举例说明，如果数据库管理人员想要知道某个MySQL数据库实例中存储空间的概况，那么可以通过执行如下的对数据字典的查询来实现：
mysql> SELECT a.schema_name db_name,

-> IFNULL(ROUND((SUM(b.data_length)+SUM(b.index_length))/1024/1024,2),0.00)

-> total_size_mb,

-> IFNULL(ROUND(((SUM(b.data_length)+SUM(b.index_length))-

-> SUM(b.data_free))/1024/1024,2),0.00) data_used_mb,

-> IFNULL(ROUND(SUM(data_free)/1024/1024,2),0.00) data_free_mb,

-> IFNULL(ROUND((((SUM(b.data_length)+SUM(b.index_length))-SUM(b.data_free))/

-> ((SUM(b.data_length)+SUM(b.index_length)))*100),2),0) pct_used,

-> COUNT(table_name) tables

-> FROM information_schema.schemata a

-> LEFT JOIN information_schema.tables b ON a.schema_name = b.table_schema

-> WHERE a.schema_name != 'information_schema'

-> GROUP BY a.schema_name

-> ORDER BY 1;

+---------+---------------+--------------+--------------+----------+--------+

| db_name | total_size_mb | data_used_mb | data_free_mb | pct_used | tables |

+---------+---------------+--------------+--------------+----------+--------+

| gim | 432.67 | 432.67 | 0.00 | 100.00 | 16 |

| gim2 | 8. | 8. | 0.00 | 100.00 | 6 |

| mysql | 0.33 | 0.33 | 0.00 | 99.69 | 18 |

| test | 0.00 | 0.00 | 0.00 | 0.00 | 0 |

| tpcc | 126.09 | 126.09 | 0.00 | 100.00 | 9 |

+---------+---------------+--------------+--------------+----------+--------+

注意上面的示例中，应该设为不查询字典数据库自身，通过where条件语句来实现。(51CTO.COM教程)