进入information_schema 数据库（存放了其他的数据库的信息）
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

 第一种情况：查询所有数据库的总大小，方法如下：
mysql> select concat(round(sum(DATA_LENGTH/1024/1024),2),'MB') as data from TABLES;
+-----------+
| data      |
+-----------+
| 3052.76MB |
+-----------+
1 row in set (0.02 sec)


 第二种情况：查看指定数据库的大小，比如说：数据库test，方法如下：
mysql> select concat(round(sum(DATA_LENGTH/1024/1024),2),'MB') as data from TABLES where table_schema='test';
+----------+
| data     |
+----------+
| 142.84MB |
+----------+
1 row in set (0.00 sec)


 第三种：查看指定数据库的表的大小，比如说：数据库test中f_apply表，方法如下：
mysql> select concat(round(sum(DATA_LENGTH/1024/1024),2),'MB') as data from TABLES where table_schema='ixinnuo_financial' and table_name='f_apply';
+--------+
| data   |
+--------+
| 0.08MB |
+--------+
1 row in set (0.00 sec)