Can you reference a foreign key?
A foreign key can reference any field defined as unique. If that unique field is itself defined as a foreign key, it makes no difference. If it is a unique field, it can also be the target of another FK.
What must a foreign key reference?
5 Answers. In the relational model of data, a foreign key must reference a candidate key. In almost all SQL dbms, a foreign key must reference a candidate key.
What is an example of a foreign key?
A foreign key is a column (or columns) that references a column (most often the primary key) of another table. For example, say we have two tables, a CUSTOMER table that includes all customer data, and an ORDERS table that includes all customer orders.
Can a relation have a foreign key that references itself?
A FOREIGN KEY constraint defines one or more columns in a table as referencing columns in a unique or primary key in another table. (A foreign key can reference a unique or primary key in the same table as the foreign key itself, but such foreign keys are rare.) Many foreign keys may exist on a table at a time.
Does a foreign key need to reference a primary key?
A foreign key must refer to an entire primary key, and not just part of it. Consider a Department table with a primary key of company_name + department_name. An Employee table should only refer to both attributes and not to department_name alone.
Do foreign keys always reference primary keys?
Originally Answered: Are foreign keys always primary keys? A foreign key constraint in a dependent or child table references either the primary key constraint or a unique key constraint on the parent or independent table. Yes it has to be.
What is primary key and foreign key with example?
Difference between Primary key and Foreign key
| Primary Key | Foreign Key |
|---|---|
| A primary key constrain is a column or group of columns that uniquely identifies every row in the table of the relational database management system. | Foreign key is a column that creates a relationship between two tables. |
How do foreign keys of relations relate to primary keys?
A foreign key is a column or a set of columns in one table that references the primary key columns in another table. The primary key is defined as a column (or set of columns) where each value is unique and identifies a single row of the table.
What is the use of foreign key in a relation?
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.
Where do we use foreign key?
Foreign keys link data in one table to the data in another table. A foreign key column in a table points to a column with unique values in another table (often the primary key column) to create a way of cross-referencing the two tables.
Can a foreign key reference same table?
FOREIGN KEY constraints can reference another column in the same table, and 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.
How do I create a foreign key reference to a table?
Use ALTER TABLE on TableA to create a foreign key reference to TableB. One special type of circular reference is a table that defines a foreign key that references the same table.
What is a foreign key?
A FOREIGN KEY is a field (or collection of fields) in one table that refers to the PRIMARY KEY in another table. The following SQL creates a FOREIGN KEY on the “PersonID” column when the “Orders” table is created:
How do I create a foreign key relationship in SQL Server?
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.
What is the FOREIGN KEY constraint?
The FOREIGN KEY constraint is a key used to link two tables together. A FOREIGN KEY is a field (or collection of fields) in one table that refers to the PRIMARY KEY in another table. The following SQL creates a FOREIGN KEY on the “PersonID” column when the “Orders” table is created: