Question

Do any queries exist that require RIGHT JOIN, or can they always be re-written with LEFT JOIN?

And more specifically, how do you re-write this one without the right join (and I guess implicitly without any subqueries or other fanciness):

 SELECT * FROM t1 LEFT JOIN t2 ON t1.k2 = t2.k2 RIGHT JOIN t3 ON t3.k3 = t2.k3 

Answer 1

You can always use only left Joins…

SELECT * FROM t1     LEFT JOIN t2 ON t1.k2 = t2.k2     RIGHT JOIN t3 ON t3.k3 = t2.k3 

is equivilent to:

Select * From t3     Left Join (t1 Left Join t2                    On t2.k2 = t1.k2)       On T2.k3 = T3.K3 

In general I always try to use only Left Joins, as the table on the left in a Left Join is the one whose rows are ALL included in the output, and I like to think of it, (The Left side) as the ‘base’ set I am performing the cartesion product (join) against … So I like to have it first in the SQL…