25.6 The INFORMATION_SCHEMA COLUMNS Table

TABLE_CATALOG

The name of the catalog to which the table containing the column belongs. This value is always def.

TABLE_SCHEMA

The name of the schema (database) to which the table containing the column belongs.

TABLE_NAME

The name of the table containing the column.

COLUMN_NAME

The name of the column.

ORDINAL_POSITION

The position of the column within the table. ORDINAL_POSITION is necessary because you might want to say ORDER BY ORDINAL_POSITION. Unlike SHOW COLUMNS, SELECT from the COLUMNS table does not have automatic ordering.

COLUMN_DEFAULT

The default value for the column. This is NULL if the column has an explicit default of NULL, or if the column definition includes no DEFAULT clause.

IS_NULLABLE

The column nullability. The value is YES if NULL values can be stored in the column, NO if not.

DATA_TYPE

The column data type.

The DATA_TYPE value is the type name only with no other information. The COLUMN_TYPE value contains the type name and possibly other information such as the precision or length.

CHARACTER_MAXIMUM_LENGTH

For string columns, the maximum length in characters.

CHARACTER_OCTET_LENGTH

For string columns, the maximum length in bytes.

NUMERIC_PRECISION

For numeric columns, the numeric precision.

NUMERIC_SCALE

For numeric columns, the numeric scale.

DATETIME_PRECISION

For temporal columns, the fractional seconds precision.

CHARACTER_SET_NAME

For character string columns, the character set name.

COLLATION_NAME

For character string columns, the collation name.

COLUMN_TYPE

The column data type.

The DATA_TYPE value is the type name only with no other information. The COLUMN_TYPE value contains the type name and possibly other information such as the precision or length.

COLUMN_KEY

Whether the column is indexed:

If more than one of the COLUMN_KEY values applies to a given column of a table, COLUMN_KEY displays the one with the highest priority, in the order PRI, UNI, MUL.

A UNIQUE index may be displayed as PRI if it cannot contain NULL values and there is no PRIMARY KEY in the table. A UNIQUE index may display as MUL if several columns form a composite UNIQUE index; although the combination of the columns is unique, each column can still hold multiple occurrences of a given value.

EXTRA

Any additional information that is available about a given column. The value is nonempty in these cases:

PRIVILEGES

The privileges you have for the column.

COLUMN_COMMENT

Any comment included in the column definition.

GENERATION_EXPRESSION

For generated columns, displays the expression used to compute column values. Empty for nongenerated columns. For information about generated columns, see Section 13.1.20.9, “CREATE TABLE and Generated Columns”.

SRS_ID

This value applies to spatial columns. It contains the column SRID value that indicates the spatial reference system for values stored in the column. See Section 11.5.1, “Spatial Data Types”, and Section 11.5.5, “Spatial Reference System Support”. The value is NULL for nonspatial columns and spatial columns with no SRID attribute.

In SHOW COLUMNS, the Type display includes values from several different COLUMNS columns.

CHARACTER_OCTET_LENGTH should be the same as CHARACTER_MAXIMUM_LENGTH, except for multibyte character sets.

CHARACTER_SET_NAME can be derived from COLLATION_NAME. For example, if you say SHOW FULL COLUMNS FROM t, and you see in the COLLATION_NAME column a value of utf8_swedish_ci, the character set is what is before the first underscore: utf8.