Restricting a LEFT JOIN

Question

I have a table, let\'s call it \"a\" that is used in a left join in a view that involves a lot of tables. However, I only want to return rows of \"a\" if they also join wit

Answer 1

First define your query between table "a" and "b" to make sure it is returning the rows you want:

Select
   a.field1,
   a.field2,
   b.field3
from
   table_a a

   JOIN table_b b
      on (b.someid = a.someid)

then put that in as a sub-query of your main query:

select
   m.field1,
   m.field2,
   m.field3,
   a.field1 as a_field1,
   b.field1 as b_field1
from
   Table_main m

   LEFT OUTER JOIN
      (
      Select
         a.field1,
         a.field2,
         b.field3
      from
         table_a a

         JOIN table_b b
            on (b.someid = a.someid)
      ) sq
   on (sq.field1 = m.field1)

that should do it.

Ahh, missed the performance problem note - what I usually end up doing is putting the query from the view in a stored procedure, so I can generate the sub-queries to temp tables and put indexes on them. Suprisingly faster than you would expect. :-)