使用MySQL时，需要了解当前数据库的情况，例如当前的数据库大小、字符集、用户等等。下面总结了一些查看数据库相关信息的命令
原文网址： https://www.cnblogs.com/jiangxiaobo/p/6110647.html
1：查看显示所有数据库
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| INVOICE |
| mysql |
| performance_schema |
| test |
+--------------------+
5 rows in set (0.00 sec)
mysql> 2：查看当前使用的数据库
mysql> select database();
+------------+
| database() |
+------------+
| INVOICE |
+------------+
1 row in set (0.00 sec)
mysql> 3：查看数据库使用端口
mysql> show variables like 'port';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| port | 3306 |
+---------------+-------+
1 row in set (0.00 sec)4：查看当前数据库大小
mysql> use information_schema
Database changed
mysql> select concat(round(sum(data_length)/(1024*1024),2) + round(sum(index_length)/(1024*1024),2),'MB') as 'DB Size'
-> from tables
-> where table_schema='INVOICE';
+-----------+
| DB Size |
+-----------+
| 7929.58MB |
+-----------+
1 row in set, 1 warning (0.00 sec)查看数据所占的空间大小
mysql> use information_schema;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql> select concat(round(sum(data_length)/(1024*1024),2),'MB') as 'DB Size'
-> from tables
-> where table_schema='INVOICE';
+-----------+
| DB Size |
+-----------+
| 6430.26MB |
+-----------+
1 row in set, 1 warning (0.00 sec)
mysql>查看索引所占的空间大小
mysql> select concat(round(sum(index_length)/(1024*1024),2),'MB') as 'DB Size'
-> from tables
-> where table_schema='INVOICE';
+-----------+
| DB Size |
+-----------+
| 1499.32MB |
+-----------+
1 row in set, 1 warning (0.13 sec)
mysql>
5：查看数据库编码
mysql> show variables like 'character%';
+--------------------------+----------------------------+
| Variable_name | Value |
+--------------------------+----------------------------+
| character_set_client | utf8 |
| character_set_connection | utf8 |
| character_set_database | utf8 |
| character_set_filesystem | binary |
| character_set_results | utf8 |
| character_set_server | latin1 |
| character_set_system | utf8 |
| character_sets_dir | /usr/share/mysql/charsets/ |
+--------------------------+----------------------------+
8 rows in set (0.00 sec)character_set_client 为客户端编码方式；
character_set_connection 为建立连接使用的编码；
character_set_database 为数据库的编码；
character_set_results 为结果集的编码；
character_set_server 为数据库服务器的编码；
只要保证以上采用的编码方式一样，就不会出现乱码问题。
mysql> show variables like 'collation%';
+----------------------+-------------------+
| Variable_name | Value |
+----------------------+-------------------+
| collation_connection | utf8_general_ci |
| collation_database | utf8_general_ci |
| collation_server | latin1_swedish_ci |
+----------------------+-------------------+
3 rows in set (0.00 sec)status也可以查看数据库的编码\
mysql> status;
--------------
mysql Ver 14.14 Distrib 5.6.20, for Linux (x86_64) using EditLine wrapper
Connection id: 1
Current database: INVOICE
Current user: root@localhost
SSL: Not in use
Current pager: stdout
Using outfile: ''
Using delimiter: ;
Server version: 5.6.20-enterprise-commercial-advanced MySQL Enterprise Server - Advanced Edition (Commercial)
Protocol version: 10
Connection: Localhost via UNIX socket
Server characterset: latin1
Db characterset: latin1
Client characterset: utf8
Conn. characterset: utf8
UNIX socket: /var/lib/mysql/mysql.sock
Uptime: 5 hours 18 min 51 sec
Threads: 1 Questions: 10884 Slow queries: 0 Opens: 650 Flush tables: 1 Open tables: 268 Queries per second avg: 0.568
--------------
mysql> 6：查看数据库的表信息
mysql> show tables;
或者使用下面SQL语句查看某个数据库的表信息。
select * from information_schema.tables where table_schema='databasename';
查看某种具体表的信息
select * from information_schema.tables where table_name ='table_name'- 查看数据库的所有用户信息
mysql> select distinct concat('user: ''',user,'''@''',host,''';') as query from mysql.user;
+-------------------------------------+
| query |
+-------------------------------------+
| user: 'root'@'127.0.0.1'; |
| user: 'root'@'::1'; |
| user: 'root'@'gettesx20.test.com'; |
| user: 'root'@'localhost'; |
+-------------------------------------+
4 rows in set (0.00 sec)
mysql> 8: 查看某个具体用户的权限
mysql> show grants for 'root'@'localhost';
2 rows in set (0.00 sec)9： 查看数据库的最大连接数
mysql> show variables like '%max_connections%';
+-----------------+-------+
| Variable_name | Value |
+-----------------+-------+
| max_connections | 151 |
+-----------------+-------+
1 row in set (0.00 sec)
mysql> 10：查看数据库当前连接数，并发数。
mysql> show status like 'Threads%';
+-------------------+-------+
| Variable_name | Value |
+-------------------+-------+
| Threads_cached | 0 |
| Threads_connected | 1 |
| Threads_created | 1 |
| Threads_running | 1 |
+-------------------+-------+
4 rows in set (0.00 sec)Threads_cached : 代表当前此时此刻线程缓存中有多少空闲线程。
Threads_connected :代表当前已建立连接的数量，因为一个连接就需要一个线程，所以也可以看成当前被使用的线程数。
Threads_created :代表从最近一次服务启动，已创建线程的数量。
Threads_running :代表当前激活的（非睡眠状态）线程数。并不是代表正在使用的线程数，有时候连接已建立，但是连接处于sleep状态，这里相对应的线程也是sleep状态。
11：查看数据文件存放路径
mysql> show variables like '%datadir%';
+---------------+-------------------+
| Variable_name | Value |
+---------------+-------------------+
| datadir | /mysqldata/mysql/ |
+---------------+-------------------+
1 row in set (0.00 sec)
mysql>
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