为了使 MySQL 在解析 NATURAL JOIN 和 JOIN ... USING SQL 时采用 SQL 2003 的标准，从 MySQL 5.0.12 开始，这两种 SQL 的解析都有做调整, NATURAL JOIN 时， 查询结果中只会显示单独的唯一一列， 即 t1.a, t2.2 两列做自然连接后在结果中只有一列 a (= COELSCE(t1.a, t2.a)); 而 JOIN ... USING时，对 USING 中 指定使用做连接的列，查询结果中也只会显示单独的唯一一列， JOIN ... USING形式的变体如 LEFT JOIN ... USING, RIGHT JOIN ... USING 等解析优化也是采用同样的处理。除此之后，其他的连接方式还是遵照以前的规则。详细说明，可查看如下引用部分或MySQL 官方文档 JOIN Syntax一节。

     Join Processing Changes in MySQL 5.0.12

     Beginning with MySQL 5.0.12, natural joins and joins with USING, including outer join variants, are processed according to the SQL:2003 standard. The goal was to align the syntax and semantics of MySQL with respect to NATURAL JOIN and JOIN ... USING according to SQL:2003. However, these changes in join processing can result in different output columns for some joins. Also, some queries that appeared to work correctly in older versions must be rewritten to comply with the standard.

     The single result column that replaces two common columns is defined using the coalesce operation. That is, for two t1.a and t2.a the resulting single join column a is defined as a = COALESCE(t1.a, t2.a), where:

     COALESCE(x, y) = (CASE WHEN V1 IS NOT NULL THEN V1 ELSE V2 END)

     If the join operation is any other join, the result columns of the join consists of the concatenation of all columns of the joined tables. This is the same as previously.

mysql> SELECT * FROM t1 LEFT JOIN t2 USING (a);
+---+------+------+
| a | b    | c    |
+---+------+------+
| 1 | a    | NULL |
| 3 | c    | NULL |
| 5 | e    | NULL |
| 7 | g    | g    |
+---+------+------+
4 rows in set (0.01 sec)

mysql> SELECT * FROM t1 LEFT JOIN t2 ON t1.a = t2.a;
+---+------+------+------+
| a | b    | a    | c    |
+---+------+------+------+
| 1 | a    | NULL | NULL |
| 3 | c    | NULL | NULL |
| 5 | e    | NULL | NULL |
| 7 | g    |    7 | g    |
+---+------+------+------+
4 rows in set (0.01 sec)

     右外连接 SQL 在解析阶段都会转换为只包含左外连接的 SQL，一般遵循如下这样的转换方式：

T1, ...) RIGHT JOIN (T2,...) ON P(T1,...,T2,...) =
(T2, ...) LEFT JOIN (T1,...) ON P(T1,...,T2,...)

    有关其详细信息，请参考 MySQL 的官方文档7.3.1.10. Outer Join Simplification。

     At the parser stage, queries with right outer joins operations are converted to equivalent queries containing only left join operations. In the general case, the conversion is performed according to the following rule:

     T1, ...) RIGHT JOIN (T2,...) ON P(T1,...,T2,...) =

     (T2, ...) LEFT JOIN (T1,...) ON P(T1,...,T2,...)

     通过EXPLAIN EXTENDED, SHOW WARNINGS, 也能查看 MySQL 解析优化后的语句：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 RIGHT JOIN t2 USING (a);
+----+-------------+-------+--------+---------------+---------+---------+-----------+------+----------+-------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra |
+----+-------------+-------+--------+---------------+---------+---------+-----------+------+----------+-------+
|  1 | SIMPLE      | t2    | ALL    | NULL          | NULL    | NULL    | NULL      |    4 |   100.00 |       |
|  1 | SIMPLE      | t1    | eq_ref | PRIMARY       | PRIMARY | 4       | test.t2.a |    1 |   100.00 |       |
+----+-------------+-------+--------+---------------+---------+---------+-----------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)

mysql> SHOW WARNINGS;
+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message                                                                                                                                                            |
+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note  | 1003 | select `test`.`t2`.`a` AS `a`,`test`.`t2`.`c` AS `c`,`test`.`t1`.`b` AS `b` from `test`.`t2` left join `test`.`t1` on((`test`.`t1`.`a` = `test`.`t2`.`a`)) where 1 |
+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)