关于MYSQL的数据类型转换机制。。

呃汗，问题还没搞清楚就急着把这个贴子结了，
http://topic.csdn.net/u/20091115/16/ae3fa852-32d7-4c97-ac9f-44e33ed3d382.html?154770355

这里再开一贴问吧。。

解释一下为什么mysql5里面select 1 union select 'a'
结果就是
1
a
呢，a为什么没有被转换？ MYSQL到底是如何进行数据类型的转换的呢？

我在官网上没找到这东西呃，， 全是鸟语，也看不大懂。。

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SQL code:
mysql> create table test3 as select 1 union select 2;
Query OK, 2 rows affected (0.30 sec)
Records: 2 Duplicates: 0 Warnings: 0

mysql> desc test3;
+-------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| 1 | bigint(20) | NO | | 0 | |
+-------+------------+------+-----+---------+-------+
1 row in set (0.00 sec)

mysql>


SQL code:
mysql> create table test4 as select 'a' union select 'a';
Query OK, 1 row affected (0.11 sec)
Records: 1 Duplicates: 0 Warnings: 0

mysql> desc test4;
+-------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+