information_schema系统表——COLUMNS

该系统表包含了所有表的列信息

The 

COLUMNS

 table
provides information about columns in tables.

INFORMATION_SCHEMA  Name	SHOW  Name	Remarks
TABLE_CATALOG		def
TABLE_SCHEMA
TABLE_NAME
COLUMN_NAME	Field
ORDINAL_POSITION		see notes
COLUMN_DEFAULT	Default
IS_NULLABLE	Null
DATA_TYPE	Type
CHARACTER_MAXIMUM_LENGTH	Type
CHARACTER_OCTET_LENGTH
NUMERIC_PRECISION	Type
NUMERIC_SCALE	Type
DATETIME_PRECISION	Type
CHARACTER_SET_NAME
COLLATION_NAME	Collation
COLUMN_TYPE	Type	MySQL extension
COLUMN_KEY	Key	MySQL extension
EXTRA	Extra	MySQL extension
PRIVILEGES	Privileges	MySQL extension
COLUMN_COMMENT	Comment	MySQL extension
GENERATION_EXPRESSION		MySQL extension

Notes:

In 

SHOW

,
the 

Type

 display includes values from
several different 

COLUMNS

 columns.

ORDINAL_POSITION

 is necessary because
you might want to say 

ORDER BY ORDINAL_POSITION

.
Unlike 

SHOW

, 

SELECT

 does
not have automatic ordering.

CHARACTER_OCTET_LENGTH

 should be the
same as 

CHARACTER_MAXIMUM_LENGTH

, except
for multibyte character sets.

CHARACTER_SET_NAME

 can be derived from 

Collation

.
For example, if you say 

SHOW FULL COLUMNS FROM
t

, and you see in the 

Collation

 column
a value of

latin1_swedish_ci

, the character
set is what is before the first underscore: 

latin1

.

GENERATION_EXPRESSION

 is nonempty for
generated columns and displays the expression used to compute column values. For information about generated columns, see Section 13.1.18.8,
“CREATE TABLE and Generated Columns”.

The 

EXTRA

 column contains 

VIRTUAL
GENERATED

 or 

VIRTUAL STORED

 for
generated columns.