MySQL是一個廣為使用的關系型數據庫管理系統。在MySQL中，有很多常用的庫，其中最常見的是兩個庫：MySQL數據庫和Information Schema。

MySQL數據庫是MySQL中的默認數據庫，它存儲了MySQL系統的內部信息、配置信息和用戶數據。在MySQL中，所有的數據庫（包括MySQL數據庫）和表都是存儲在文件系統上的文件。在MySQL 5.5版本以前，MySQL數據存儲在一個名為“mysql”文件夾下，而在5.5版本以后，MySQL數據存儲在一個名為“data”文件夾下。

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| performance_schema |
| test               |
+--------------------+
4 rows in set (0.00 sec)

Information Schema是一個特殊的庫，它存儲了MySQL系統中的元數據信息，如數據庫、表、列、索引、用戶、權限等。通常，我們通過查詢Information Schema庫可以獲得MySQL系統的很多有用信息。

mysql> select * from information_schema.tables where table_schema='mysql';
+--------------+---------------------+---------------------+------+--------+---------+------------+
| TABLE_CATALOG | TABLE_SCHEMA        | TABLE_NAME          | TABLE_TYPE | ENGINE | VERSION | ROW_FORMAT |
+--------------+---------------------+---------------------+------+--------+---------+------------+
| def          | mysql               | columns_priv        | BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | db                  | BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | event               | BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | func                | BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | help_category       | BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | help_keyword        | BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | help_relation       | BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | help_topic          | BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | host                | BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | ndb_binlog_index    | BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | plugin              | BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | proc                | BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | procs_priv          | BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | proxies_priv        | BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | roles_mapping       | BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | servers             | BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | table_constraints  | BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | tables_priv         | BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | time_zone           | BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | time_zone_leap_second|BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | time_zone_name      | BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | time_zone_transition|BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | time_zone_transition_type|BASE TABLE| MyISAM |      10 | Dynamic    |
| def          | mysql               | user                | BASE TABLE| MyISAM |      10 | Dynamic    |
+--------------+---------------------+---------------------+------+--------+---------+------------+
23 rows in set (0.00 sec)