Quick Answer: Does A Foreign Key Have To Be Unique?

What is the difference between primary key and unique key?

Primary Key is a column that is used to uniquely identify each tuple of the table.

It is used to add integrity constraints to the table.

Only one primary key is allowed to be used in a table.

Unique key is a constraint that is used to uniquely identify a tuple in a table..

How do you set a foreign key to null?

A foreign key with “set null on delete” means that if a record in the parent table is deleted, then the corresponding records in the child table will have the foreign key fields set to NULL.

Can primary key have same values?

Since both primary key and unique columns do not accept duplicate values, they can be used for uniquely identifying a record in the table. This means that, for each value in the primary or unique key column, only one record will be returned.

Can foreign key be part of composite primary key?

This is not possible. The foreign key can not refer to part of composite primary key of other table. Because it is supposed to be one-to-one relationship and if you refer just part of primary-key, there might be more than one parent record available, which is neither allowed nor possible.

What are three major types of constraints?

An informational constraint is an attribute of a certain type of constraint, but one that is not enforced by the database manager.NOT NULL constraints.Unique constraints.Primary key constraints.(Table) Check constraints.Foreign key (referential) constraints.Informational constraints.

Does foreign key have to be primary?

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. So in your case if you make AnotherID unique, it will be allowed.

Can a primary key be non unique?

3 Answers. Primary keys are unique by deifinition; a better approach would be to have either a surrogate key (like an auto numbered value), or a composite key which covers multiple columns, such as the postal code and property name/number.

Can foreign key be duplicate?

Unlike primary keys, foreign keys can contain duplicate values. Also, it is OK for them to 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.

Is primary key always unique?

Primary key is always unique in every SQL. … Also you can have only one primary key constraint in the table(as the point of creating a primary key is to uniquely identify the row in your table) but you can more than one unique key constraint in your table.

Can a table have two primary keys?

A table can have only one primary key, which may consist of single or multiple fields. When multiple fields are used as a primary key, they are called a composite key. If a table has a primary key defined on any field(s), then you cannot have two records having the same value of that field(s).

Can a column be a primary key and a foreign key?

Technically database will not allow you to create a foreign key on same primary key column key. Because foreign key should point to primary key column of another. … Basically foreign key is primary key of the related table. So you cannot have the same column with primary key and foreign key.

Why foreign keys are not redundant?

Primary and foreign keys are a way in which to constrain related data together to ensure data in your database remains consistent and to ensure no redundant data is in the database as a result of deleting a table or row in one table that affects data in other tables that may perhaps rely on that information.

Is a foreign key unique?

A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables. It uniquely identifies a record in the relational database table. It refers to the field in a table which is the primary key of another table. Only one primary key is allowed in a table.

Can foreign key be null?

A foreign key makes its table dependent on another table called a parent table. … 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.

Does every table need a primary key?

Every table can have (but does not have to have) a primary key. The column or columns defined as the primary key ensure uniqueness in the table; no two rows can have the same key. The primary key of one table may also help to identify records in other tables, and be part of the second table’s primary key.

Can foreign key be not unique?

InnoDB allows a foreign key constraint to reference a non-unique key. This is an InnoDB extension to standard SQL. However, there is a pratical reason to avoid foreign keys on non-unique columns of referenced table. … You are advised to use foreign keys that reference only UNIQUE (including PRIMARY) and NOT NULL keys.

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.

What is a foreign key example?

Definition: Foreign keys are the columns of a table that points to the primary key of another table. They act as a cross-reference between tables. For example: In the below example the Stu_Id column in Course_enrollment table is a foreign key as it points to the primary key of the Student table.