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APPLIES TO: SQL Server 2016 and later Azure SQL Database Azure Synapse Analytics (SQL DW) Parallel Data Warehouse

This article describes how to create foreign key relationships in SQL Server 2019 (15.x) by using SQL Server Management Studio or Transact-SQL. You create a relationship between two tables when you want to associate rows of one table with rows of another.

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Before You Begin! Limits and Restrictions

• A foreign key constraint does not have to be linked only to a primary key constraint in another table; it can also be defined to reference the columns of a UNIQUE constraint in another table.

• When a value other than NULL is entered into the column of a FOREIGN KEY constraint, the value must exist in the referenced column; otherwise, a foreign key violation error message is returned. To make sure that all values of a composite foreign key constraint are verified, specify NOT NULL on all the participating columns.

• FOREIGN KEY constraints can reference only tables within the same database on the same server. Cross-database referential integrity must be implemented through triggers. For more information, see CREATE TRIGGER.

• FOREIGN KEY constraints can reference another column in the same table. This is referred to as a self-reference.

• A FOREIGN KEY constraint specified at the column level can list only one reference column. This column must have the same data type as the column on which the constraint is defined.

• A FOREIGN KEY constraint specified at the table level must have the same number of reference columns as the number of columns in the constraint column list. The data type of each reference column must also be the same as the corresponding column in the column list.

• The Database Engine does not have a predefined limit on either the number of FOREIGN KEY constraints a table can contain that reference other tables, or the number of FOREIGN KEY constraints that are owned by other tables that reference a specific table. Nevertheless, the actual number of FOREIGN KEY constraints that can be used is limited by the hardware configuration and by the design of the database and application. A table can reference a maximum of 253 other tables and columns as foreign keys (outgoing references). SQL Server 2016 (13.x) increases the limit for the number of other table and columns that can reference columns in a single table (incoming references), from 253 to 10,000. (Requires at least 130 compatibility level.) The increase has the following restrictions:

  • Greater than 253 foreign key references are supported for DELETE and UPDATE DML operations. MERGE operations are not supported.
  • A table with a foreign key reference to itself is still limited to 253 foreign key references.
  • Greater than 253 foreign key references are not currently available for columnstore indexes, memory-optimized tables, or Stretch Database.

• FOREIGN KEY constraints are not enforced on temporary tables.

• If a foreign key is defined on a CLR user-defined type column, the implementation of the type must support binary ordering. For more information, see CLR User-Defined Types.

• A column of type varchar(max) can participate in a FOREIGN KEY constraint only if the primary key it references is also defined as type varchar(max).

Permissions

Creating a new table with a foreign key requires CREATE TABLE permission in the database and ALTER permission on the schema in which the table is being created.

Creating a foreign key in an existing table requires ALTER permission on the table.

• No, there is no implicit index on foreign key fields, otherwise why would Microsoft say 'Creating an index on a foreign key is often useful'. Your colleague may be confusing the foreign key field in the referring table with the primary key in the referred-to table - primary keys do create an implicit index.
• Rails 3.1 makes migrations smarter by providing a new change method. This method is preferred for writing constructive migrations (adding columns or tables). The migration knows how to migrate your database and reverse it when the migration is rolled back without the need to.

Create a foreign key relationship in Table Designer

Using SQL Server Management Studio

1. In Object Explorer, right-click the table that will be on the foreign-key side of the relationship and click Design.

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   The table opens in Table Designer.

2. From the Table Designer menu, click Relationships.

3. In the Foreign-key Relationships dialog box, click Add.

   The relationship appears in the Selected Relationship list with a system-provided name in the format FK_<tablename>_<tablename>, where tablename is the name of the foreign key table.

4. Click the relationship in the Selected Relationship list.

5. Click Tables and Columns Specification in the grid to the right and click the ellipses (..) to the right of the property.

6. In the Tables and Columns dialog box, in the Primary Key drop-down list, choose the table that will be on the primary-key side of the relationship.

7. In the grid beneath, choose the columns contributing to the table's primary key. In the adjacent grid cell to the left of each column, choose the corresponding foreign-key column of the foreign-key table.

   Table Designer suggests a name for the relationship. To change this name, edit the contents of the Relationship Name text box.

8. Choose OK to create the relationship.

Create a foreign key in a new table

Using Transact-SQL

The following example creates a table and defines a foreign key constraint on the column TempID that references the column SalesReasonID in the Sales.SalesReason table in the AdventureWorks database. The ON DELETE CASCADE and ON UPDATE CASCADE clauses are used to ensure that changes made to Sales.SalesReason table are automatically propagated to the Sales.TempSalesReason table.

Create a foreign key in an existing table

Using Transact-SQL

The following example creates a foreign key on the column TempID and references the column SalesReasonID in the Sales.SalesReason table in the AdventureWorks database.

For more information, see:

• table_constraint.

Immigrant gives Rails a foreign key migration generator so you caneffortlessly find and add missing keys. This is particularly helpfulwhen you decide to add keys to an established Rails app.

Installation

Add the following to your Gemfile:

If you're using a version of Rails prior to 4.2, you'll also need theForeigner gem.

Usage

This will create a migration named AddKeys which will have add_foreign_keystatements for any missing foreign keys. Immigrant infers missing ones byevaluating the associations in your models (e.g. belongs_to, has_many, etc.).Only missing keys will be added; existing ones will never be altered orremoved.

Rake Task

To help you remember to add keys in the future, there's a handy raketask you can add to your CI setup. Just run rake immigrant:check_keys,and if anything is missing it will tell you about it and exit with anon-zero status.

Skipping associations

Immigrant.ignore_keys allows you to specify a list of keys that shouldbe ignored (both in the migration generator and the rake task). This isuseful if you have associations spanning databases.

Just create an config/initializers/immigrant.rb file with something likethe following:

Considerations

If the data in your tables is bad, then the migration will fail to run(obviously). IOW, ensure you don't have orphaned records before you try toadd foreign keys.

Known Issues

Add Foreign Key To Rails Generate Key

Immigrant currently only looks for foreign keys in ActiveRecord::Base'sdatabase. So if a model is using a different database connection and it hasforeign keys, Immigrant will incorrectly include them again in the generatedmigration. Immigrant.ignore_keys can be used to work around this.

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Copyright (c) 2012-2015 Jon Jensen, released under the MIT license