MySQL学习笔记19：系统信息函数

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系统信息函数用来查询mysql数据库的系统信息

VERSION()返回数据库版本号

mysql> SELECT VERSION();+-------------------------+| VERSION()               |+-------------------------+| 5.5.28-0ubuntu0.12.10.2 |+-------------------------+1 row in set (0.00 sec)

我这里用的是基于ubuntu发行版，Linux Mint

CONNECTION_ID()返回数据库的连接次数

mysql> SELECT CONNECTION_ID();+-----------------+| CONNECTION_ID() |+-----------------+|              36 |+-----------------+1 row in set (0.00 sec)

其实每次连接到mysql的时候就会有显示

DATABASE()、SCHEMA()返回当前数据库名

mysql> SELECT DATABASE(), SCHEMA();+------------+----------+| DATABASE() | SCHEMA() |+------------+----------+| person     | person   |+------------+----------+1 row in set (0.00 sec)

USER()、SYSTEM_USER()、SESSION_USER()返回当前用户

mysql> SELECT USER(), SYSTEM_USER(), SESSION_USER();+----------------+----------------+----------------+| USER()         | SYSTEM_USER()  | SESSION_USER() |+----------------+----------------+----------------+| root@localhost | root@localhost | root@localhost |+----------------+----------------+----------------+1 row in set (0.00 sec)

CURRENT_USER()、CURRENT_USER返回当前用户

mysql> SELECT CURRENT_USER(), CURRENT_USER;+----------------+----------------+| CURRENT_USER() | CURRENT_USER   |+----------------+----------------+| root@localhost | root@localhost |+----------------+----------------+1 row in set (0.00 sec)

上面的三个和这两个功能是一样的

CHARSET(str)返回字符串str的字符集

mysql> SELECT CHARSET('张三');+-------------------+| CHARSET('张三')   |+-------------------+| utf8              |+-------------------+1 row in set (0.00 sec)

COLLATION(str)返回字符串str的字符排列方式

mysql> SELECT COLLATION('张三');+---------------------+| COLLATION('张三')   |+---------------------+| utf8_general_ci     |+---------------------+1 row in set (0.00 sec)

LAST_INSERT_ID()返回最后生成的AUTO_INCREMENT值

mysql> CREATE TABLE t1(id INT PRIMARY KEY AUTO_INCREMENT);Query OK, 0 rows affected (0.10 sec)mysql> INSERT INTO t1 VALUES(NULL);Query OK, 1 row affected (0.04 sec)mysql> INSERT INTO t1 VALUES(NULL);Query OK, 1 row affected (0.03 sec)mysql> INSERT INTO t1 VALUES(NULL);Query OK, 1 row affected (0.04 sec)mysql> SELECT * FROM t1;+----+| id |+----+|  1 ||  2 ||  3 |+----+3 rows in set (0.00 sec)mysql> SELECT LAST_INSERT_ID();+------------------+| LAST_INSERT_ID() |+------------------+|                3 |+------------------+1 row in set (0.00 sec)

上面的语句首先创建了一张表t1，其中有一个自增字段id

然后分三次插入NULL，使其自增

确认已经存在数据之后，使用LAST_INSERT_ID()获取最后自动生成的值