System information function of MySQL notes

The system information function queries the system information of the mysql database

VERSION() returns the database version number

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

I'm using the ubuntu based distribution here, Linux Mint

CONNECTION_ID() returns the number of connections to the database

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

It actually shows up every time you connect to mysql

DATABASE(), SCHEMA() returns the current database name

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

USER(), SYSTEM_USER(), SESSION_USER() return the current user

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

CURRENT_USER(), CURRENT_USER return the current user

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

The three above are identical to these two functions

CHARSET(str) returns the character set of the string str

mysql> SELECT CHARSET(' zhang 3');
+-------------------+
| CHARSET(' zhang 3')   |
+-------------------+
| utf8              |
+-------------------+
 row in set (0.00 sec)

COLLATION(str) returns the character arrangement of the string str

mysql> SELECT COLLATION(' zhang 3');
+---------------------+
| COLLATION(' zhang 3')   |
+---------------------+
| utf8_general_ci     |
+---------------------+
 row in set (0.00 sec)

LAST_INSERT_ID() returns the last generated AUTO_INCREMENT value

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 |
+----+
 rows in set (0.00 sec)
mysql> SELECT LAST_INSERT_ID();
+------------------+
| LAST_INSERT_ID() |
+------------------+
|                3 |
+------------------+
 row in set (0.00 sec)

The above statement first creates a table t1 with a self-incrementing field id

Then insert NULL three times to make it self-increment

After confirming that the data already exists, use LAST_INSERT_ID() to get the last automatically generated value