MySQL Error 6125: Missing Unique Parent Key

MySQL Error 6125 (HY000, ER_FK_NO_UNIQUE_INDEX_PARENT) means a foreign key references parent columns that do not form a complete unique key. MySQL 8.4 and 9.7 reject non-unique or partial parent keys by default. See the MySQL 8.4 error reference and foreign-key requirements.

Check the referenced parent key

Inspect the live table definition and its indexes:

SHOW CREATE TABLE parent\G
SHOW INDEX FROM parent;

The referenced columns must match all columns of a PRIMARY KEY or UNIQUE key in the same order. A leftmost prefix of a longer composite unique key is only a partial key. For example, if the parent has UNIQUE (a, b), a foreign key that references only a is not a complete unique-key reference:

CREATE TABLE parent (
  a INT NOT NULL,
  b INT NOT NULL,
  UNIQUE KEY uq_parent_a_b (a, b)
) ENGINE = InnoDB;

CREATE TABLE child (
  id INT PRIMARY KEY,
  parent_a INT NOT NULL,
  CONSTRAINT fk_child_parent
    FOREIGN KEY (parent_a) REFERENCES parent (a)
) ENGINE = InnoDB;

Before changing an index, check whether the proposed single column is actually unique:

SELECT a, COUNT(*) AS row_count
FROM parent
WHERE a IS NOT NULL
GROUP BY a
HAVING COUNT(*) > 1;

The local MySQL foreign-key checker can compare common type, engine, and parent-key conditions without sending schema values to a server. It does not inspect your live indexes, so verify the actual parent definition with SHOW CREATE TABLE or SHOW INDEX.

Choose a key that matches the data model

If a is meant to identify one parent row, check for duplicate non-NULL values, then create a unique index. A NULL child key does not require a matching parent row, and MySQL unique indexes permit multiple NULL values:

CREATE UNIQUE INDEX uq_parent_a ON parent (a);

If the parent row is identified by the pair (a, b), include both columns in the child foreign key and reference them in the same order:

CREATE TABLE child (
  id INT PRIMARY KEY,
  parent_a INT NOT NULL,
  parent_b INT NOT NULL,
  CONSTRAINT fk_child_parent
    FOREIGN KEY (parent_a, parent_b)
    REFERENCES parent (a, b)
) ENGINE = InnoDB;

For an existing child table, populate and verify the additional key value for every row before adding the composite constraint. Do not add a unique index unless the data model requires those values to be unique; an index should enforce the intended relationship, not just suppress an error.

Do not rely on non-standard parent keys

Check restrict_fk_on_non_standard_key only when diagnosing a legacy schema:

SHOW VARIABLES LIKE 'restrict_fk_on_non_standard_key';

In MySQL 8.4 and 9.7, the default is ON. Setting it to OFF permits deprecated foreign keys that reference non-unique or partial parent keys, but MySQL warns that support may be removed. Prefer changing the schema to use a complete unique parent key. See the system-variable reference.

Distinguish Error 6125 from related errors

• Error 6125: a non-unique or partial parent key is not allowed under the current settings.
• Error 1822: the referenced columns do not have a suitable parent index. See Error 1822 troubleshooting.
• Error 3780: corresponding child and parent columns have incompatible definitions. See Error 3780 troubleshooting.
• Error 1215: a generic foreign-key definition error used by older versions or other failure cases. See Error 1215 troubleshooting.

For other constraint rules, see the MySQL foreign-key guide.