Can I join to a table in ORACLE (10g) using a CASE clause in the ON statement (or even where clause as it’s an inner join)

5 thoughts on “Can I join to a table in ORACLE (10g) using a CASE clause in the ON statement (or even where clause as it’s an inner join)”

1. user November 30, -0001 at 12:00 am

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2. user November 30, -0001 at 12:00 am

   You can achieve this by using left join and where condition

   select a.*
   from table1 a
   left join tablea2 b on a."cola2" = b."colb2"
   WHERE a."cola1" = 'valuea2'
     and ( b."colb2" is null or b."colb3" = 'valueb3' )
   

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3. user November 30, -0001 at 12:00 am

   I think CASE statements work in join conditions, but I’m not sure. But would this work for you?

   select *
     from table1 a
    where a.cola1 = 'valuea1'
       or (a.cola1 = 'valuea2'
           and Exists(select 1
                        from table2 b
                       where a.cola2 = b.colb2
                         and b.colb3 = 'valueb3'
                      )
           )
   

   ---

   Edit: Wouldn’t this simply work?

   select a.*
     from table1 a
     Left Outer Join table2 b On (a.cola2 = b.colb2)
    where a.cola1 = 'valuea1'
       or (a.cola1 = 'valuea2' and b.colb3 = 'valueb3')
   

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4. user November 30, -0001 at 12:00 am

   OP: I’ve got a mini-workaround which goes close (This may only work given this is an inner join.)

   select a.* from table1 a
   inner join table2 b on a.”cola1″ = b.”COLb1″
   WHERE
   (a.”cola2″ = ‘valuea1’)
   OR (a.”cola2″ = ‘valuea2’ and b.”colb1″ = ‘valueb3’)

   Sometimes writing code out can prompt some alternative thinking. Self-Therapy sort of. Thanks for your input.

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5. user November 30, -0001 at 12:00 am

   Overall you should follow Hosam’s suggestion of rewriting the predicate entirely. But to explain your original problem further, the issue is that in SQL, CASE .. END is an expression and can only be used where any other expression could be used. A condition like “a=b” is two expressions connected by a logical operator. You may want to think of it as a boolean expression but that’s not the way SQL views it.

   You could accomplish what you want with CASE by using it as one of the two expressions in the condition, like so:

   WHERE a."cola2" IN ('valuea1','valuea2')
     and 
     b."colb1" = CASE 
                   WHEN a."cola2" = 'valuea2' THEN 'valueb3'
                   ELSE b."colb1"
                 END CASE
   

   (If it is possible for colb1 to include NULLs you would need to modify to handle that.)

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