In MySQL, a **foreign key constraint is not enforced if any column in the foreign key is `NULL`**. That means **if at least one column of a composite key is `NULL`**, MySQL **ignores the foreign key check** and allows the insert.
---
### Example: Composite Primary Key with Foreign Key
```sql -- Parent table with a composite primary key CREATE TABLE tab1 ( id INT, name VARCHAR(20), cc VARCHAR(20), PRIMARY KEY (id, name) );
DESCRIBE tab1; ------------------------------------------------------ | Field | Type | Null | Key | Default | Extra | | ----- | ----------- | ---- | --- | ------- | ----- | | id | int | NO | PRI | NULL | | | name | varchar(20) | NO | PRI | NULL | | | cc | varchar(20) | YES | | NULL | | ------------------------------------------------------
-- Child table referencing the composite key of tab1 CREATE TABLE tab2 ( id INT, name VARCHAR(25), tid INT PRIMARY KEY, FOREIGN KEY (id, name) REFERENCES tab1(id, name) );
DESCRIBE tab2; ------------------------------------------------------ | Field | Type | Null | Key | Default | Extra | | ----- | ----------- | ---- | --- | ------- | ----- | | id | int | YES | MUL | NULL | | | name | varchar(25) | YES | | NULL | | | tid | int | NO | PRI | NULL | | ------------------------------------------------------
-- Inserting NULL for both foreign key columns INSERT INTO tab2 VALUES (NULL, NULL, 35); SELECT * FROM tab2; --------------------- | id | name | tid | | ---- | ---- | --- | | NULL | NULL | 35 | ---------------------
-- Inserting NULL for one column INSERT INTO tab2 VALUES (1, NULL, 36); INSERT INTO tab2 VALUES (10, NULL, 39); SELECT * FROM tab2; --------------------- | id | name | tid | | ---- | ---- | --- | | NULL | NULL | 35 | | 1 | NULL | 36 | | 10 | NULL | 39 | ---------------------
-- Inserting a fully non-null foreign key that doesn't exist in parent table INSERT INTO tab2 VALUES (10, 'HELLO', 40); -- ERROR 1452: Cannot add or update a child row: a foreign key constraint fails
-- Inserting with one NULL column bypasses the foreign key check INSERT INTO tab2 VALUES (NULL, 'HELLO', 40);
SELECT * FROM tab2; ---------------------- | id | name | tid | | ---- | ----- | --- | | NULL | NULL | 35 | | NULL | HELLO | 40 | | 1 | NULL | 36 | | 10 | NULL | 39 | ----------------------
``` NULL in Foreign Key: MySQL allows NULL values in foreign key columns. If any column in a composite foreign key is NULL, the foreign key constraint is ignored for that row.
Composite Keys: All columns must match a parent key to enforce the constraint. If one is NULL, the check is skipped.
Enforcement Happens Only When Fully Non-NULL: Only when all columns of a composite foreign key are non-NULL does MySQL check the existence of the referenced parent row.

In MySQL, a **foreign key constraint is not enforced if any column in the foreign key is `NULL`**. That means **if at least one column of a composite key is `NULL`**, MySQL **ignores the foreign key check** and allows the insert.
---
### Example: Composite Primary Key with Foreign Key
```sql -- Parent table with a composite primary key CREATE TABLE tab1 ( id INT, name VARCHAR(20), cc VARCHAR(20), PRIMARY KEY (id, name) );
DESCRIBE tab1; ------------------------------------------------------ | Field | Type | Null | Key | Default | Extra | | ----- | ----------- | ---- | --- | ------- | ----- | | id | int | NO | PRI | NULL | | | name | varchar(20) | NO | PRI | NULL | | | cc | varchar(20) | YES | | NULL | | ------------------------------------------------------
-- Child table referencing the composite key of tab1 CREATE TABLE tab2 ( id INT, name VARCHAR(25), tid INT PRIMARY KEY, FOREIGN KEY (id, name) REFERENCES tab1(id, name) );
DESCRIBE tab2; ------------------------------------------------------ | Field | Type | Null | Key | Default | Extra | | ----- | ----------- | ---- | --- | ------- | ----- | | id | int | YES | MUL | NULL | | | name | varchar(25) | YES | | NULL | | | tid | int | NO | PRI | NULL | | ------------------------------------------------------
-- Inserting NULL for both foreign key columns INSERT INTO tab2 VALUES (NULL, NULL, 35); SELECT * FROM tab2; --------------------- | id | name | tid | | ---- | ---- | --- | | NULL | NULL | 35 | ---------------------
-- Inserting NULL for one column INSERT INTO tab2 VALUES (1, NULL, 36); INSERT INTO tab2 VALUES (10, NULL, 39); SELECT * FROM tab2; --------------------- | id | name | tid | | ---- | ---- | --- | | NULL | NULL | 35 | | 1 | NULL | 36 | | 10 | NULL | 39 | ---------------------
-- Inserting a fully non-null foreign key that doesn't exist in parent table INSERT INTO tab2 VALUES (10, 'HELLO', 40); -- ERROR 1452: Cannot add or update a child row: a foreign key constraint fails
-- Inserting with one NULL column bypasses the foreign key check INSERT INTO tab2 VALUES (NULL, 'HELLO', 40);
SELECT * FROM tab2; ---------------------- | id | name | tid | | ---- | ----- | --- | | NULL | NULL | 35 | | NULL | HELLO | 40 | | 1 | NULL | 36 | | 10 | NULL | 39 | ----------------------
``` NULL in Foreign Key: MySQL allows NULL values in foreign key columns. If any column in a composite foreign key is NULL, the foreign key constraint is ignored for that row.
Composite Keys: All columns must match a parent key to enforce the constraint. If one is NULL, the check is skipped.
Enforcement Happens Only When Fully Non-NULL: Only when all columns of a composite foreign key are non-NULL does MySQL check the existence of the referenced parent row.