MySQL是一款開源的關(guān)系型數(shù)據(jù)庫(kù)系統(tǒng)。當(dāng)我們安裝了MySQL并成功啟動(dòng)之后，會(huì)發(fā)現(xiàn)有一些默認(rèn)庫(kù)已經(jīng)被創(chuàng)建。這篇文章會(huì)介紹MySQL默認(rèn)庫(kù)的種類和作用。

1. mysql庫(kù)

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| performance_schema |
| sys                |
+--------------------+

mysql庫(kù)中存儲(chǔ)了MySQL的權(quán)限和用戶信息，也包含了一些系統(tǒng)變量和函數(shù)。我們可以通過(guò)該庫(kù)進(jìn)行用戶管理。

2. information_schema庫(kù)

mysql> use information_schema;
Database changed
mysql> show tables;
+---------------------------------------+
| Tables_in_information_schema          |
+---------------------------------------+
| CHARACTER_SETS                         |
| COLLATIONS                             |
| COLLATION_CHARACTER_SET_APPLICABILITY |
| COLUMNS                                |
| COLUMN_PRIVILEGES                      |
| ENGINES                                |
| EVENTS                                 |
| FILES                                  |
| GLOBAL_STATUS                          |
| GLOBAL_VARIABLES                       |
| KEY_COLUMN_USAGE                       |
| OPTIMIZER_TRACE                        |
| PARAMETERS                             |
| PARTITIONS                             |
| PLUGINS                                |
| PROCESSLIST                            |
| PROFILING                              |
| REFERENTIAL_CONSTRAINTS               
……

information_schema庫(kù)中包含了關(guān)于MySQL表和數(shù)據(jù)庫(kù)元數(shù)據(jù)的信息。我們可以通過(guò)該庫(kù)獲得數(shù)據(jù)庫(kù)、表、列的相關(guān)信息。

3. performance_schema庫(kù)

mysql> use performance_schema;
Database changed
mysql> show tables;
+-------------------------------------------------------+
| Tables_in_performance_schema                          |
+-------------------------------------------------------+
| accounts                                              |
| binary_log_transaction_compression_stats               |
| cond_instances                                        |
| data_locks                                            |
| events_stages_current                                 |
| events_stages_history                                 |
| events_stages_history_long                            |
| events_statements_current                             |
| events_statements_history                             |
| events_statements_history_long                        |
| events_transactions_current                           |
| events_transactions_history                           |
| events_transactions_history_long                      |
| exported_foreign_keys                                 |
| file_instances                                        |
| file_summary_by_event_name                            
……

performance_schema庫(kù)用于收集MySQL性能相關(guān)信息，包括線程、鎖、SQL語(yǔ)句執(zhí)行次數(shù)等。我們可以通過(guò)該庫(kù)進(jìn)行性能調(diào)優(yōu)。

4. sys庫(kù)

mysql> use sys;
Database changed
mysql> show tables;
+----------------------+
| Tables_in_sys        |
+----------------------+
| all_grants           |
| all_roles            |
| all_role_edges       |
| all_routine_privs    |
| all_sequences        |
| all_tables           |
| all_views            |
| any_user_privileges  |
| applicable_roles     |
| attributes           |
| character_sets       |
| check_constraints    |
| collations           |
| column_privileges    |
| column_statistics    |
| columns              |
| column_types         |
| ……

sys庫(kù)是MySQL 5.7版本以后出現(xiàn)的一個(gè)庫(kù)，用于提供一些方便的視圖和存儲(chǔ)過(guò)程來(lái)查詢數(shù)據(jù)。我們可以通過(guò)該庫(kù)進(jìn)行數(shù)據(jù)查看和分析。

總結(jié)

MySQL默認(rèn)庫(kù)的種類和作用分別是mysql庫(kù)（用戶權(quán)限和系統(tǒng)變量管理）、information_schema庫(kù)（數(shù)據(jù)庫(kù)和表元數(shù)據(jù)查詢）、performance_schema庫(kù)（性能數(shù)據(jù)收集和分析）和sys庫(kù)（數(shù)據(jù)查看和分析）。根據(jù)我們的實(shí)際需求，選擇合適的庫(kù)進(jìn)行操作和查詢。