User() 这个函数, 是取得  当前登陆的用户.

相当于问 MySQL  我是谁 的意思。

mysql> SELECT USER();
+----------------+
| USER()         |
+----------------+
| root@localhost |
+----------------+ 

1 row in set (0.00 sec)

通过mysql客户端查看到的帮助信息,直接贴解释吧.

user:Returns the current MySQL username and hostname as a string in the utf8 character set.//phpfensi.com

system_user:SYSTEM_USER() is a synonym for USER().

session_user:SESSION_USER() is a synonym for USER().

current_user：

Returns the username and hostname combination for the MySQL account that the server used to authenticate the current client. This account determines your access privileges. As of MySQL 5.0.10, within a stored routine that is defined with the SQL SECURITY DEFINER characteristic,CURRENT_USER() returns the creator of the routine. The return value is

a string in the utf8 character set.

The value of CURRENT_USER() can differ from the value of USER().

其实想翻一下再放出来的,怎奈自己那见不人的英语水平,就不翻译了,总的来说,user()、system_user()、session_user()，这三是同一个东西（synonym ）,然后current_user()跟前三个也很像,不过他偶尔又会有点区别.