By: Greg Robidoux Updated: 2017-04-04 Comments (7) Related: More >Constraints
Dec 30, 2015 For example, they can be used to facilitate automated generation and validation of foreign key constraints. I don't always remember to create foreign key constraints when I add new tables and columns to a database. And even if I do remember, I'm not always the only person who makes schema changes to a database, and others can forget too.
Problem
I need to create a Foreign Key relationship between two SQL Server tables and I would like to know how this is done using the SQL Server Management Studio (SSMS) GUI as well as using T-SQL scripts. I already have the tables created, but how do I create the Foreign Key relationship.
Solution
Creating a Foreign Key relationship should be a pretty straightforward task, but understanding how to use the GUI to do this is not as simple as you might think.
- Jul 02, 2013 Dr. Soper shows how to create simple and composite primary keys and foreign key relationships in a SQL Server database using SQL Server Management Studio. Several different methods of.
- However, in SQL this raises the error: MSG 1763 Cross-Database foreign key references are not supported. I have read the most common work around is to add a trigger, but seeings how this is not that safe and there are MANY instances of this I was wondering if there was another way, perhaps a stored procedure or something of the sort.
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- Jun 07, 2012 This video shows you how to create a new table, set a primary key, and apply default value to your column in SQL Server Management Studio 2008.
Let's say we have these two tables in our database.
We want to create a Foreign Key relationship between Product.ProductCategoryID and ProductCategory.ProductCategoryID to make sure the ProductCategoryID is a valid option for any entries entered into the Product table.
To help with understanding the relationships, ProductCategory will be the referenced table and Product will be the referencing table.
When creating a Foreign Key there are a few options to enforce rules when changes are made to the referenced table:
- Delete Rule - this determines what happens if the record is deleted in the referenced table.
- Update Rule - this determines what happens if the row key is updated in the referenced table.
The options for the Delete and Update rules are:
- NO ACTION - if the change breaks the referential integrity, the change is rolled back.
- CASCADE - the change should also occur on the referencing table. If it is for DELETE the referencing rows will be deleted too. If this if for an UPDATE the referencing table row values will be updated to match the new value.
- SET NULL - the value in the referencing table should be set to NULL as long as NULL values are allowed on that column in the referencing table. If not, the change is rolled back.
- SET DEFAULT - the value in the referencing table would be set to a default value. This value would also need to exist in the referenced table. If the value does not exist in the referenced table, the change would be rolled back.
Create Foreign Key Using T-SQL
To create the Foreign Key using T-SQL, the statement would be written as follows. This is showing that we want to CASCADE the changes for both DELETEs and UPDATEs.
Here is a description for each line above:
- Since the Product table already exists, we are using the ALTER TABLE command on the dbo.Product table.
- Foreign Keys are constraints, so we are adding a Constraint of type Foreign Key named FK_Product_ProductCategoryID using the ProductCategoryID column
- The Foreign Key references table dbo.ProductCategory using the ProductCategoryID column
- For the DELETE rule we are using CASCADE
- For the UPDATE rule we are using CASCADE
That's all there is to it.
Create Foreign Key Using SSMS GUI
To create a Foreign Key using the SSMS GUI, using Object Explorer select the referencing table dbo.Product, go to Keys, right click on Keys and select New Foreign Key..:
The table designer will open as well as a new window like below. Click on the ellipse (..) next to Tables and Columns Specification.
Another window will open like below.
We can change the Relationship Name (Foreign Key name) and also need to select the appropriate tables and columns. So on the left we select the referenced table ProductCategory and underneath we select the ProductCategoryID column. On the right, the referencing table Product is already selected, but underneath we need to select the column ProductCategoryID. After making the changes, click OK.
At this point we are back to the first Foreign Key screen that we saw. If you scroll down on the right pane, we can see the Delete and Update rules as shown in the image below. Select the appropriate values from the dropdown list.
You can see below there are two other options in the GUI as shown below:
Sql Server Default Foreign Key Generation Logic Free
- Enforce For Replication - if you are using replication and don't want the foreign keys enforced at the subscriber for the replicated data you would select No.
- Enforce Foreign Key Constraint - if you do not want to the foreign key to be checked you would select No. Setting this to No defeats the purpose of having a foreign key setup.
Now click Close to accept the Foreign Key changes.
To actually save the changes to the database, you can click on the Save button in SSMS, press Ctrl-S or use the SSMS menu and select File > Save Product (will be table name you are changing to save the Foreign Key constraint.
Reviewing the New Foreign Key
After the Foreign Key has been created, we can see it in Object Explorer as shown below. You can right click on the Foreign Key and either generate a script, modify it, delete it or rename it.
Next Steps
- Next time you are creating a Foreign Key, refer to this simple tip.
Last Updated: 2017-04-04
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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.
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.
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Creating a foreign key in an existing table requires ALTER permission on the table.
Create a foreign key relationship in Table Designer
Using SQL Server Management Studio
In Object Explorer, right-click the table that will be on the foreign-key side of the relationship and click Design.
The table opens in Table Designer.
From the Table Designer menu, click Relationships.
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.
Click the relationship in the Selected Relationship list.
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Click Tables and Columns Specification in the grid to the right and click the ellipses (..) to the right of the property.
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.
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.
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.