Mysql Let Insert Continue Even With Unique Constraint Violation
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A foreign key is a column (or combination of columns) in a table whose values must match values of a column in some other table. FOREIGN KEY constraints enforce referential integrity, which essentially says that if column value A refers to column value B, then column value B must exist.
For example, given an orders table and a customers table, if you create a column orders.customer_id that references the customers.id primary key:
- Each value inserted or updated in
orders.customer_idmust exactly match a value incustomers.id, or beNULL. - Values in
customers.idthat are referenced byorders.customer_idcannot be deleted or updated, unless you have cascading actions. However, values ofcustomers.idthat are not present inorders.customer_idcan be deleted or updated.
To learn more about the basics of foreign keys, watch the following video:
Details
Rules for creating foreign keys
Foreign Key Columns
- Foreign key columns must use their referenced column's type.
- A foreign key column cannot be a virtual computed column, but it can be a stored computed column.
- A single column can have multiple foreign key constraints. For an example, see Add multiple foreign key constraints to a single column.
- A foreign key column can reference the
crdb_regioncolumn inREGIONAL BY ROWtables even if thecrdb_regioncolumn is not explicitly part of aUNIQUEconstraint. This is possible becausecrdb_regionis implicitly included in every index onREGIONAL BY ROWtables as the partitioning key. This applies to whichever column is used as the partitioning column, in case a different name is used viaREGIONAL BY ROW AS.
Referenced Columns
- Referenced columns must contain only unique sets of values. This means the
REFERENCESclause must use exactly the same columns as aUNIQUEorPRIMARY KEYconstraint on the referenced table. For example, the clauseREFERENCES tbl (C, D)requirestblto have either the constraintUNIQUE (C, D)orPRIMARY KEY (C, D). The order of the columns in the foreign key definition does not need to match the order of the columns in the correspondingUNIQUEorPRIMARY KEYconstraint. - In the
REFERENCESclause, if you specify a table but no columns, CockroachDB references the table's primary key. In these cases, theFOREIGN KEYconstraint and the referenced table's primary key must contain the same number of columns. - By default, referenced columns must be in the same database as the referencing foreign key column. To enable cross-database foreign key references, set the
sql.cross_db_fks.enabledcluster setting totrue.
Null values
Single-column foreign keys accept null values.
Multiple-column (composite) foreign keys only accept null values in the following scenarios:
- The write contains null values for all foreign key columns (if
MATCH FULLis specified). - The write contains null values for at least one foreign key column (if
MATCH SIMPLEis specified).
For more information about composite foreign keys, see the composite foreign key matching section.
Note that allowing null values in either your foreign key or referenced columns can degrade their referential integrity, since any key with a null value is never checked against the referenced table. To avoid this, you can use a NOT NULL constraint on foreign keys when creating your tables.
Note:
A NOT NULL constraint cannot be added to existing tables.
Composite foreign key matching
By default, composite foreign keys are matched using the MATCH SIMPLE algorithm (which is the same default as PostgreSQL). MATCH FULL is available if specified. You can specify both MATCH FULL and MATCH SIMPLE.
All composite key matches defined prior to version 19.1 use the MATCH SIMPLE comparison method. If you had a composite foreign key constraint and have just upgraded to version 19.1, then please check that MATCH SIMPLE works for your schema and consider replacing that foreign key constraint with a MATCH FULL one.
How it works
For matching purposes, composite foreign keys can be in one of three states:
-
Valid: Keys that can be used for matching foreign key relationships.
-
Invalid: Keys that will not be used for matching (including for any cascading operations).
-
Unacceptable: Keys that cannot be inserted at all (an error is signalled).
MATCH SIMPLE stipulates that:
-
Valid keys may not contain any null values.
-
Invalid keys contain one or more null values.
-
Unacceptable keys do not exist from the point of view of
MATCH SIMPLE; all composite keys are acceptable.
MATCH FULL stipulates that:
-
Valid keys may not contain any null values.
-
Invalid keys must have all null values.
-
Unacceptable keys have any combination of both null and non-null values. In other words,
MATCH FULLrequires that if any column of a composite key isNULL, then all columns of the key must beNULL.
For examples showing how these key matching algorithms work, see Match composite foreign keys with MATCH SIMPLE and MATCH FULL.
Note:
CockroachDB does not support MATCH PARTIAL. For more information, see issue #20305.
Foreign key actions
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.
| Parameter | Description |
|---|---|
ON DELETE NO ACTION | Default action. If there are any existing references to the key being deleted, the transaction will fail at the end of the statement. The key can be updated, depending on the ON UPDATE action. Alias: |
ON UPDATE NO ACTION | Default action. If there are any existing references to the key being updated, the transaction will fail at the end of the statement. The key can be deleted, depending on the ON DELETE action. Alias: |
ON DELETE RESTRICT / ON UPDATE RESTRICT | RESTRICT and NO ACTION are currently equivalent until options for deferring constraint checking are added. To set an existing foreign key action to RESTRICT, the foreign key constraint must be dropped and recreated. |
ON DELETE CASCADE / ON UPDATE CASCADE | When a referenced foreign key is deleted or updated, all rows referencing that key are deleted or updated, respectively. If there are other alterations to the row, such as a SET NULL or SET DEFAULT, the delete will take precedence. Note that |
ON DELETE SET NULL / ON UPDATE SET NULL | When a referenced foreign key is deleted or updated, respectively, the columns of all rows referencing that key will be set to NULL. The column must allow NULL or this update will fail. |
ON DELETE SET DEFAULT / ON UPDATE SET DEFAULT | When a referenced foreign key is deleted or updated, the columns of all rows referencing that key are set to the default value for that column. If the default value for the column is null, or if no default value is provided and the column does not have a |
Note:
If a foreign key column has multiple constraints that reference the same column, the foreign key action that is specified by the first foreign key takes precedence. For an example, see Add multiple foreign key constraints to a single column.
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.
To improve query performance, we recommend doing the following:
-
Create a secondary index on all referencing foreign key columns that are not already indexed.
-
For bulk inserts into new tables with foreign key or referenced columns, use the
IMPORTstatement instead ofINSERT.
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.
Note:
You can also add the FOREIGN KEY constraint to existing tables through ADD CONSTRAINT.
Column level
| Parameter | Description |
|---|---|
table_name | The name of the table you're creating. |
column_name | The name of the foreign key column. |
column_type | The foreign key column's data type. |
parent_table | The name of the table the foreign key references. |
ref_column_name | The name of the column the foreign key references. If you do not include the |
column_constraints | Any other column-level constraints you want to apply to this column. |
column_def | Definitions for any other columns in the table. |
table_constraints | Any table-level constraints you want to apply. |
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 ) ); Warning:
CASCADE does not list objects it drops or updates, so it should be used cautiously.
Table level
| Parameter | Description |
|---|---|
table_name | The name of the table you're creating. |
column_def | Definitions for the table's columns. |
name | The name of the constraint. |
fk_column_name | The name of the foreign key column. |
parent_table | The name of the table the foreign key references. |
ref_column_name | The name of the column the foreign key references. If you do not include the |
table_constraints | Any other table-level constraints you want to apply. |
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 ); 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 (2 rows) 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 (1 row) Use a Foreign Key Constraint with CASCADE
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 (3 rows)
> SELECT * FROM orders_2 ; id | customer_id +-----+-------------+ 100 | 23 101 | 2 102 | 3 103 | 23 (4 rows) 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 ; Let's check to make sure the rows in orders_2 where customers_id = 23 were also deleted:
> SELECT * FROM orders_2 ; id | customer_id +-----+-------------+ 101 | 2 102 | 3 (2 rows) Use a Foreign Key Constraint with SET NULL
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 orders_3 ; id | customer_id +-----+-------------+ 100 | 1 101 | 2 102 | 3 103 | 1 (4 rows) 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 (3 rows)
> SELECT * FROM orders_3 ; id | customer_id +-----+-------------+ 100 | NULL 101 | 2 102 | 3 103 | NULL (4 rows) 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 ; 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 (4 rows) Use a Foreign Key Constraint with SET DEFAULT
In this example, we'll create a table with a FOREIGN 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 );
> SELECT * FROM orders_4 ; id | customer_id +-----+-------------+ 100 | 1 101 | 2 102 | 3 103 | 1 (4 rows) 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 (4 rows)
> SELECT * FROM orders_4 ; id | customer_id +-----+-------------+ 100 | 9999 101 | 2 102 | 3 103 | 9999 (4 rows) 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 (3 rows) 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 (4 rows) If the default value for the customer_id column is not set, and the column does not have a NOT NULL constraint, ON UPDATE SET DEFAULT and ON DELETE SET DEFAULT actions set referenced column values to NULL.
For example, let's create a new customers_5 table and insert some values:
> CREATE TABLE customers_5 ( id INT PRIMARY KEY );
> INSERT INTO customers_5 VALUES ( 1 ), ( 2 ), ( 3 ), ( 4 ); Then we can create a new orders_5 table that references the customers_5 table, but with no default value specified for the ON UPDATE SET DEFAULT and ON DELETE SET DEFAULT actions:
> CREATE TABLE orders_5 ( id INT PRIMARY KEY , customer_id INT REFERENCES customers_5 ( id ) ON UPDATE SET DEFAULT ON DELETE SET DEFAULT );
> INSERT INTO orders_5 VALUES ( 200 , 1 ), ( 201 , 2 ), ( 202 , 3 ), ( 203 , 4 ); Deleting and updating values in the customers_5 table sets the referenced values in orders_5 to NULL:
> DELETE FROM customers_5 WHERE id = 3 ;
> UPDATE customers_5 SET id = 0 WHERE id = 1 ;
> SELECT * FROM orders_5 ; id | customer_id +-----+-------------+ 200 | NULL 201 | 2 202 | NULL 203 | 4 (4 rows) Add multiple foreign key constraints to a single column
You can add more than one foreign key constraint to a single column.
For example, if you create the following tables:
> CREATE TABLE customers ( id INT PRIMARY KEY , name STRING , email STRING );
> CREATE TABLE orders ( id INT PRIMARY KEY , customer_id INT UNIQUE , item_number INT ); You can create a table with a column that references columns in both the customers and orders tables:
> CREATE TABLE shipments ( tracking_number UUID DEFAULT gen_random_uuid () PRIMARY KEY , carrier STRING , status STRING , customer_id INT , CONSTRAINT fk_customers FOREIGN KEY ( customer_id ) REFERENCES customers ( id ), CONSTRAINT fk_orders FOREIGN KEY ( customer_id ) REFERENCES orders ( customer_id ) ); Inserts into the shipments table must fulfill both foreign key constraints on customer_id (fk_customers and fk_customers_2).
Let's insert a record into each table:
> INSERT INTO customers VALUES ( 1001 , 'Alexa' , 'a@co.tld' ), ( 1234 , 'Evan' , 'info@cockroachlabs.com' );
> INSERT INTO orders VALUES ( 1 , 1001 , 25 ), ( 2 , 1234 , 15 ), ( 3 , 2000 , 5 );
> INSERT INTO shipments ( carrier , status , customer_id ) VALUES ( 'USPS' , 'Out for delivery' , 1001 ); The last statement succeeds because 1001 matches a unique id value in the customers table and a unique customer_id value in the orders table. If 1001 was in neither of the referenced columns, or in just one of them, the statement would return an error.
For instance, the following statement fulfills just one of the foreign key constraints and returns an error:
> INSERT INTO shipments ( carrier , status , customer_id ) VALUES ( 'DHL' , 'At facility' , 2000 ); ERROR: insert on table "shipments" violates foreign key constraint "fk_customers" SQLSTATE: 23503 DETAIL: Key (customer_id)=(2000) is not present in table "customers". CockroachDB allows you to add multiple foreign key constraints on the same column, that reference the same column:
> ALTER TABLE shipments ADD CONSTRAINT fk_customers_2 FOREIGN KEY ( customer_id ) REFERENCES customers ( id ) ON DELETE CASCADE ;
> SHOW CONSTRAINTS FROM shipments ; table_name | constraint_name | constraint_type | details | validated -------------+-----------------+-----------------+----------------------------------------------------------------------+------------ shipments | fk_customers | FOREIGN KEY | FOREIGN KEY (customer_id) REFERENCES customers(id) | true shipments | fk_customers_2 | FOREIGN KEY | FOREIGN KEY (customer_id) REFERENCES customers(id) ON DELETE CASCADE | true shipments | fk_orders | FOREIGN KEY | FOREIGN KEY (customer_id) REFERENCES orders(customer_id) | true shipments | shipments_pkey | PRIMARY KEY | PRIMARY KEY (tracking_number ASC) | true (4 rows) There are now two foreign key constraints on customer_id that reference the customers(id) column (i.e., fk_customers and fk_customers_2).
In the event of a DELETE or UPDATE to the referenced column (customers(id)), the action for the first foreign key specified takes precedence. In this case, that will be the default action (ON UPDATE NO ACTION ON DELETE NO ACTION) on the first foreign key constraint (fk_customers). This means that DELETEs on referenced columns will fail, even though the second foreign key constraint (fk_customer_2) is defined with the ON DELETE CASCADE action.
> DELETE FROM orders WHERE customer_id = 1001 ; ERROR: delete on table "orders" violates foreign key constraint "fk_orders" on table "shipments" SQLSTATE: 23503 DETAIL: Key (customer_id)=(1001) is still referenced from table "shipments". Match composite foreign keys with MATCH SIMPLE and MATCH FULL
The examples in this section show how composite foreign key matching works for both the MATCH SIMPLE and MATCH FULL algorithms. For a conceptual overview, see Composite foreign key matching.
First, let's create some tables. parent is a table with a composite key:
> CREATE TABLE parent ( x INT , y INT , z INT , UNIQUE ( x , y , z )); full_test has a foreign key on parent that uses the MATCH FULL algorithm:
> CREATE TABLE full_test ( x INT , y INT , z INT , FOREIGN KEY ( x , y , z ) REFERENCES parent ( x , y , z ) MATCH FULL ON DELETE CASCADE ON UPDATE CASCADE ); simple_test has a foreign key on parent that uses the MATCH SIMPLE algorithm (the default):
> CREATE TABLE simple_test ( x INT , y INT , z INT , FOREIGN KEY ( x , y , z ) REFERENCES parent ( x , y , z ) ON DELETE CASCADE ON UPDATE CASCADE ); Next, we populate parent with some values:
> INSERT INTO parent VALUES ( 1 , 1 , 1 ), ( 2 , 1 , 1 ), ( 1 , 2 , 1 ), ( 1 , 1 , 2 ), ( NULL , NULL , NULL ), ( 1 , NULL , NULL ), ( NULL , 1 , NULL ), ( NULL , NULL , 1 ), ( 1 , 1 , NULL ), ( 1 , NULL , 1 ), ( NULL , 1 , 1 ); Now let's look at some INSERT statements to see how the different key matching algorithms work.
- MATCH SIMPLE
- MATCH FULL
MATCH SIMPLE
Inserting values into the table using the MATCH SIMPLE algorithm (described above) gives the following results:
| Statement | Can insert? | Throws error? | Notes |
|---|---|---|---|
INSERT INTO simple_test VALUES (1,1,1) | Yes | No | References parent (1,1,1). |
INSERT INTO simple_test VALUES (NULL,NULL,NULL) | Yes | No | Does not reference parent. |
INSERT INTO simple_test VALUES (1,NULL,NULL) | Yes | No | Does not reference parent. |
INSERT INTO simple_test VALUES (NULL,1,NULL) | Yes | No | Does not reference parent. |
INSERT INTO simple_test VALUES (NULL,NULL,1) | Yes | No | Does not reference parent. |
INSERT INTO simple_test VALUES (1,1,NULL) | Yes | No | Does not reference parent. |
INSERT INTO simple_test VALUES (1,NULL,1) | Yes | No | Does not reference parent. |
INSERT INTO simple_test VALUES (NULL,1,1) | Yes | No | Does not reference parent. |
INSERT INTO simple_test VALUES (2,2,NULL) | Yes | No | Does not reference parent. |
INSERT INTO simple_test VALUES (2,2,2) | No | Yes | No parent reference exists. |
MATCH FULL
Inserting values into the table using the MATCH FULL algorithm (described above) gives the following results:
| Statement | Can insert? | Throws error? | Notes |
|---|---|---|---|
INSERT INTO full_test VALUES (1,1,1) | Yes | No | References parent(1,1,1). |
INSERT INTO full_test VALUES (NULL,NULL,NULL) | Yes | No | Does not reference parent. |
INSERT INTO full_test VALUES (1,NULL,NULL) | No | Yes | Can't mix null and non-null values in MATCH FULL. |
INSERT INTO full_test VALUES (NULL,1,NULL) | No | Yes | Can't mix null and non-null values in MATCH FULL. |
INSERT INTO full_test VALUES (NULL,NULL,1) | No | Yes | Can't mix null and non-null values in MATCH FULL. |
INSERT INTO full_test VALUES (1,1,NULL) | No | Yes | Can't mix null and non-null values in MATCH FULL. |
INSERT INTO full_test VALUES (1,NULL,1) | No | Yes | Can't mix null and non-null values in MATCH FULL. |
INSERT INTO full_test VALUES (NULL,1,1) | No | Yes | Can't mix null and non-null values in MATCH FULL. |
INSERT INTO full_test VALUES (2,2,NULL) | No | Yes | Can't mix null and non-null values in MATCH FULL. |
INSERT INTO full_test VALUES (2,2,2) | No | Yes | No parent reference exists. |
See also
- Constraints
-
DROP CONSTRAINT -
ADD CONSTRAINT -
CHECKconstraint -
DEFAULTconstraint -
NOT NULLconstraint -
PRIMARY KEYconstraint -
UNIQUEconstraint -
SHOW CONSTRAINTS - What is a Foreign Key? (With SQL Examples)
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Source: https://www.cockroachlabs.com/docs/stable/foreign-key.html
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