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25.6 The INFORMATION_SCHEMA COLUMNS Table

Inhoudsopgave

The COLUMNS table provides information about columns in tables. The related ST_GEOMETRY_COLUMNS table provides information about table columns that store spatial data. See Section 25.27, “The INFORMATION_SCHEMA ST_GEOMETRY_COLUMNS Table”.

The COLUMNS table has these columns:

  • 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 COLUMN_KEY is empty, the column either is not indexed or is indexed only as a secondary column in a multiple-column, nonunique index.

    • If COLUMN_KEY is PRI, the column is a PRIMARY KEY or is one of the columns in a multiple-column PRIMARY KEY.

    • If COLUMN_KEY is UNI, the column is the first column of a UNIQUE index. (A UNIQUE index permits multiple NULL values, but you can tell whether the column permits NULL by checking the Null column.)

    • If COLUMN_KEY is MUL, the column is the first column of a nonunique index in which multiple occurrences of a given value are permitted within the column.

    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:

    • auto_increment for columns that have the AUTO_INCREMENT attribute.

    • on update CURRENT_TIMESTAMP for TIMESTAMP or DATETIME columns that have the ON UPDATE CURRENT_TIMESTAMP attribute.

    • VIRTUAL GENERATED or VIRTUAL STORED for generated columns.

    • DEFAULT_GENERATED for columns that have an expression default value.

  • 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.

Notes

  • 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.

Column information is also available from the SHOW COLUMNS statement. See Section 13.7.6.5, “SHOW COLUMNS Syntax”. The following statements are nearly equivalent:

  1. SELECT COLUMN_NAME, DATA_TYPE, IS_NULLABLE, COLUMN_DEFAULT
  2.   FROM INFORMATION_SCHEMA.COLUMNS
  3.   WHERE table_name = 'tbl_name'
  4.   [AND table_schema = 'db_name']
  5.   [AND column_name LIKE 'wild']
  6.  
  7. SHOW COLUMNS
  8.   FROM tbl_name
  9.   [FROM db_name]
  10.   [LIKE 'wild']

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Document heeft de 26/06/2006 gemaakt, de laatste keer de 26/10/2018 gewijzigd
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