只能說我菜，在網上找了下沒發現誰解釋過他們四個的區別，那就只能自己來了。下面是通過mysql客戶端查看到的幫助信息，直接貼解釋吧。
user：Returns the current MySQL username and hostname as a string in the utf8 character set.
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().
其實想翻一下再放出來的，怎奈自己那見不人的英語水平，就不翻譯了。總的來說：user()、system_user()、session_user()，這三是同一個東西(synonym )。然後current_user()跟前三個也很像，不過他偶爾又會有點區別。
