Foreign Key Constraint

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The Foreign Key constraint specifies that all of a column's values must exactly match existing values from the column it references, enforcing referential integrity.

For example, if you create a foreign key on orders.customer that references customers.id:

• Each value inserted or updated in orders.customer must exactly match a value in customers.id.
• Values in customers.id that are referenced by orders.customer cannot be deleted or updated. However, customers.id values that aren't present in orders.customer can be.

Details

Rules for Creating Foreign Keys

Foreign Key Columns

• Foreign key columns must use their referenced column's type.
• Each column cannot belong to more than 1 Foreign Key constraint.
• Cannot be a computed column.
• Foreign key columns must be indexed. This is required because updates and deletes on the referenced table will need to search the referencing table for any matching records to ensure those operations would not violate existing references. In practice, such indexes are likely also needed by applications using these tables, since finding all records which belong to some entity, for example all orders for a given customer, is very common.
  • To meet this requirement when creating a new table, there are a few options:
    • Create indexes explicitly using the INDEX clause of CREATE TABLE.
    • Rely on indexes created by the Primary Key or Unique constraints.
    • Have CockroachDB automatically create an index of the foreign key columns for you. However, it's important to note that if you later remove the Foreign Key constraint, this automatically created index is not removed.
    • Using the foreign key columns as the prefix of an index's columns also satisfies the requirement for an index. For example, if you create foreign key columns (A, B), an index of columns (A, B, C) satisfies the requirement for an index.
  • To meet this requirement when adding the Foreign Key constraint to an existing table, if the columns you want to constrain are not already indexed, use CREATE INDEX to index them and only then use the ADD CONSTRAINT statement to add the Foreign Key constraint to the columns.

Referenced Columns

• Referenced columns must contain only unique sets of values. This means the REFERENCES clause must use exactly the same columns as a Unique or Primary Key constraint on the referenced table. For example, the clause REFERENCES tbl (C, D) requires tbl to have either the constraint UNIQUE (C, D) or PRIMARY KEY (C, D).
• In the REFERENCES clause, if you specify a table but no columns, CockroachDB references the table's primary key. In these cases, the Foreign Key constraint and the referenced table's primary key must contain the same number of columns.

NULL Values

Single-column foreign keys accept NULL values.

Multiple-column foreign keys only accept NULL values in these scenarios:

• The row you're ultimately referencing—determined by the statement's other values—contains NULL as the value of the referenced column (i.e., NULL is valid from the perspective of referential integrity)
• The write contains NULL values for all foreign key columns

For example, if you have a Foreign Key constraint on columns (A, B) and try to insert (1, NULL), the write would fail unless the row with the value 1 for (A) contained a NULL value for (B). However, inserting (NULL, NULL) would succeed.

However, allowing NULL values in either your foreign key or referenced columns can degrade their referential integrity. To avoid this, you can use the Not Null constraint on both sets of columns when creating your tables. (The Not Null constraint cannot be added to existing tables.)

Foreign Key Actions New in v2.0

When you set a foreign key constraint, you can control what happens to the constrained column when the column it's referencing (the foreign key) is deleted or updated.

Performance

Because the Foreign Key constraint requires per-row checks on two tables, statements involving foreign key or referenced columns can take longer to execute. You're most likely to notice this with operations like bulk inserts into the table with the foreign keys.

We're currently working to improve the performance of these statements, though.

Syntax

Foreign Key constraints can be defined at the table level. However, if you only want the constraint to apply to a single column, it can be applied at the column level.

Column Level

Example

> CREATE TABLE IF NOT EXISTS orders (
    id INT PRIMARY KEY,
    customer INT NOT NULL REFERENCES customers (id) ON DELETE CASCADE,
    orderTotal DECIMAL(9,2),
    INDEX (customer)
  );

Table Level

Example

CREATE TABLE packages (
    customer INT,
    "order" INT,
    id INT,
    address STRING(50),
    delivered BOOL,
    delivery_date DATE,
    PRIMARY KEY (customer, "order", id),
    CONSTRAINT fk_order FOREIGN KEY (customer, "order") REFERENCES orders
    ) INTERLEAVE IN PARENT orders (customer, "order")
  ;

Usage Examples

Use a Foreign Key Constraint with Default Actions

In this example, we'll create a table with a foreign key constraint with the default actions (ON UPDATE NO ACTION ON DELETE NO ACTION).

First, create the referenced table:

> CREATE TABLE customers (id INT PRIMARY KEY, email STRING UNIQUE);

Next, create the referencing table:

> CREATE TABLE IF NOT EXISTS orders (
    id INT PRIMARY KEY,
    customer INT NOT NULL REFERENCES customers (id),
    orderTotal DECIMAL(9,2),
    INDEX (customer)
  );

Let's insert a record into each table:

> INSERT INTO customers VALUES (1001, 'a@co.tld'), (1234, 'info@cockroachlabs.com');

> INSERT INTO orders VALUES (1, 1002, 29.99);

pq: foreign key violation: value [1002] not found in customers@primary [id]

The second record insertion returns an error because the customer 1002 doesn't exist in the referenced table.

Let's insert a record into the referencing table and try to update the referenced table:

> INSERT INTO orders VALUES (1, 1001, 29.99);

> UPDATE customers SET id = 1002 WHERE id = 1001;

pq: foreign key violation: value(s) [1001] in columns [id] referenced in table "orders"

The update to the referenced table returns an error because id = 1001 is referenced and the default foreign key action is enabled (ON UPDATE NO ACTION). However, id = 1234 is not referenced and can be updated:

> UPDATE customers SET id = 1111 WHERE id = 1234;

> SELECT * FROM customers;

+------+------------------------+
|  id  |         email          |
+------+------------------------+
| 1001 | a@co.tld               |
| 1111 | info@cockroachlabs.com |
+------+------------------------+

Now let's try to delete a referenced row:

> DELETE FROM customers WHERE id = 1001;

pq: foreign key violation: value(s) [1001] in columns [id] referenced in table "orders"

Similarly, the deletion returns an error because id = 1001 is referenced and the default foreign key action is enabled (ON DELETE NO ACTION). However, id = 1111 is not referenced and can be deleted:

> DELETE FROM customers WHERE id = 1111;

> SELECT * FROM customers;

+------+----------+
|  id  |  email   |
+------+----------+
| 1001 | a@co.tld |
+------+----------+

Use a Foreign Key Constraint with CASCADE New in v2.0

In this example, we'll create a table with a foreign key constraint with the foreign key actions ON UPDATE CASCADE and ON DELETE CASCADE.

First, create the referenced table:

> CREATE TABLE customers_2 (
    id INT PRIMARY KEY
  );

Then, create the referencing table:

> CREATE TABLE orders_2 (
    id INT PRIMARY KEY,
    customer_id INT REFERENCES customers_2(id) ON UPDATE CASCADE ON DELETE CASCADE
  );

Insert a few records into the referenced table:

> INSERT INTO customers_2 VALUES (1), (2), (3);

Insert some records into the referencing table:

> INSERT INTO orders_2 VALUES (100,1), (101,2), (102,3), (103,1);

Now, let's update an id in the referenced table:

> UPDATE customers_2 SET id = 23 WHERE id = 1;

> SELECT * FROM customers_2;

+----+
| id |
+----+
|  2 |
|  3 |
| 23 |
+----+

> SELECT * FROM orders_2;

+-----+--------------+
| id  | customers_id |
+-----+--------------+
| 100 |           23 |
| 101 |            2 |
| 102 |            3 |
| 103 |           23 |
+-----+--------------+

When id = 1 was updated to id = 23 in customers_2, the update propagated to the referencing table orders_2.

Similarly, a deletion will cascade. Let's delete id = 23 from customers_2:

> DELETE FROM customers_2 WHERE id = 23;

> SELECT * FROM customers_2;

+----+
| id |
+----+
|  2 |
|  3 |
+----+

Let's check to make sure the rows in orders_2 where customers_id = 23 were also deleted:

> SELECT * FROM orders_2;

+-----+--------------+
| id  | customers_id |
+-----+--------------+
| 101 |            2 |
| 102 |            3 |
+-----+--------------+

Use a Foreign Key Constraint with SET NULL New in v2.0

In this example, we'll create a table with a foreign key constraint with the foreign key actions ON UPDATE SET NULL and ON DELETE SET NULL.

First, create the referenced table:

> CREATE TABLE customers_3 (
    id INT PRIMARY KEY
  );

Then, create the referencing table:

> CREATE TABLE orders_3 (
    id INT PRIMARY KEY,
    customer_id INT REFERENCES customers_3(id) ON UPDATE SET NULL ON DELETE SET NULL
  );

Insert a few records into the referenced table:

> INSERT INTO customers_3 VALUES (1), (2), (3);

Insert some records into the referencing table:

> INSERT INTO orders_3 VALUES (100,1), (101,2), (102,3), (103,1);

> SELECT * FROM customers_3;

+-----+-------------+
| id  | customer_id |
+-----+-------------+
| 100 |           1 |
| 101 |           2 |
| 102 |           3 |
| 103 |           1 |
+-----+-------------+

Now, let's update an id in the referenced table:

> UPDATE customers_3 SET id = 23 WHERE id = 1;

> SELECT * FROM customers_3;

+----+
| id |
+----+
|  2 |
|  3 |
| 23 |
+----+

> SELECT * FROM orders_3;

+-----+-------------+
| id  | customer_id |
+-----+-------------+
| 100 |        NULL |
| 101 |           2 |
| 102 |           3 |
| 103 |        NULL |
+-----+-------------+

When id = 1 was updated to id = 23 in customers_3, the referencing customer_id was set to NULL.

Similarly, a deletion will set the referencing customer_id to NULL. Let's delete id = 2 from customers_3:

> DELETE FROM customers_3 WHERE id = 2;

> SELECT * FROM customers_3;

+----+
| id |
+----+
|  3 |
| 23 |
+----+

Let's check to make sure the row in orders_3 where customers_id = 2 was updated to NULL:

> SELECT * FROM orders_3;

+-----+-------------+
| id  | customer_id |
+-----+-------------+
| 100 |        NULL |
| 101 |        NULL |
| 102 |           3 |
| 103 |        NULL |
+-----+-------------+

Use a Foreign Key Constraint with SET DEFAULT New in v2.0

In this example, we'll create a table with a foreign key constraint with the foreign key actions ON UPDATE SET DEFAULT and ON DELETE SET DEFAULT.

First, create the referenced table:

> CREATE TABLE customers_4 (
    id INT PRIMARY KEY
  );

Then, create the referencing table with the DEFAULT value for customer_id set to 9999:

> CREATE TABLE orders_4 (
    id INT PRIMARY KEY,
    customer_id INT DEFAULT 9999 REFERENCES customers_4(id) ON UPDATE SET DEFAULT ON DELETE SET DEFAULT
  );

Insert a few records into the referenced table:

> INSERT INTO customers_4 VALUES (1), (2), (3), (9999);

Insert some records into the referencing table:

> INSERT INTO orders_4 VALUES (100,1), (101,2), (102,3), (103,1);

+-----+-------------+
| id  | customer_id |
+-----+-------------+
| 100 |           1 |
| 101 |           2 |
| 102 |           3 |
| 103 |           1 |
+-----+-------------+

Now, let's update an id in the referenced table:

> UPDATE customers_4 SET id = 23 WHERE id = 1;

> SELECT * FROM customers_4;

+------+
|  id  |
+------+
|    2 |
|    3 |
|   23 |
| 9999 |
+------+

> SELECT * FROM orders_4;

+-----+-------------+
| id  | customer_id |
+-----+-------------+
| 100 |        9999 |
| 101 |           2 |
| 102 |           3 |
| 103 |        9999 |
+-----+-------------+

When id = 1 was updated to id = 23 in customers_4, the referencing customer_id was set to DEFAULT (i.e., 9999). You can see this in the first and last rows of orders_4, where id = 100 and the customer_id is now 9999

Similarly, a deletion will set the referencing customer_id to the DEFAULT value. Let's delete id = 2 from customers_4:

> DELETE FROM customers_4 WHERE id = 2;

> SELECT * FROM customers_4;

+------+
|   id |
+------+
|    3 |
|   23 |
| 9999 |
+------+

Let's check to make sure the corresponding customer_id value to id = 101, was updated to the DEFAULT value (i.e., 9999) in orders_4:

> SELECT * FROM orders_4;

+-----+-------------+
| id  | customer_id |
+-----+-------------+
| 100 |        9999 |
| 101 |        9999 |
| 102 |           3 |
| 103 |        9999 |
+-----+-------------+

See Also

• Constraints
• DROP CONSTRAINT
• ADD CONSTRAINT
• Check constraint
• Default Value constraint
• Not Null constraint
• Primary Key constraint
• Unique constraint
• SHOW CONSTRAINTS

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