MySQL :: MySQL 8.3 Reference Manual :: 28.3.16 The INFORMATION_SCHEMA KEY_COLUMN

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28.3.16 The INFORMATION_SCHEMA KEY_COLUMN_USAGE Table

The KEY_COLUMN_USAGE table describes which key columns have constraints. This table provides no information about functional key parts because they are expressions and the table provides information only about columns.

The KEY_COLUMN_USAGE table has these columns:

CONSTRAINT_CATALOG

The name of the catalog to which the constraint belongs. This value is always def.
CONSTRAINT_SCHEMA

The name of the schema (database) to which the constraint belongs.
CONSTRAINT_NAME

The name of the constraint.
TABLE_CATALOG

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

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

The name of the table that has the constraint.
COLUMN_NAME

The name of the column that has the constraint.

If the constraint is a foreign key, then this is the column of the foreign key, not the column that the foreign key references.
ORDINAL_POSITION

The column's position within the constraint, not the column's position within the table. Column positions are numbered beginning with 1.
POSITION_IN_UNIQUE_CONSTRAINT

NULL for unique and primary-key constraints. For foreign-key constraints, this column is the ordinal position in key of the table that is being referenced.
REFERENCED_TABLE_SCHEMA

The name of the schema referenced by the constraint.
REFERENCED_TABLE_NAME

The name of the table referenced by the constraint.
REFERENCED_COLUMN_NAME

The name of the column referenced by the constraint.

Suppose that there are two tables name t1 and t3 that have the following definitions:

CREATE TABLE t1
(
    s1 INT,
    s2 INT,
    s3 INT,
    PRIMARY KEY(s3)
) ENGINE=InnoDB;

CREATE TABLE t3
(
    s1 INT,
    s2 INT,
    s3 INT,
    KEY(s1),
    CONSTRAINT CO FOREIGN KEY (s2) REFERENCES t1(s3)
) ENGINE=InnoDB;

For those two tables, the KEY_COLUMN_USAGE table has two rows:

One row with CONSTRAINT_NAME = 'PRIMARY', TABLE_NAME = 't1', COLUMN_NAME = 's3', ORDINAL_POSITION = 1, POSITION_IN_UNIQUE_CONSTRAINT = NULL.

For NDB: This value is always NULL.
One row with CONSTRAINT_NAME = 'CO', TABLE_NAME = 't3', COLUMN_NAME = 's2', ORDINAL_POSITION = 1, POSITION_IN_UNIQUE_CONSTRAINT = 1.

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