SELECT Statement is chosen as the value of the coalesced column. A simple way to interpret this is to consider that a coalesced column of an outer join is represented by the common column of the inner table of a  JOIN . Suppose that the tables  t1(a, b)  and  t2(a, c)  have the following contents: Then, for this join, column  a  contains the values of  t1.a : By contrast, for this join, column  a  contains the values of  t2.a . Compare those results to the otherwise equivalent queries with  JOIN ... ON : a LEFT JOIN b USING (c1, c2, c3) •A  USING  clause can be rewritten as an  ON  clause that compares corresponding columns. However, although  USING  and  ON  are similar, they are not quite the same. Consider the following two queries: 2675 t1 t2 ---- ---- 1 x 2 z 2 y 3 w mysql>  SELECT * FROM t1 NATURAL LEFT JOIN t2; +------+------+------+ | a | b | c | +------+------+------+ | 1 | x | NULL | | 2 | y | z | +------+------+------+ mysql>  SELECT * FROM t1 NATURAL RIGHT JOIN t2; +------+------+------+ | a | c | b | +------+------+------+ | 2 | z | y | | 3 | w | NULL | +------+------+------+ mysql>  SELECT * FROM t1 LEFT JOIN t2 ON (t1.a = t2.a); +------+------+------+------+ | a | b | a | c | +------+------+------+------+ | 1 | x | NULL | NULL | | 2 | y | 2 | z | +------+------+------+------+ mysql>  SELECT * FROM t1 RIGHT JOIN t2 ON (t1.a = t2.a); +------+------+------+------+ | a | b | a | c | +------+------+------+------+ | 2 | y | 2 | z | | NULL | NULL | 3 | w | +------+------+------+------+