1 Answer. 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.
Likewise, can a foreign key reference unique key?
Primary keys always need to be unique, foreign keys need to allow non-unique values if the table is a one-to-many relationship. 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.
Furthermore, can a foreign key have a different name? Most commonly, a foreign key references the primary key field of the other table. The foreign key field (CustomerNum) and referenced field (CustID) may have different names (or the same name), but must have the same data type and field constraints.
Subsequently, one may also ask, can a foreign key reference the same table?
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.
Can a foreign key be updated?
However, if the update rule is NO ACTION, parent unique keys can be updated as long as every child has a parent key by the time the update statement completes. A non-null update value of a foreign key must be equal to a value of the primary key of the parent table of the relationship.
18 Related Question Answers Found
Are foreign keys mandatory?
Since the purpose of the foreign key is to identify a particular row of referenced table, it is generally required that the foreign key is equal to the candidate key in some row of the primary table, or else have no value (the NULL value.). This rule is called a referential integrity constraint between the two tables.
Can foreign key have duplicate values?
Unlike primary keys, foreign keys can contain duplicate values. Also, it is OK for them contain NULL values. Indexes aren’t automatically created for foreign keys; however, as a DBA, you can define them. A table is allowed to contain more than one foreign key.
Can I create a foreign key without primary key?
Yes. We can create a Foreign key without having Primary key in the parent table. Most of us are in a misconception that we can not create a Foreign key without primary key in the parent table. That unique id can be primary key or unique key.
Can a foreign key be null?
A foreign key containing null values cannot match the values of a parent key, since a parent key by definition can have no null values. However, a null foreign key value is always valid, regardless of the value of any of its non-null parts. A table can have many foreign keys.
What is a foreign key example?
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.
What is the difference between primary key unique key and foreign key?
Unique key can accept only one null value. By default, Primary key is clustered index and data in the database table is physically organized in the sequence of clustered index. By default, Unique key is a unique non-clustered index. Primary key can be made foreign key into another table.
Is foreign key a subset of primary key?
Yes , There can be a foreign key which is unique key in other table as Unique key is subset of primary key but not the exact primary key. So that’s possible that foreign key is unique key in aother table. General standard answer is no. It is only possible if foreign key refers any column uniquely in other table.
How do I reference a foreign key in SQL?
Summary: Every value of Foreign key has to be part of Primary Key of other tables. The Foreign key can reference to another column in the same table. This reference is known as a self-reference. You can create a Foreign Key using Create Table, Alter Table, or SQL Server Management Studio.
Can 2 tables have the same primary key?
Generally if two tables have the same primary key, it’s because the data within the tables refer to the same object.
Can we create primary key and foreign key on same table?
Yes it possible 2 have foriegn key constraint on the same table where there is a primary key. this is can done by adding a constraint to the table using alter table tblname . the refernce column and table being the base table. thus one table can have both primary key as well as foreign key constraint.
What is a foreign key constraint?
SQL FOREIGN KEY Constraint. A FOREIGN KEY 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 FOREIGN KEY constraint is used to prevent actions that would destroy links between tables.
How many foreign keys can be there in a table?
A table can reference a maximum of 253 other tables and columns as foreign keys (Outgoing Foreign Key References). SQL Server 2016 increases the limit for the number of other table and columns that can reference columns in a single table (Incoming Foreign Key References), from 253 to 10,000.
Can primary key be null?
A primary key is a field in a table which uniquely identifies each row/record in a database table. Primary keys must contain unique values. A primary key column cannot have NULL values. A table can have only one primary key, which may consist of single or multiple fields.
What is foreign key in MS Access?
Primary Keys and Foreign Keys in Microsoft Access. The term Foreign Key (FK) in a Relational Database System refers to a field that in a table that stores values from a Primary Key field in another table, to relate the two records to each other.
Can foreign key refer to two tables?
A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables. A foreign key acts as a cross-reference between tables in that it references the primary key or unique key columns of another table, and thus establishes a link between them.
Are foreign keys unique?
Each table can have more than one Unique Constraint. By default, Unique key is a unique non-clustered index. Unique Constraint can not be related with another table’s as a Foreign Key.
Can a table have a foreign key to itself?
a table that has a foreign key that references itself. You cannot use TRUNCATE TABLE on tables that: Are referenced by a FOREIGN KEY constraint. (You can truncate a table that has a foreign key that references itself.)
What is foreign key violation?
Occurs when an update or delete on a primary key table would violate a foreign key constraint. Occurs when an insert or update on a foreign key table is performed without a matching value in the primary key table.