有如下两张表
a
+------+------+---------+
| uid | name | addtime |
+------+------+---------+
| 1 | tom | 2001 |
| 2 | jack | 2002 |
| 3 | alex | 2003 |
+------+------+---------+ b
+------+-------+---------+
| uid | title | addtime |
+------+-------+---------+
| 1 | rain | 2004 |
| 2 | snow | 2005 |
| 3 | wind | 2006 |
+------+-------+---------+
join是将两个表横向合并，就是通过on的条件将一个表追加在另一个表后边，新表相对于原来的两个表字段增多了。
mysql> select * from b left join a on b.uid = a.uid;
+------+-------+---------+------+------+---------+
| uid | title | addtime | uid | name | addtime |
+------+-------+---------+------+------+---------+
| 1 | rain | 2004 | 1 | tom | 2001 |
| 2 | snow | 2005 | 2 | jack | 2002 |
| 3 | wind | 2006 | 3 | alex | 2003 |
+------+-------+---------+------+------+---------+
union是将两个表竖向合并，新表字段不会多于原表。
mysql> select * from b union select * from a;
+------+-------+---------+
| uid | title | addtime |
+------+-------+---------+
| 1 | rain | 2004 |
| 2 | snow | 2005 |
| 3 | wind | 2006 |
| 1 | tom | 2001 |
| 2 | jack | 2002 |
| 3 | alex | 2003 |
+------+-------+---------+ mysql> select title,addtime from b union select name,addtime from a;
+-------+---------+
| title | addtime |
+-------+---------+
| rain | 2004 |
| snow | 2005 |
| wind | 2006 |
| tom | 2001 |
| jack | 2002 |
| alex | 2003 |
+-------+---------+
参考：http://bbs.csdn.net/topics/390410010