Question

In my query i want to join two tables based on the value of a field (say field1). Depending on the value of the field1 the join would EITHER be:

field3 = field4 OR field5 = field6

Something like

join on    
  CASE FIELD1    
   When 1 THEN FIELD 2 = FIELD3    
   When 2 THEN FIELD 4 = FIELD5    
  END

I am doing something like this at the moment

 .... join on  (field1=1 AND field2=field3) OR (field1=2 AND field4=field5)

but it takes ages to run the query. The two conditions individually take less than 7 secs each

How can i do this?

Answer 1

I would highly recommend against this approach. You are forcing the db to run a query it can’t do anything to optimise. Instead look at your tables and see if there isn’t something that can be done to split the data so this conditional isn’t needed.

Alternatively if you really need to do this then the fastest way to do it is run two queries and union the results:

select * from table as x where x.field1 = 1 AND x.field2 = x.field3
union [ALL]
select * from table as y where x.field1 = 2 AND y.field4 = y.field5

This should be far faster.