ASA SQL User's Guide
  Joins: Retrieving Data from Several Tables
    Key joins

Key joins with an ON phrase

When you specify a KEY JOIN and put a join condition in an ON phrase, the result is the conjunction of the two join conditions. For example,

SELECT *
FROM A KEY JOIN B
ON A.x = B.y

If the join condition generated by the key join of A and B is A.w    =    B.z, then this query is equivalent to

SELECT *
FROM A JOIN B
ON A.x = B.y AND A.w = B.z