information_schema 是一个信息数据库。其中保存着关于MySQL服务器所维护的所有其他数据库的信息。如数据库名，数据库的表，表栏的数据类型与访问权 限等。在INFORMATION_SCHEMA中，有数个只读表。它们实际上是视图，而不是基本表，因此，你将无法看到与之相关的任何文件。
information_schema数据库表说明:
SCHEMATA表：提供了当前mysql实例中所有数据库的信息。是show databases的结果取之此表。
TABLES表：提供了关于数据库中的表的信息（包括视图）。详细表述了某个表属于哪个schema，表类型，表引擎，创建时间等信息。是show tables from schemaname的结果取之此表。
COLUMNS表：提供了表中的列信息。详细表述了某张表的所有列以及每个列的信息。是show columns from schemaname.tablename的结果取之此表。
STATISTICS表：提供了关于表索引的信息。是show index from schemaname.tablename的结果取之此表。
USER_PRIVILEGES（用户权限）表：给出了关于全程权限的信息。该信息源自mysql.user授权表。是非标准表。
SCHEMA_PRIVILEGES（方案权限）表：给出了关于方案（数据库）权限的信息。该信息来自mysql.db授权表。是非标准表。
TABLE_PRIVILEGES（表权限）表：给出了关于表权限的信息。该信息源自mysql.tables_priv授权表。是非标准表。
COLUMN_PRIVILEGES（列权限）表：给出了关于列权限的信息。该信息源自mysql.columns_priv授权表。是非标准表。
CHARACTER_SETS（字符集）表：提供了mysql实例可用字符集的信息。是SHOW CHARACTER SET结果集取之此表。
COLLATIONS表：提供了关于各字符集的对照信息。
COLLATION_CHARACTER_SET_APPLICABILITY表：指明了可用于校对的字符集。这些列等效于SHOW COLLATION的前两个显示字段。
TABLE_CONSTRAINTS表：描述了存在约束的表。以及表的约束类型。
KEY_COLUMN_USAGE表：描述了具有约束的键列。
ROUTINES表：提供了关于存储子程序（存储程序和函数）的信息。此时，ROUTINES表不包含自定义函数（UDF）。名为“mysql.proc name”的列指明了对应于INFORMATION_SCHEMA.ROUTINES表的mysql.proc表列。
VIEWS表：给出了关于数据库中的视图的信息。需要有show views权限，否则无法查看视图信息。
TRIGGERS表：提供了关于触发程序的信息。必须有super权限才能查看该表
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| MariaDB [information_schema]> show tables;
+---------------------------------------+
| Tables_in_information_schema |
+---------------------------------------+
| ALL_PLUGINS |
| APPLICABLE_ROLES |
| CHARACTER_SETS |
| CHECK_CONSTRAINTS |
| COLLATIONS |
| COLLATION_CHARACTER_SET_APPLICABILITY |
| COLUMNS |
| COLUMN_PRIVILEGES |
| ENABLED_ROLES |
| ENGINES |
| EVENTS |
| FILES |
| GLOBAL_STATUS |
| GLOBAL_VARIABLES |
| KEY_CACHES |
| KEY_COLUMN_USAGE |
| PARAMETERS |
| PARTITIONS |
| PLUGINS |
| PROCESSLIST |
| PROFILING |
| REFERENTIAL_CONSTRAINTS |
| ROUTINES |
| SCHEMATA |
| SCHEMA_PRIVILEGES |
| SESSION_STATUS |
| SESSION_VARIABLES |
| STATISTICS |
| SYSTEM_VARIABLES |
| TABLES |
| TABLESPACES |
| TABLE_CONSTRAINTS |
| TABLE_PRIVILEGES |
| TRIGGERS |
| USER_PRIVILEGES |
| VIEWS |
| GEOMETRY_COLUMNS |
| SPATIAL_REF_SYS |
| CLIENT_STATISTICS |
| INDEX_STATISTICS |
| INNODB_SYS_DATAFILES |
| USER_STATISTICS |
| INNODB_SYS_TABLESTATS |
| INNODB_LOCKS |
| INNODB_MUTEXES |
| INNODB_CMPMEM |
| INNODB_CMP_PER_INDEX |
| INNODB_CMP |
| INNODB_FT_DELETED |
| INNODB_CMP_RESET |
| INNODB_LOCK_WAITS |
| TABLE_STATISTICS |
| INNODB_TABLESPACES_ENCRYPTION |
| INNODB_BUFFER_PAGE_LRU |
| INNODB_SYS_FIELDS |
| INNODB_CMPMEM_RESET |
| INNODB_SYS_COLUMNS |
| INNODB_FT_INDEX_TABLE |
| INNODB_CMP_PER_INDEX_RESET |
| user_variables |
| INNODB_FT_INDEX_CACHE |
| INNODB_SYS_FOREIGN_COLS |
| INNODB_FT_BEING_DELETED |
| INNODB_BUFFER_POOL_STATS |
| INNODB_TRX |
| INNODB_SYS_FOREIGN |
| INNODB_SYS_TABLES |
| INNODB_FT_DEFAULT_STOPWORD |
| INNODB_FT_CONFIG |
| INNODB_BUFFER_PAGE |
| INNODB_SYS_TABLESPACES |
| INNODB_METRICS |
| INNODB_SYS_INDEXES |
| INNODB_SYS_VIRTUAL |
| INNODB_TABLESPACES_SCRUBBING |
| INNODB_SYS_SEMAPHORE_WAITS |
+---------------------------------------+
76 rows in set (0.000 sec)
MariaDB [information_schema]> describe TABLES;
+------------------+---------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------------+---------------------+------+-----+---------+-------+
| TABLE_CATALOG | varchar(512) | NO | | | |
| TABLE_SCHEMA | varchar(64) | NO | | | |
| TABLE_NAME | varchar(64) | NO | | | |
| TABLE_TYPE | varchar(64) | NO | | | |
| ENGINE | varchar(64) | YES | | NULL | |
| VERSION | bigint(21) unsigned | YES | | NULL | |
| ROW_FORMAT | varchar(10) | YES | | NULL | |
| TABLE_ROWS | bigint(21) unsigned | YES | | NULL | |
| AVG_ROW_LENGTH | bigint(21) unsigned | YES | | NULL | |
| DATA_LENGTH | bigint(21) unsigned | YES | | NULL | |
| MAX_DATA_LENGTH | bigint(21) unsigned | YES | | NULL | |
| INDEX_LENGTH | bigint(21) unsigned | YES | | NULL | |
| DATA_FREE | bigint(21) unsigned | YES | | NULL | |
| AUTO_INCREMENT | bigint(21) unsigned | YES | | NULL | |
| CREATE_TIME | datetime | YES | | NULL | |
| UPDATE_TIME | datetime | YES | | NULL | |
| CHECK_TIME | datetime | YES | | NULL | |
| TABLE_COLLATION | varchar(32) | YES | | NULL | |
| CHECKSUM | bigint(21) unsigned | YES | | NULL | |
| CREATE_OPTIONS | varchar(2048) | YES | | NULL | |
| TABLE_COMMENT | varchar(2048) | NO | | | |
| MAX_INDEX_LENGTH | bigint(21) unsigned | YES | | NULL | |
| TEMPORARY | varchar(1) | YES | | NULL | |
+------------------+---------------------+------+-----+---------+-------+
|
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| MariaDB [information_schema]> describe COLUMNS;
+--------------------------+---------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------------------------+---------------------+------+-----+---------+-------+
| TABLE_CATALOG | varchar(512) | NO | | | |
| TABLE_SCHEMA | varchar(64) | NO | | | |
| TABLE_NAME | varchar(64) | NO | | | |
| COLUMN_NAME | varchar(64) | NO | | | |
| ORDINAL_POSITION | bigint(21) unsigned | NO | | 0 | |
| COLUMN_DEFAULT | longtext | YES | | NULL | |
| IS_NULLABLE | varchar(3) | NO | | | |
| DATA_TYPE | varchar(64) | NO | | | |
| CHARACTER_MAXIMUM_LENGTH | bigint(21) unsigned | YES | | NULL | |
| CHARACTER_OCTET_LENGTH | bigint(21) unsigned | YES | | NULL | |
| NUMERIC_PRECISION | bigint(21) unsigned | YES | | NULL | |
| NUMERIC_SCALE | bigint(21) unsigned | YES | | NULL | |
| DATETIME_PRECISION | bigint(21) unsigned | YES | | NULL | |
| CHARACTER_SET_NAME | varchar(32) | YES | | NULL | |
| COLLATION_NAME | varchar(32) | YES | | NULL | |
| COLUMN_TYPE | longtext | NO | | | |
| COLUMN_KEY | varchar(3) | NO | | | |
| EXTRA | varchar(30) | NO | | | |
| PRIVILEGES | varchar(80) | NO | | | |
| COLUMN_COMMENT | varchar(1024) | NO | | | |
| IS_GENERATED | varchar(6) | NO | | | |
| GENERATION_EXPRESSION | longtext | YES | | NULL | |
+--------------------------+---------------------+------+-----+---------+-------+
|
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| MariaDB [information_schema]> select TABLE_NAME from information_schema.TABLES where TABLE_SCHEMA='test';
+------------+
| TABLE_NAME |
+------------+
| flag |
| student |
| people |
| php |
+------------+
4 rows in set (0.000 sec)
MariaDB [information_schema]> select COLUMN_NAME from information_schema.COLUMNS where TABLE_SCHEMA='test';
+-------------+
| COLUMN_NAME |
+-------------+
| flag |
| id |
| name |
| score |
| pass |
| name |
| age |
| user |
| pw |
+-------------+
|
