Mysql查看数据库和表占用空间
1、查看数据库的大小
select sum(DATA_LENGTH)+sum(INDEX_LENGTH) from information_schema.TABLES where TABLE_SCHEMA='数据库名';
结果是以字节为单位，除(1024*1024)为M。
mysql> select sum(DATA_LENGTH)+sum(INDEX_LENGTH) as 'hellodb_use_Total(bytes)' from information_schema.TABLES where TABLE_SCHEMA='hellodb';
+--------------------------+
| hellodb_use_Total(bytes) |
+--------------------------+
| 262144 |
+--------------------------+
1 row in set (0.00 sec
2、concat()函数：联合字段
mysql> select concat(10,' Mb') as Lenge;
+-------+
| Lenge |
+-------+
| 10 Mb |
+-------+
1 row in set (0.00 sec)
3、ROUND()函数
ROUND函数用于把数值字段舍入为指定的小数位数。语法如下：
SELECT ROUND(column_name,decimals) FROM table_name
column_name 必需，要舍入的字段
decimals 必需，要返回的小数位数
mysql> select round(10.1234,2) as '10.231';
+--------+
| 10.231 |
+--------+
| 10.12 |
+--------+
1 row in set (0.00 sec)
mysql> select round(10.1234,0) as '10.231';
+--------+
| 10.231 |
+--------+
| 10 |
+--------+
1 row in set (0.00 sec)
4、truncate(X,D)函数：截取字段函数
X算数操作；D截取字段，D小于0，将小数点前|D|位置零；D大于0，截取小数点后D位；
truncate()函数用法示例
mysql> SELECT TRUNCATE(1.223343*100,2);
+--------------------------+
| TRUNCATE(1.223343*100,2) |
+--------------------------+
| 122.33 |
+--------------------------+
1 row in set (0.00 sec)
mysql> SELECT TRUNCATE(1.223343*100,0);
+--------------------------+
| TRUNCATE(1.223343*100,0) |
+--------------------------+
| 122 |
+--------------------------+
1 row in set (0.00 sec)
mysql> SELECT TRUNCATE(1.223343*100,-1);
+---------------------------+
| TRUNCATE(1.223343*100,-1) |
+---------------------------+
| 120 |
+---------------------------+
1 row in set (0.03 sec)
mysql> SELECT TRUNCATE(1.223343*100,-2);
+---------------------------+
| TRUNCATE(1.223343*100,-2) |
+---------------------------+
| 100 |
+---------------------------+
1 row in set (0.00 sec)
示例：查询数据库占用多少G
select concat(truncate((sum(DATA_LENGTH)+sum(INDEX_LENGTH) )/1024/1024/1024,2) ,' G') from information_schema.TABLES where TABLE_SCHEMA=’数据库名’;
5、information_schema 数据库中的TABLES 表参数说明
mysql> use information_schema
Database changed
information_schema库中的tables表字段如下：
mysql> select * from TABLES limit 71,3;
+---------------+--------------+----------------+------------+--------+---------+------------+------------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+------------+-------------------+----------+----------------+-----------------------------+
| TABLE_CATALOG | TABLE_SCHEMA | TABLE_NAME | TABLE_TYPE | ENGINE | VERSION | ROW_FORMAT | TABLE_ROWS | AVG_ROW_LENGTH | DATA_LENGTH | MAX_DATA_LENGTH | INDEX_LENGTH | DATA_FREE | AUTO_INCREMENT | CREATE_TIME | UPDATE_TIME | CHECK_TIME | TABLE_COLLATION | CHECKSUM | CREATE_OPTIONS | TABLE_COMMENT |
+---------------+--------------+----------------+------------+--------+---------+------------+------------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+------------+-------------------+----------+----------------+-----------------------------+
| NULL | mysql | user | BASE TABLE | MyISAM | 10 | Dynamic | 7 | 81 | 572 | 281474976710655 | 2048 | 0 | NULL | 2017-11-26 17:19:46 | 2018-06-08 21:45:38 | NULL | utf8_bin | NULL | | Users and global privileges |
| NULL | smp11 | mcn_user_major | BASE TABLE | InnoDB | 10 | Compact | 0 | 0 | 16384 | 0 | 16384 | 0 | NULL | 2018-06-10 20:40:45 | NULL | NULL | latin1_swedish_ci | NULL | | |
| NULL | testslave | id | BASE TABLE | InnoDB | 10 | Compact | 2 | 8192 | 16384 | 0 | 0 | 0 | NULL | 2017-11-26 17:37:17 | NULL | NULL | latin1_swedish_ci | NULL | | |
+---------------+--------------+----------------+------------+--------+---------+------------+------------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+------------+-------------------+----------+----------------+-----------------------------+
3 rows in set (0.05 sec)
MySQL的 information_schema 数据库中的TABLES 表记录了MySQL数据库中每个表占用的空间、表记录的行数，更新时间，说明等，这个表主要字段如下：
TABLE_SCHEMA : 数据库名
TABLE_NAME：表名
ENGINE：所使用的存储引擎
TABLES_ROWS：记录数，即表的行数
DATA_LENGTH：数据大小
INDEX_LENGTH：索引大小
CREATE_TIME：创建时间
UPDATE_TIME：最近更新时间
DATA_FREE：如果是共享表空间，该字段表示共享表空间的大小而非数据的大小。只有使用独占表空间时，该字段才表示该表的剩余空间；
说明：该参数与mysql碎片有关，当MySQL从列表中删除一行内容，该段空间就会被留空。在一段时间内执行大量删除操作后，往往会使留空的空间变得比存储列表内容所使用的空间更大。
通俗的讲：Data_free字段即为多占的物理空间，通过‘show table status’可以查看指定表的Data_free字段，对应的值就是多占用的物理空间，当drop表重建或重新导入可以释放这部分空间。
此时可以使用optimize整理表的碎片：
注意：该操作执行的时候会把该表格先写入一个tmp临时表，所以磁盘剩余空间必须大于 表空间，否则会执行失败。mysql> optimize table classes,students;
5.6.X以前的版本会提示该表不支持optimize，5.6.X的版本已经支持Innodb了。
6、查询一个表或库占用总的空间的大小
使用sum(数据大小)+sum(索引大小) 即可，SQL如下：
1）、查询库
select TABLE_SCHEMA,sum(DATA_LENGTH)+sum(INDEX_LENGTH) as use_total ,TABLE_ROWS FROM information_schema.TABLES where TABLE_SCHEMA='数据库名'
mysql> select TABLE_SCHEMA,sum(DATA_LENGTH)+sum(INDEX_LENGTH) FROM information_schema.TABLES where TABLE_SCHEMA='hellodb';
+--------------+------------------------------------+
| TABLE_SCHEMA | sum(DATA_LENGTH)+sum(INDEX_LENGTH) |
+--------------+------------------------------------+
| hellodb | 262144 |
+--------------+------------------------------------+
1 row in set (0.02 sec)
2）、查询表
select TABLE_NAME,DATA_LENGTH+INDEX_LENGTH,TABLE_ROWS FROM information_schema.TABLES where TABLE_SCHEMA='数据库名' and TABLE_NAME='表名'
mysql> select TABLE_NAME,DATA_LENGTH+INDEX_LENGTH,TABLE_ROWS FROM information_schema.TABLES where TABLE_SCHEMA='hellodb' and TABLE_NAME='classes';
+------------+--------------------------+------------+
| TABLE_NAME | DATA_LENGTH+INDEX_LENGTH | TABLE_ROWS |
+------------+--------------------------+------------+
| classes | 16384 | 8 |
+------------+--------------------------+------------+
1 row in set (0.00 sec)
7、查看表的最后更新时间
select TABLE_NAME,UPDATE_TIME from information_schema.TABLES where TABLE_SCHEMA='数据库名';
可以通过查看数据库中表的更新时间，确定mysql数据库是使用情况。
8、案例：修改指定数据库名即可查看表格占用空间
案例1：
数据库所占磁盘空间，查询所有数据库占用磁盘空间大小的SQL语句如下：
mysql> select TABLE_SCHEMA, concat(truncate(sum(data_length)/1024/1024,2),' MB') as data_size,
-> concat(truncate(sum(index_length)/1024/1024,2),'MB') as index_size
-> from information_schema.tables
-> group by TABLE_SCHEMA
-> order by data_length desc;
+--------------------+-----------+------------+
| TABLE_SCHEMA | data_size | index_size |
+--------------------+-----------+------------+
| smp11 | 0.01 MB | 0.01MB |
| mydb | 0.06 MB | 0.00MB |
| ibdatax | 0.03 MB | 0.00MB |
| hellodb | 0.18 MB | 0.06MB |
| db1 | 43.62 MB | 13.04MB |
| testslave | 0.01 MB | 0.00MB |
| mysql | 0.53 MB | 0.08MB |
| information_schema | 0.00 MB | 0.00MB |
+--------------------+-----------+------------+
8 rows in set (0.27 sec)
案例2：各个表所占的磁盘空间大小
查询单个数据库里面各个表所占磁盘空间大小包括其索引的大小，SQL语句如下：
mysql> select TABLE_NAME, concat(truncate(data_length/1024/1024,2),' MB') as data_size,
-> concat(truncate(index_length/1024/1024,2),' MB') as index_size
-> from information_schema.tables where TABLE_SCHEMA = 'hellodb'
-> group by TABLE_NAME
-> order by data_length desc;
+------------+-----------+------------+
| TABLE_NAME | data_size | index_size |
+------------+-----------+------------+
| classes | 0.01 MB | 0.00 MB |
| classes2 | 0.01 MB | 0.00 MB |
| classes3 | 0.01 MB | 0.00 MB |
| classes4 | 0.01 MB | 0.00 MB |
| coc | 0.01 MB | 0.00 MB |
| courses | 0.01 MB | 0.00 MB |
| paixi | 0.01 MB | 0.00 MB |
| scores | 0.01 MB | 0.00 MB |
| students | 0.01 MB | 0.03 MB |
| students1 | 0.01 MB | 0.03 MB |
| teachers | 0.01 MB | 0.00 MB |
| toc | 0.01 MB | 0.00 MB |
| test | NULL | NULL |
+------------+-----------+------------+
13 rows in set (0.00 sec)
此处只需将SQL语句中的hellodb换成其它的数据库名称即可。