIN (select catid, count(*) as countB from tableb group by catid) b 
ON a.catid = b.catid
LEFT JOIN (select catid, count(*) as countC from tablec group by catid) c
ON a.catid = c.catid
where somecolumn = 'whatever'

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0
 
LVL 60

Expert Comment

by:Kevin Cross
ID: 22708295
An alternative would be to make your original query a derived query and only do the second count like this:
SELECT a,b,c,d, 
case count when 0 then 
(select count(*)
from tablec c
where c.catid = derived.catid)
else count end AS count
FROM (select a,b,c,d,
(select count(*)
from tableb b
where b.catid = a.catid) as count
from tablea a
where somecolumn='whatever') derived

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0
 
LVL 17

Expert Comment

by:HuyBD
ID: 22708322
0
 
LVL 17

Accepted Solution

by:
HuyBD earned 2000 total points
ID: 22708337
try this
select a,b,c,d,
case count1 when 0 then count2 else count1 end as count
from (select a,b,c,d,(select count(*)
from tableb b
where b.catid = a.catid) as count1
(select count(*)
from tablec c
where c.catid = a.catid) as count2
from tablea a) as T

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0
 
LVL 5

Expert Comment

by:harwantgrewal
ID: 22708415
My Suggestion that as you are checking the count from a table and then if its greater then 0 then you want to output that else count from table c.

Harry
Declare @countTableA int
 
select @countTableA = count(*) from tablec b where b.catid = a.catid
 
if @countTableA > 0
BEGIN
   Select @countTableA
END
ELSE
BEGIN
  select count(*) from tablec c where c.catid = a.catid
END

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0
 
LVL 60

Expert Comment

by:Kevin Cross
ID: 22708431
dgrafx, I would use a join to both sets of data as shown here: http:#22708286; however, you could also use derived table like the suggestion following the one linked here OR a common table expression allowing you to do the count once and then do additional logic based on that result in an outer query.

Hopefully that helps.

Regards,
Kevin
0
 
LVL 25

Author Comment

by:dgrafx
ID: 22708453
looks good - will test some more tomorrow - it's late here

harwantgrewal: could you explain how you would 'blend' that into an example like:
select a,b,c,d,(select count(*) from tableb) as count
from tablea
0
 
LVL 25

Author Closing Comment

by:dgrafx
ID: 31505766
this one seemed to work out the best and thanks to everyone
0
 
LVL 60

Expert Comment

by:Kevin Cross
ID: 22710424
LOL.

Glad we were able to help you find an alternative solution.

Regards,
Kevin
0
 
LVL 25

Author Comment

by:dgrafx
ID: 22710435
thanks Kevin
0

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