The KEY_COLUMN_USAGE table describes which key columns have constraints.
mysql>mysql>mysql> desc INFORMATION_SCHEMA.KEY_COLUMN_USAGE;+-------------------------------+--------------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------------------------------+--------------+------+-----+---------+-------+| CONSTRAINT_CATALOG | varchar(512) | YES | | NULL | || CONSTRAINT_SCHEMA | varchar(64) | NO | | | || CONSTRAINT_NAME | varchar(64) | NO | | | || TABLE_CATALOG | varchar(512) | YES | | NULL | || TABLE_SCHEMA | varchar(64) | NO | | | || TABLE_NAME | varchar(64) | NO | | | || COLUMN_NAME | varchar(64) | NO | | | || ORDINAL_POSITION | bigint(10) | NO | | 0 | || POSITION_IN_UNIQUE_CONSTRAINT | bigint(10) | YES | | NULL | || REFERENCED_TABLE_SCHEMA | varchar(64) | YES | | NULL | || REFERENCED_TABLE_NAME | varchar(64) | YES | | NULL | || REFERENCED_COLUMN_NAME | varchar(64) | YES | | NULL | |+-------------------------------+--------------+------+-----+---------+-------+12 rows in set (0.00 sec)
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