前言：在mysql中有一個預設的資料表information_schema，information_schema這張資料表儲存了MySQL伺服器所有資料庫的資訊。如資料庫名，資料庫的表，表欄的資料類型與通路權限等。再簡單點，這台MySQL伺服器上，到底有哪些資料庫、各個資料庫有哪些表，每張表的字段類型是什麼，各個資料庫要什麼權限才能通路，等等資訊都儲存在information_schema表裡面，是以請勿删改此表。
代碼：
1，切換資料庫
use information_schema;
2，檢視資料庫使用大小
select concat(round(sum(data_length/1024/1024),2),'MB') as data from tables where table_schema='DB_Name';
3，檢視表使用大小
select concat(round(sum(data_length/1024/1024),2),'MB') as data from tables where table_schema='DB_Name' and table_name='Table_Name';
示例：
mysql> select data_length,index_length from tables where table_schema='dbname' and table_name = 'tablename';
+-------------+--------------+
| data_length | index_length |
+-------------+--------------+
| 166379520 | 235782144 |
+-------------+--------------+
mysql> select concat(round(sum(data_length/1024/1024),2),'MB') as data_length_MB, concat(round(sum(index_length/1024/1024),2),'MB') as index_length_MB from tables where table_schema='xiaoneiwai' and table_name = 'course_promotion_agents';
+----------------+-----------------+
| data_length_MB | index_length_MB |
+----------------+-----------------+
| 158.67MB | 224.86MB |
+----------------+-----------------+
1.檢視所有資料庫容量大小
select table_schema as '資料庫',sum(table_rows) as '記錄數',sum(truncate(data_length/1024/1024, 2)) as '資料容量(MB)',sum(truncate(index_length/1024/1024, 2)) as '索引容量(MB)' from information_schema.tables group by table_schema order by sum(data_length) desc, sum(index_length) desc;
+--------------------+-----------+------------------+------------------+
| 資料庫 | 記錄數 | 資料容量(MB) | 索引容量(MB) |
+--------------------+-----------+------------------+------------------+
| xiaoneiwai | 469948640 | 60724.32 | 61246.20 |
| admin | 86341747 | 39631.26 | 7924.72 |
| smsgw | 7367058 | 2125.15 | 462.67 |
| back | 4447309 | 956.16 | 1078.14 |
| gs1 | 2426963 | 71.89 | 74.82 |
| mysql | 2034 | 0.50 | 0.03 |
| uc | 14 | 0.00 | 0.00 |
| test | 1 | 0.00 | 0.00 |
| information_schema | NULL | 0.00 | 0.00 |
| performance_schema | 23014 | 0.00 | 0.00 |
+--------------------+-----------+------------------+------------------+
2.檢視所有資料庫各表容量大小
select table_schema as '資料庫',table_name as '表名',table_rows as '記錄數',truncate(data_length/1024/1024, 2) as '資料容量(MB)',truncate(index_length/1024/1024, 2) as '索引容量(MB)' from information_schema.tables order by data_length desc, index_length desc;
+--------------------+----------------------------------------------+-----------+------------------+------------------+
| 資料庫 | 表名 | 記錄數 | 資料容量(MB) | 索引容量(MB) |
+--------------------+----------------------------------------------+-----------+------------------+------------------+
| admin | user_active_data | 43441854 | 19513.23 | 3779.16 |
| admin | qun_active_data | 41141839 | 18794.00 | 3679.30 |
| xiaoneiwai | notification | 26199895 | 6102.00 | 3385.84 |
| xiaoneiwai | weibo_footprint | 122371009 | 4856.00 | 9398.00 |
| xiaoneiwai | attachment | 9452395 | 3770.00 | 2605.15 |
| xiaoneiwai | weibo_pic_video | 2763042 | 3437.96 | 0.00 |
+--------------------+----------------------------------------------+-----------+------------------+------------------+
3.檢視指定資料庫容量大小
select table_schema as '資料庫',sum(table_rows) as '記錄數',sum(truncate(data_length/1024/1024, 2)) as '資料容量(MB)',sum(truncate(index_length/1024/1024, 2)) as '索引容量(MB)' from information_schema.tables where table_schema='mysql';
+-----------+-----------+------------------+------------------+
| 資料庫 | 記錄數 | 資料容量(MB) | 索引容量(MB) |
+-----------+-----------+------------------+------------------+
| mysql | 2034 | 0.50 | 0.03 |
+-----------+-----------+------------------+------------------+
4.檢視指定資料庫各表容量大小
select table_schema as '資料庫',table_name as '表名',table_rows as '記錄數',truncate(data_length/1024/1024, 2) as '資料容量(MB)',truncate(index_length/1024/1024, 2) as '索引容量(MB)' from information_schema.tables where table_schema='mysql' order by data_length desc, index_length desc;
+-----------+---------------------------+-----------+------------------+------------------+
| 資料庫 | 表名 | 記錄數 | 資料容量(MB) | 索引容量(MB) |
+-----------+---------------------------+-----------+------------------+------------------+
| mysql | help_topic | 505 | 0.40 | 0.01 |
| mysql | help_keyword | 453 | 0.08 | 0.01 |
| mysql | help_category | 38 | 0.02 | 0.00 |
| mysql | help_relation | 992 | 0.00 | 0.01 |
| mysql | db | 17 | 0.00 | 0.00 |
| mysql | user | 23 | 0.00 | 0.00 |
| mysql | proxies_priv | 2 | 0.00 | 0.00 |
| mysql | procs_priv | 0 | 0.00 | 0.00 |
| mysql | tables_priv | 0 | 0.00 | 0.00 |
| mysql | columns_priv | 0 | 0.00 | 0.00 |
| mysql | proc | 0 | 0.00 | 0.00 |
| mysql | host | 0 | 0.00 | 0.00 |
| mysql | event | 0 | 0.00 | 0.00 |
| mysql | time_zone | 0 | 0.00 | 0.00 |
| mysql | plugin | 0 | 0.00 | 0.00 |
| mysql | ndb_binlog_index | 0 | 0.00 | 0.00 |
| mysql | servers | 0 | 0.00 | 0.00 |
| mysql | func | 0 | 0.00 | 0.00 |
| mysql | time_zone_transition_type | 0 | 0.00 | 0.00 |
| mysql | time_zone_transition | 0 | 0.00 | 0.00 |
| mysql | time_zone_name | 0 | 0.00 | 0.00 |
| mysql | time_zone_leap_second | 0 | 0.00 | 0.00 |
| mysql | slow_log | 2 | 0.00 | 0.00 |
| mysql | general_log | 2 | 0.00 | 0.00 |
+-----------+---------------------------+-----------+------------------+------------------+