标题：MySQL 获取服务器元数据

如果想要获取服务器的元数据，可以使用下表的语句

命令	说明
SELECT VERSION()	返回服务器版本信息
SELECT DATABASE()	返回当前数据库名 (或者返回空)
SELECT USER()	返回当前用户名
SHOW STATUS	返回服务器状态
SHOW VARIABLES	返回服务器配置变量

SELECT VERSION() 获取服务器版本信息

MariaDB [twle]> SELECT VERSION();
+-----------------+
| VERSION()       |
+-----------------+
| 10.2.13-MariaDB |
+-----------------+
1 row in set (0.01 sec)

SELECT DATABASE() 获取当前数据库名

MariaDB [twle]> SELECT DATABASE();
+------------+
| DATABASE() |
+------------+
| twle       |
+------------+
1 row in set (0.00 sec)

如果当前没有选择任何数据库，则返回 NULL

MariaDB [(none)]> SELECT DATABASE();
+------------+
| DATABASE() |
+------------+
| NULL       |
+------------+
1 row in set (0.00 sec)

SELECT USER() 获取当前用户名

MariaDB [(none)]> SELECT USER();
+----------------+
| USER()         |
+----------------+
| root@localhost |
+----------------+
1 row in set (0.00 sec)

SHOW STATUS 获取服务器状态

MariaDB [(none)]> SHOW STATUS;
+----------------------+----------------------------------+
| Variable_name        | Value                            |
+----------------------+----------------------------------+
| Aborted_clients      | 14                               |
| Aborted_connects     | 0                                |
| Access_denied_errors | 0                                |
| Acl_column_grants    | 0                                |
| Acl_database_grants  | 2                                |
| Acl_function_grants  | 0                                |
| Acl_procedure_grants | 
| Binlog_commits       | 0                                |
....
....
483 rows in set (0.02 sec)

非常多数据，看不完，我们一般会使用 LIKE 语句查询某些具体的值，比如使用下面的语句查询缓存命中率相关的信息

SHOW STATUS LIKE '%Qcache%';

运行结果如下

MariaDB [(none)]> SHOW STATUS LIKE '%Qcache%';
+-------------------------+---------+
| Variable_name           | Value   |
+-------------------------+---------+
| Qcache_free_blocks      | 1       |
| Qcache_free_memory      | 1031312 |
| Qcache_hits             | 0       |
| Qcache_inserts          | 0       |
| Qcache_lowmem_prunes    | 0       |
| Qcache_not_cached       | 0       |
| Qcache_queries_in_cache | 0       |
| Qcache_total_blocks     | 1       |
+-------------------------+---------+
8 rows in set (0.00 sec)

SHOW VARIABLES 获取服务器配置变量

MariaDB [(none)]> SHOW VARIABLES\G;
*********************** 626. row ***********************
Variable_name: wsrep_slave_threads
        Value: 1
*********************** 627. row ***********************
Variable_name: wsrep_sst_auth
        Value: 
*********************** 628. row ***********************
Variable_name: wsrep_sst_donor
        Value: 
*********************** 629. row ***********************
Variable_name: wsrep_sst_donor_rejects_queries
        Value: OFF
*********************** 630. row ***********************
Variable_name: wsrep_sst_method
        Value: rsync
*********************** 631. row ***********************
Variable_name: wsrep_sst_receive_address
        Value: AUTO
*********************** 632. row ***********************
Variable_name: wsrep_start_position
        Value: 00000000-0000-0000-0000-000000000000:-1
*************************** 633. row ********************
Variable_name: wsrep_sync_wait
        Value: 0
633 rows in set (0.01 sec)
...
...
633 rows in set (0.02 sec)

非常多数据，看不完，我们一般会使用 LIKE 语句查询某些具体的值，比如使用下面的语句查询编码相关信息

SHOW VARIABLES LIKE '%character%';

运行结果如下

MariaDB [(none)]> 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     | utf8                                                    |
| character_set_system     | utf8                                                    |
| character_sets_dir       | .../share/mysql/charsets/      |
+--------------------------+--------------------------------+
8 rows in set (0.00 sec)