Question

Let’s say there are 4 tables , such that
t1.id = t2.pid
and
t3.id = t4.pid

How do I fetch records where t1.val1 != t3.val1 AND t2.val2 != t4.val2?

Answer 1

You need to use LEFT OUTER JOIN to find non matching records from two tables.

SELECT *
FROM t1
     INNER JOIN t2
           ON t1.id = t2.pid
     LEFT OUTER JOIN t3
           ON t2.pid = t3.id
     LEFT OUTER JOIN t4
           ON t3.id = t4.pid
WHERE (t1.val1 IS NULL OR t3.val1 IS NULL)
      AND (t2.val2 IS NULL OR t4.val2 IS NULL)

Visit: http://www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html