MySQL 8.4 Reference Manual Including MySQL NDB Cluster 8.4

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:

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:

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