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: 

t1 t2 ---- ---- 1 x 2 z 2 y 3 w 

Then, for this join, column a  contains the values of t1.a : 

mysql> SELECT * FROM t1 NATURAL LEFT JOIN t2; +------+------+------+ | a | b | c | +------+------+------+ | 1 | x | NULL | | 2 | y | z | +------+------+------+ 

By contrast, for this join, column a  contains the values of t2.a . 

mysql> SELECT * FROM t1 NATURAL RIGHT JOIN t2; +------+------+------+ | a | c | b | +------+------+------+ | 2 | z | y | | 3 | w | NULL | +------+------+------+ 

Compare those results to the otherwise equivalent queries with JOIN ... ON : 

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 | +------+------+------+------+ 

•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: 

a LEFT JOIN b USING (c1, c2, c3) 

