• Why Doesn't Rails Generate A Foreign Key In Florida
• Why Doesn't Rails Generate A Foreign Key Code

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

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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.

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

1. 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.

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.

Summary: in this tutorial, you will learn about MySQL foreign key and how to create, drop, and disable a foreign key constraint.

Introduction to MySQL foreign key

A foreign key is a column or group of columns in a table that links to a column or group of columns in another table. The foreign key places constraints on data in the related tables, which allows MySQL to maintain referential integrity.

Let’s take a look at the following customers and orders tables from the sample database.

In this diagram, each customer can have zero or many orders and each order belongs to one customer.

The relationship between customers table and orders table is one-to-many. And this relationship is established by the foreign key in the orders table specified by the customerNumber column.

The customerNumber column in the orders table links to the customerNumber primary key column in the customers table.

The customers table is called the parent table or referenced table, and the orders table is known as the child table or referencing table.

Typically, the foreign key columns of the child table often refer to the primary key columns of the parent table.

A table can have more than one foreign key where each foreign key references to a primary key of the different parent tables.

Once a foreign key constraint is in place, the foreign key columns from the child table must have the corresponding row in the parent key columns of the parent table or values in these foreign key column must be NULL (see the SET NULL action example below).

For example, each row in the orders table has a customerNumber that exists in the customerNumber column of the customers table. Multiple rows in the orders table can have the same customerNumber.

Why Doesn't Rails Generate A Foreign Key In Florida

Self-referencing foreign key

Sometimes, the child and parent tables may refer to the same table. In this case, the foreign key references back to the primary key within the same table.

See the following employees table from the sample database.

The reportTo column is a foreign key that refers to the employeeNumber column which is the primary key of the employees table.

This relationship allows the employees table to store the reporting structure between employees and managers. Each employee reports to zero or one employee and an employee can have zero or many subordinates.

The foreign key on the column reportTo is known as a recursive or self-referencing foreign key.

MySQL FOREIGN KEY syntax

Here is the basic syntax of defining a foreign key constraint in the CREATE TABLE or ALTER TABLE statement:

In this syntax:

First, specify the name of foreign key constraint that you want to create after the CONSTRAINT keyword. If you omit the constraint name, MySQL automatically generates a name for the foreign key constraint.

Second, specify a list of comma-separated foreign key columns after the FOREIGN KEY keywords. The foreign key name is also optional and is generated automatically if you skip it.

Third, specify the parent table followed by a list of comma-separated columns to which the foreign key columns reference.

Finally, specify how foreign key maintains the referential integrity between the child and parent tables by using the ON DELETE and ON UPDATE clauses. The reference_option determines action which MySQL will take when values in the parent key columns are deleted (ON DELETE) or updated (ON UPDATE).

MySQL has five reference options: CASCADE, SET NULL, NO ACTION, RESTRICT, and SET DEFAULT.

• CASCADE: if a row from the parent table is deleted or updated, the values of the matching rows in the child table automatically deleted or updated.
• SET NULL: if a row from the parent table is deleted or updated, the values of the foreign key column (or columns) in the child table are set to NULL.
• RESTRICT: if a row from the parent table has a matching row in the child table, MySQL rejects deleting or updating rows in the parent table.
• NO ACTION: is the same as RESTRICT.
• SET DEFAULT: is recognized by the MySQL parser. However, this action is rejected by both InnoDB and NDB tables.

In fact, MySQL fully supports three actions: RESTRICT, CASCADE and SET NULL.

If you don’t specify the ON DELETE and ON UPDATE clause, the default action is RESTRICT.

MySQL FOREIGN KEY examples

Let’s create a new database called fkdemo for the demonstration.

RESTRICT & NO ACTION actions

Inside the fkdemo database, create two tables categories and products:

The categoryId in the products table is the foreign key column that refers to the categoryId column in the categories table.

Because we don’t specify any ON UPDATE and ON DELETE clauses, the default action is RESTRICT for both update and delete operation.

The following steps illustrate the RESTRICT action.

1) Insert two rows into the categories table:

2) Select data from the categories table:

3) Insert a new row into the products table:

It works because the categoryId 1 exists in the categories table.

4) Attempt to insert a new row into the products table with a categoryId value does not exist in the categories table:

MySQL issued the following error:

5) Update the value in the categoryId column in the categories table to 100:

MySQL issued this error:

Because of the RESTRICT option, you cannot delete or update categoryId 1 since it is referenced by the productId1 in the products table.

CASCADE action

These steps illustrate how ON UPDATE CASCADE and ON DELETE CASCADE actions work.

1) Drop the products table:

2) Create the products table with the ON UPDATE CASCADE and ON DELETE CASCADE options for the foreign key:

3) Insert four rows into the products table:

4) Select data from the products table:

5) Update categoryId 1 to 100 in the categories table:

6) Verify the update:

7) Get data from the products table:

As you can see, two rows with value 1 in the categoryId column of the products table were automatically updated to 100 because of the ON UPDATE CASCADE action.

8) Delete categoryId 2 from the categories table:

9) Verify the deletion:

10) Check the products table:

All products with categoryId 2 from the products table were automatically deleted because of the ON DELETE CASCADE action.

SET NULL action

These steps illustrate how the ON UPDATE SET NULL and ON DELETE SET NULL actions work.

1) Drop both categories and products tables:

2) Create the categories and products tables:

The foreign key in the products table changed to ON UPDATE SET NULL and ON DELETE SET NULL options.

3) Insert rows into the categories table:

4) Insert rows into the products table:

5) Update categoryId from 1 to 100 in the categories table:

6) Verify the update:

7) Select data from the products table:

The rows with the categoryId 1 in the products table were automatically set to NULL due to the ON UPDATE SET NULL action.

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8) Delete the categoryId 2 from the categories table:

9) Check the products table:

The values in the categoryId column of the rows with categoryId 2 in the products table were automatically set to NULL due to the ON DELETE SET NULL action.

Drop MySQL foreign key constraints

To drop a foreign key constraint, you use the ALTER TABLE statement:

In this syntax:

• First, specify the name of the table from which you want to drop the foreign key after the ALTER TABLE keywords.
• Second, specify the constraint name after the DROP FOREIGN KEY keywords.

Notice that constraint_name is the name of the foreign key constraint specified when you created or added the foreign key constraint to the table.

To obtain the generated constraint name of a table, you use the SHOW CREATE TABLE statement:

For example, to see the foreign keys of the products table, you use the following statement:

The following is the output of the statement:

As you can see clearly from the output, the table products table has one foreign key constraint: fk_category

And this statement drops the foreign key constraint of the products table:

To ensure that the foreign key constraint has been dropped, you can view the structure of the products table:

Disabling foreign key checks

Sometimes, it is very useful to disable foreign key checks e.g., when you import data from a CSV file into a table. If you don’t disable foreign key checks, you have to load data into a proper order i.e., you have to load data into parent tables first and then child tables, which can be tedious. However, if you disable the foreign key checks, you can load data into tables in any order.

To disable foreign key checks, you use the following statement:

And you can enable it by using the following statement:

Why Doesn't Rails Generate A Foreign Key Code

In this tutorial, you have learned about the MySQL foreign key and how to create a foreign key constraint with various reference options.