MySQL - JOIN a OR b

Question

Let\'s say I have a TABLE a in which a COLUMN data is the one to join on for 2 other tables (TABLE b and TABLE

Answer 1

If data is guaranteed to be UNIQUE in tables b and c

We can use outer join to attempt to get matching row from each of the tables, and then use expression in the SELECT list to test whether we got a matching row. For example:

SELECT a.data
     , IF(b.data IS NULL,IF(c.data IS NULL,NULL,'c'),'b') AS info_src
     , IF(b.data IS NULL,c.info,b.info) AS b_or_c_info
  FROM a
  LEFT
  JOIN b 
    ON b.data = a.data
  LEFT
  JOIN c
    ON c.data = a.data

If we find a matching row in b, then b.data is guaranteed to be non-NULL. We know the value is non-NULL because it satisfied an equality (=) comparison.

Note that IF is MySQL specific syntax. A more portable ANSI-standards compliant version

       CASE
         WHEN b.data IS NULL
         THEN c.info
         ELSE b.info
       END  AS b_or_c_info