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MySQL Study之--MySQL schema_information数据库
information_schema数据库是在mysql的版本5.0之后产生的,一个虚拟数据库,物理上并不存在。
information_schema数据库类似与“数据字典”,提供了访问数据库元数据的方式,即数据的数据。比如数据库名或表名,列类型,访问权限(更加细化的访问方式)。
案例:
mysql> show databases;
+--------------------+ | Database | +--------------------+ | information_schema | | mysql | | performance_schema | | prod | | test | +--------------------+ 5 rows in set (0.00 sec)访问information_schema:
+---------------------------------------+ | 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 | | ROUTINES | | SCHEMATA | | SCHEMA_PRIVILEGES | | SESSION_STATUS | | SESSION_VARIABLES | | STATISTICS | | TABLES | | TABLESPACES | | TABLE_CONSTRAINTS | | TABLE_PRIVILEGES | | TRIGGERS | | USER_PRIVILEGES | | VIEWS | | INNODB_LOCKS | | INNODB_TRX | | INNODB_SYS_DATAFILES | | INNODB_LOCK_WAITS | | INNODB_SYS_TABLESTATS | | INNODB_CMP | | INNODB_METRICS | | INNODB_CMP_RESET | | INNODB_CMP_PER_INDEX | | INNODB_CMPMEM_RESET | | INNODB_FT_DELETED | | INNODB_BUFFER_PAGE_LRU | | INNODB_SYS_FOREIGN | | INNODB_SYS_COLUMNS | | INNODB_SYS_INDEXES | | INNODB_FT_DEFAULT_STOPWORD | | INNODB_SYS_FIELDS | | INNODB_CMP_PER_INDEX_RESET | | INNODB_BUFFER_PAGE | | INNODB_CMPMEM | | INNODB_FT_INDEX_TABLE | | INNODB_FT_BEING_DELETED | | INNODB_SYS_TABLESPACES | | INNODB_FT_INDEX_CACHE | | INNODB_SYS_FOREIGN_COLS | | INNODB_SYS_TABLES | | INNODB_BUFFER_POOL_STATS | | INNODB_FT_CONFIG | +---------------------------------------+ 59 rows in set (0.00 sec)mysql> desc SCHEMATA;
+----------------------------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +----------------------------+--------------+------+-----+---------+-------+ | CATALOG_NAME | varchar(512) | NO | | | | | SCHEMA_NAME | varchar(64) | NO | | | | | DEFAULT_CHARACTER_SET_NAME | varchar(32) | NO | | | | | DEFAULT_COLLATION_NAME | varchar(32) | NO | | | | | SQL_PATH | varchar(512) | YES | | NULL | | +----------------------------+--------------+------+-----+---------+-------+ 5 rows in set (0.00 sec)mysql> select SCHEMA_NAME,DEFAULT_CHARACTER_SET_NAME from SCHEMATA;
+--------------------+----------------------------+ | SCHEMA_NAME | DEFAULT_CHARACTER_SET_NAME | +--------------------+----------------------------+ | information_schema | utf8 | | mysql | latin1 | | performance_schema | utf8 | | prod | latin1 | | test | latin1 | +--------------------+----------------------------+ 5 rows in set (0.00 sec)mysql> desc 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(255) | YES | | NULL | | | TABLE_COMMENT | varchar(2048) | NO | | | | +-----------------+---------------------+------+-----+---------+-------+ 21 rows in set (0.00 sec)
mysql> select table_schema,table_name,table_type,engine,table_rows from tables where table_schema='prod';
+--------------+------------+------------+--------+------------+ | table_schema | table_name | table_type | engine | table_rows | +--------------+------------+------------+--------+------------+ | prod | t1 | BASE TABLE | InnoDB | 2 | | prod | t2 | BASE TABLE | InnoDB | 2 | +--------------+------------+------------+--------+------------+ 2 rows in set (0.00 sec)mysql> select * from prod.t1;
+------+-------+ | id | name | +------+-------+ | 10 | tom | | 20 | jerry | | 30 | rose | | 40 | ellen | +------+-------+ 4 rows in set (0.00 sec)mysql> explain select * from prod.t1;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-------+ | 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 2 | NULL | +----+-------------+-------+------+---------------+------+---------+------+------+-------+ 1 row in set (0.00 sec)
其中重要的表有:
SCHEMATA表:
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:
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,等等信息。是show tables from schemaname的结果取之此表。
COLUMNS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:
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:
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;
COLLATIONS表提供了关于各字符集的对照信息。
COLLATION_CHARACTER_SET_APPLICABILITY表:
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;
TABLE_CONSTRAINTS表描述了存在约束的表。以及表的约束类型。
KEY_COLUMN_USAGE表:
KEY_COLUMN_USAGE表描述了具有约束的键列。
ROUTINES表:
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:
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;
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。
另外,在未来的版本中会有新的表添加。
【注】information_schema是一个由数据库的元数据组成的数据库。里面存储的是mysql的数据库基本信息。并随时改变。用于查看信息以及系统决策时作为重要的信息提供者。