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Information Schema KEY_COLUMN_USAGE Table

The Information Schema KEY_COLUMN_USAGE table shows which key columns have constraints.

It contains the following columns:

Column Description
CONSTRAINT_CATALOG Always def.
CONSTRAINT_SCHEMA Database name of the constraint.
CONSTRAINT_NAME Name of the constraint (PRIMARY for the primary key).
TABLE_CATALOG Always #def.
TABLE_SCHEMA Database name of the column constraint.
TABLE_NAME Table name of the column constraint.
COLUMN_NAME Column name of the constraint.
ORDINAL_POSITION Position of the column within the constraint.
POSITION_IN_UNIQUE_CONSTRAINT For foreign keys, the position in the unique constraint.
REFERENCED_TABLE_SCHEMA For foreign keys, the referenced database name.
REFERENCED_TABLE_NAME For foreign keys, the referenced table name.
REFERENCED_COLUMN_NAME For foreign keys, the referenced column name.

Example

SELECT * FROM information_schema.KEY_COLUMN_USAGE LIMIT 1 \G
********************** 1. row **********************
           CONSTRAINT_CATALOG: def
            CONSTRAINT_SCHEMA: my_website
              CONSTRAINT_NAME: PRIMARY
                TABLE_CATALOG: def
                 TABLE_SCHEMA: users
                  COLUMN_NAME: user_id
             ORDINAL_POSITION: 1
POSITION_IN_UNIQUE_CONSTRAINT: NULL
      REFERENCED_TABLE_SCHEMA: NULL
        REFERENCED_TABLE_NAME: NULL
       REFERENCED_COLUMN_NAME: NULL

See Also

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https://mariadb.com/kb/en/information-schema-key_column_usage-table/