It is possible to reference a UNIQUE constraint in a FOREIGN KEY. You could have a Primary key and an Unique key, and you would like to validate both. Yes, you can reference a column (or columns) governed by either a primary key constraint or a unique constraint.
Can an unique key be a foreign key?
Foreign Key is a column that refers to the primary key/unique key of other table. So it demonstrates relationship between tables and act as cross reference among them. Table in which foreign key is defined is called Foreign table/Referencing table.
What are the characteristics of foreign key?
- Column pattern: the regular expression that will match a column name that you want to use as a virtual foreign key. ...
- Target column pattern: the replacement pattern that uses the match from the Column pattern expression and is interpreted as a regular expression. ...
- Source column: an example of a column name that you want to use as a virtual foreign key.
How do I create a foreign key?
Create Foreign Key Using SSMS GUI. To create a Foreign Key using the SSMS GUI, using Object Explorer select the referencing table dbo.Product, go to Keys, right click on Keys and select New Foreign Key...: The table designer will open as well as a new window like below. Click on the ellipse (...) next to Tables and Columns Specification.
What is difference between foreign key and reference key?
CONTENTS
- Overview and Key Difference
- What is Foreign key
- What is Primary key
- Side by Side Comparison – Foreign key and Primary key in Tabular Form
- Summary
Is unique same as foreign key?
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 foreign key be part of a unique constraint?
A UNIQUE constraint can be referenced by a FOREIGN KEY constraint. When a UNIQUE constraint is added to an existing column or columns in the table, by default, the Database Engine examines the existing data in the columns to make sure all values are unique.
Can foreign key not be unique?
No, foreign keys do not have to be unique. Indeed, a lack of uniqueness is requisite for one-to-many or many-to-many relations. Show activity on this post. Foreign key(s) must reference a unique set of attributes in the referenced table.
Can we create foreign key on unique key in SQL?
To create a SQL foreign key constraint, the parent table should have primary key column or column with UNIQUE constraint. In this case, table Dept is parent table which has Primary key and will be referenced in child tables having foreign key.
Can a unique key be NULL?
Key Differences Between Primary key and Unique key: Primary key will not accept NULL values whereas Unique key can accept NULL values. A table can have only one primary key whereas there can be multiple unique key on a table.
Can a column be both primary and foreign key?
You can create a column having both keys (primary and foreign) but then it will be one to one mapping and add uniqueness to this column.
Can a primary key be a foreign key at the same time?
A Foreign Key is used for referential integrity, to make sure that a value exists in another table. The Foreign key needs to reference the primary key in another table. If you want to have a foreign key that is also unique, you could make a FK constraint and add a unique index/constraint to that same field.
Can a foreign key exist without primary key?
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 candidate key be a foreign key?
In simpler words, a foreign key is a set of attributes that references a candidate key. For example, a table called TEAM may have an attribute, MEMBER_NAME, which is a foreign key referencing a candidate key, PERSON_NAME, in the PERSON table.
Do foreign keys have to be unique mysql?
Every column with an key (primary, foreign) needs an index. Same with column being unique. You probably created two indexes (one when creating FK and one on Unique constraint).
What is a foreign key?
Foreign Key. Foreign key is a field in the table that is Primary key in another table. Foreign key can accept multiple null value. Foreign key do not automatically create an index, clustered or non-clustered. You can manually create an index on foreign key.
Can a unique constraint have more than one value?
Unique Constraint may have a NULL value. 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 primary key be null?
Primary Key. Primary key cannot have a NULL value. Each table can have only one primary key. By default, Primary key is clustered index, and the data in database table is physically organized in the sequence of clustered index. Primary key can be related to another tables as a Foreign Key.
Can you create an index on a foreign key?
You can manually create an index on foreign key. We can have more than one foreign key in a table. Foreign keys do not automatically create an index, clustered or non-clustered. You must manually create an index on foreign keys.
