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Modify Foreign Key Relationships

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Applies to: SQL Server 2016 (13.x) and later Azure SQL Database Azure SQL Managed Instance

You can modify the foreign key side of a relationship in SQL Server by using SQL Server Management Studio or Transact-SQL. Modifying a table's foreign key changes which columns are related to columns in the primary key table.

In This Topic

Before You Begin

Limitations and Restrictions

The new foreign key column must match the data type and size of the primary key column to which it relates, with these exceptions:

  • A char column or sysname column can relate to a varchar column.

  • A binary column can relate to a varbinary column.

  • An alias data type can relate to its base type.

Security

Permissions

Requires ALTER permission on the table.

Using SQL Server Management Studio

To modify a foreign key

  1. In Object Explorer, expand the table with the foreign key and then expand Keys.

  2. Right-click the foreign key to be modified and select Modify.

  3. In the Foreign Key Relationships dialog box, you can make the following modifications.

    Selected Relationship
    Lists existing relationships. Select a relationship to show its properties in the grid to the right. If the list is empty, no relationships have been defined for the table.

    Add
    Create a new relationship. The Tables and Columns Specifications must be set before the relationship will be valid.

    Delete
    Delete the relationship selected in the Selected Relationships list. To cancel the addition of a relationship, use this button to remove the relationship.

    General Category
    Expand to show Check Existing Data on Creation or RE-Enabling and Tables and Columns Specifications.

    Check Existing Data on Creation or Re-Enabling
    Verify all existing data in the table before the constraint was created or re-enabled, against the constraint.

    Tables and Columns Specifications Category
    Expand to show which columns from which tables act as the foreign key and primary (or unique) key in the relationship. To edit or define these values, click the ellipsis button (...) to the right of the property field.

    Foreign Key Base Table
    Shows which table contains the column acting as a foreign key in the selected relationship.

    Foreign Key Columns
    Shows which column acts as a foreign key in the selected relationship.

    Primary/Unique Key Base Table
    Shows which table contains the column acting as a primary (or unique) key in the selected relationship.

    Primary/Unique Key Columns
    Shows which column acts as a primary (or unique) key in the selected relationship.

    Identity Category
    Expand to show the property fields for Name and Description.

    Name
    Shows the name of the relationship. When a new relationship is created, it is given a default name based on the table in the active window in Table Designer. You can change the name at any time.

    Description
    Describe the relationship. To write a more detailed description, click Description and then click the ellipsis (...) that appears to the right of the property field. This provides a larger area in which to write text.

    Table Designer Category
    Expand to show information for Check Existing Data on Creation or Re-Enabling and Enforce for Replication.

    Enforce For Replication
    Indicates whether to enforce the constraint when a replication agent performs an insert, update, or delete on this table.

    Enforce Foreign Key Constraint
    Specify whether changes are allowed to the data of the columns in the relationship if those changes would invalidate the integrity of the foreign key relationship. Choose Yes if you do not want to allow such changes, and choose No if you do want to allow them.

    INSERT and UPDATE Specification Category
    Expand to show information for the Delete Rule and the Update Rule for the relationship.

    Delete Rule
    Specify what happens if a user tries to delete a row with data that is involved in a foreign key relationship:

    • No Action An error message tells the user that the deletion is not allowed and the DELETE is rolled back.

    • Cascade Deletes all rows containing data involved in the foreign key relationship. Do not specify CASCADE if the table will be included in a merge publication that uses logical records.

    • Set Null Sets the value to null if all foreign key columns for the table can accept null values.

    • Set Default Sets the value to the default value defined for the column if all foreign key columns for the table have defaults defined for them.

    Update Rule
    Specify what occurs if a user tries to update a row with data that is involved in a foreign key relationship:

    • No Action An error message tells the user that the update is not allowed and the UPDATE is rolled back.

    • Cascade Updates all rows that contain data involved in the foreign key relationship. Do not specify CASCADE if the table will be included in a merge publication that uses logical records.

    • Set Null Sets the value to null if all foreign key columns for the table can accept null values.

    • Set Default Sets the value to the default value that is defined for the column if all foreign key columns for the table have defaults defined for them.

  4. On the File menu, click Savetable name.

Using Transact-SQL

To modify a foreign key

To modify a FOREIGN KEY constraint by using Transact-SQL, you must first delete the existing FOREIGN KEY constraint and then re-create it with the new definition. For more information, see Delete Foreign Key Relationships and Create Foreign Key Relationships.