Creating and managing foreign key relationships

This page describes how to manage foreign key relationships in your database.

Foreign keys allow you to define relationships between tables. Cloud Spanner ensures the referential integrity between these tables is maintained. The following diagram illustrates a simple database schema that we'll use in this guide.

Figure 1. Diagram of an order processing database schema

There are three tables in the schema shown in figure 1:

• The Customers table records the names of each customer.
• The Orders tables keeps track of all orders made.
• The Products table stores the product information for every product.

There are two foreign key relationships between these tables:

• A foreign key relationship is defined between the Orders table and the Customers table to ensure that an order can't be created unless there is a corresponding customer.

• A foreign key relationship between the Orders table and the Products table ensures that an order can't be created for a product that doesn't exist.

With this schema as a reference, let's look at the Data Definition Language (DDL) statements you can use to manage these constraints in your database.

Adding a foreign key when creating a new table

Let's assume we've already created the Customers table in our simple product ordering database. We need an Orders table to store information about the orders that customers make. To ensure all orders are valid, we don't want to allow the system to insert rows into the Orders table that do not have a matching entry in our Customers table.

Here's the CREATE TABLE DDL statement for the Orders table that includes the foreign key constraint referencing the Customers table.

CREATE TABLE Orders (
  OrderID INT64 NOT NULL,
  CustomerID INT64 NOT NULL,
  Quantity INT64 NOT NULL,
  ProductID INT64 NOT NULL,
  CONSTRAINT FK_CustomerOrder FOREIGN KEY (CustomerID) REFERENCES Customers (CustomerID)
) PRIMARY KEY (OrderID);

The preceding statement contains a CONSTRAINT clause that we can describe as follows:

• Use of the CONSTRAINT syntax enables you to name a constraint, making it easier to drop using the name you've chosen.

• The constraint has the name FK_CustomerOrder. Constraint names are scoped to the schema and must be unique within the schema.

• The Orders table, on which we define the constraint, is called the referencing table. The Customers table is the referenced table.

• The referencing column in the referencing table is CustomerID. It references the CustomerID field in the Customers table. If someone tries to insert a row into Orders with a CustomerID that does not exist in Customers, the insert will fail.

Note: Often the referenced columns of the referenced table correspond to the primary key columns, but this is not required. A referencing table may also reference non-primary columns of a referenced table.

The following snippet shows an alternative table creation statement. Here, the foreign key constraint is defined without a name. When you use this syntax, Cloud Spanner generates a name for you. To discover the names of all foreign keys, refer to View properties of a foreign key relationship.

CREATE TABLE Orders (
  OrderID INT64 NOT NULL,
  CustomerID INT64 NOT NULL,
  ProductID INT64 NOT NULL,
  FOREIGN KEY (CustomerID) REFERENCES Customers (CustomerID)
) PRIMARY KEY (OrderID);

Adding a foreign key to an existing table

We also want to make sure that orders can only be created for products that exist. We'll use ALTER TABLE to add another foreign key constraint to the orders table as follows:

ALTER TABLE Orders
  ADD CONSTRAINT FK_ProductOrder FOREIGN KEY (ProductID) REFERENCES Products (ProductID);

The referencing column in Orders is ProductID, and it references the ProductID column in Products. Again, if you are fine with Cloud Spanner naming these constraints for you, use the following syntax:

ALTER TABLE Orders
  ADD FOREIGN KEY (ProductID) REFERENCES Products (ProductID);

Querying data across foreign key relationships

SELECT * FROM Orders
  INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID
  INNER JOIN Products ON Orders.ProductsID = Products.ProductID;

Example of referential integrity

The main reason for adding foreign key relationships is so that Cloud Spanner can maintain the referential integrity of your data. If you modify data in a way that breaks a foreign key constraint, the update will fail with an error.

Consider the data in figure 2 below. Some customers have ordered products, as shown in the orders table. Because of our foreign keys that are in place, we can guarantee that the data that was inserted into the Orders table has referential integrity.

Figure 2. Sample data for in our ordering database.

Let's look at what happens when we attempt to modify the data in a way that would break referential integrity.

Adding a row into the Orders table with a CustomerID value that does not exist in Customers

What happens if we try the following modification, given the sample data from the preceding diagram?

INSERT INTO Orders (OrderID, ProductID, Quantity, CustomerID)
  VALUES (19, 337876, 4, 447);

In this case, we are attempting to insert a row into Orders with a CustomerID (447) that doesn't exist in the Customers table. If we allowed this, we would have an invalid order in our system. However, the foreign key constraint we defined from the Orders table to the Customers table protects us, and the INSERT fails with the following message, assuming the constraint is called FK_CustomerOrder.

Foreign key constraint `FK_CustomerOrder` is violated on table `Orders`. Cannot find referenced values in Customers(CustomerID).

Attempting to delete a row from the Customers table when the customer is referenced in a foreign key constraint.

Let's imagine a situation where a customer unsubscribes from our online store. We want to remove the customer from our backend, so we attempt the following operation.

DELETE FROM Customers WHERE CustomerID = 721;

In this example, Cloud Spanner detects through the foreign key constraint that there are still records in the Orders table that reference the customer row we are trying to delete. The following error is displayed in this case.

Foreign key constraint violation when deleting or updating referenced row(s): referencing row(s) found in table `Orders`.

To fix this issue, we would delete all referencing entries in Orders first. Another solution, had we defined the CustomerID as NULLABLE, would be to set the CustomerID field to NULL to remove the reference. CASCADE DELETE is not supported on foreign keys.

Viewing properties of a foreign key relationship

Cloud Spanner's INFORMATION_SCHEMA contains information about foreign keys and their backing indexes. The following are some examples of the questions you can answer by querying the INFORMATION SCHEMA.

For more information on backing indexes, see Foreign keys backing indexes.

What constraints are defined in my database?

SELECT tc.CONSTRAINT_NAME, tc.TABLE_NAME, tc.CONSTRAINT_TYPE
FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS as tc
WHERE tc.CONSTRAINT_TYPE = "FOREIGN KEY";

What foreign keys are defined in my database?

SELECT rc.CONSTRAINT_NAME, rc.UNIQUE_CONSTRAINT_NAME, rc.SPANNER_STATE
FROM INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS as rc;

Which indexes are secondary indexes for foreign keys, also known as backing indexes?

Foreign key backing indexes are managed by Cloud Spanner , so querying for SPANNER_IS_MANAGED on the INDEXES view will return all backing indexes.

SELECT i.TABLE_NAME, i.INDEX_NAME, i.INDEX_TYPE, i.INDEX_STATE,
  i.IS_UNIQUE, i.IS_NULL_FILTERED, i.SPANNER_IS_MANAGED
FROM INFORMATION_SCHEMA.INDEXES as i
WHERE SPANNER_IS_MANAGED is true;

For more information, see Information Schema.

Removing a foreign key relationship

The following DDL drops a foreign key constraint from the Orders table.

ALTER TABLE Orders
  DROP CONSTRAINT FK_CustomerOrder;

The foreign key backing indexes are dropped automatically when the constraint itself is dropped.

Support for more complex foreign key relationships

Multiple columns

Foreign keys can reference multiple columns. The list of columns form a key that corresponds to a table's primary key or to a backing index. The referencing table contains foreign keys of the referenced table key.

In the following example, the foreign key definitions tell Cloud Spanner that each SongName value in the TopHits table must have a matching value in the Songs table; and each SingerFirstName and SingerLastName pair of values must have a matching FirstName and LastName pair of values in the Singers table.

CREATE TABLE TopHits (
  Rank INT64 NOT NULL,
  SongName STRING(MAX),
  SingerFirstName STRING(MAX),
  SingerLastName STRING(MAX),

  -- Song names must either be NULL or have matching values in Songs.
  FOREIGN KEY (SongName) REFERENCES Songs (SongName),

  -- Singer names must either be NULL or have matching values in Singers.
  FOREIGN KEY (SingerFirstName, SingerLastName)
  REFERENCES Singers (FirstName, LastName)

) PRIMARY KEY (Rank);

Circular references

Occasionally tables have circular dependencies, perhaps for legacy reasons or due to denormalization. Cloud Spanner foreign keys permit circular references. Since a referenced table must exist before a foreign key can reference it, one of the foreign keys must be added with an ALTER TABLE statement. Here's an example

1. Create TableA, without a foreign key
2. Create TableB with a foreign key constraint on TableA.
3. Use ALTER TABLE on TableA to create a foreign key reference to TableB.

Self-referencing tables

One special type of circular reference is a table that defines a foreign key that references the same table. For example, the following snippet shows a foreign key to enforce that an employee's ManagerId is also an employee.

CREATE TABLE Employees (
  EmployeeId INT64 NOT NULL,
  EmployeeName STRING(MAX) NOT NULL,
  ManagerId INT64,
  FOREIGN KEY (ManagerId) REFERENCES Employees (EmployeeId)
) PRIMARY KEY (EmployeeId);

What's next

• Learn more about foreign keys support in Cloud Spanner.

• Learn more about Cloud Spanner's INFORMATION SCHEMA.