New Features of MySQL5 (data dictionary)

Because the use of metadata (metadata is the data about how to database) has become more and more common, MySQL 5 also designs a dedicated database in the product, the fixed name of this database is information_schema. This database acts as a central data dictionary and contains all the object information of the database and other database-related projects (such as security ), these objects and other information have been fixed in the MySQL server product. Database administrators and related developers can use the information_schema data dictionary database to obtain metadata related to one or more data libraries on the MySQL server.
ExampleNote: If the database administrator wants to know the overview of the storage space in a MySQL database instance, you can query the data dictionary as follows:

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    |          8.64 |         8.64 |         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 |+---------+---------------+--------------+--------------+----------+--------+

Note that in the above example, you should set it to not query the dictionary database itself, and use the where Condition Statement. (Bkjia.com tutorial)