Question

I need to add value condition to the FULL OUTER JOIN.

I.e. I’m triyng to do this:

SELECT *
FROM Table1
FULL OUTER JOIN Table2 ON Table1.Field1 = Table2.Field1 AND Table2.Field2 > 5

But this script doesn’t work. Actually it looks like the condition (Table2.Field2 > 5) was never applied at all.

The same issue appears for the RIGHT OUTER JOIN, so I think the reason is that when there is RIGHT or FULL join no value conditions applied to the right table in join.
Why is this happening? Is there a conceptual explanation of such behaviour?

And of course the main question is how to solve this issue.

Is there a way to solve this without using subqueries?

SELECT *
FROM Table1
FULL OUTER JOIN (SELECT * FROM Table2 WHERE Table2.Field2 > 5) AS t2 ON Table1.Field1 = t2.Field1

Answer 1

Pretty convulated but no subqueries

SELECT  Table1.*
        , CASE WHEN Table2.Field2 > 5 THEN Table2.Field1 ELSE NULL END
        , CASE WHEN Table2.Field2 > 5 THEN Table2.Field2 ELSE NULL END        
FROM    Table1
        FULL OUTER JOIN Table2 ON Table1.Field1 = Table2.Field1
WHERE   COALESCE(Table2.Field2, 6) > 5
        OR Table1.Field1 = Table2.Field1

Test script

;WITH Table1 AS (
  SELECT * FROM (VALUES
    (1, 1)
    , (2, 2)
    , (5, 5)
    , (6, 6)
  ) AS Table1 (Field1, Field2)
)
, Table2 AS (
  SELECT * FROM (VALUES
    (1, 1)
    , (3, 3)
    , (4, 4)
    , (5, 5)
    , (7, 7)
  ) AS Table2 (Field1, Field2)
)
SELECT  Table1.*
        , CASE WHEN Table2.Field2 > 5 THEN Table2.Field1 ELSE NULL END
        , CASE WHEN Table2.Field2 > 5 THEN Table2.Field2 ELSE NULL END        
FROM    Table1
        FULL OUTER JOIN Table2 ON Table1.Field1 = Table2.Field1
WHERE   COALESCE(Table2.Field2, 6) > 5
        OR Table1.Field1 = Table2.Field1

Results

 Field1 Field2 Field1 Field2
 1      1      NULL   NULL
 5      5      NULL   NULL
 NULL   NULL   7      7
 6      6      NULL   NULL
 1      1      NULL   NULL
 2      2      NULL   NULL