Is Foreign Key Unique?

by | Last updated on January 24, 2024

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S.NO. PRIMARY KEY FOREIGN KEY 4 It is a combination of UNIQUE and Not Null constraints. It can contain duplicate values and a table in a relational database.

Can foreign key be not 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. Foreign key(s) must reference a unique set of attributes in the referenced table. … Note, the referenced columns in this case must be unique to qualify as Primary Keys.

Can foreign key be duplicate?

Unlike primary keys,

foreign keys can contain duplicate values

. Also, it is OK for them to contain NULL values. Though not automatically created for foreign keys, it is a good idea to define them. You can define several foreign key within a table.

Can foreign key not be primary key?

A foreign key can refer to either a unique or a primary key of the parent table. If the foreign key refers to a non-primary unique key, you

must specify

the column names of the key explicitly.

Is a foreign key always unique?


Any primary key must be unique and non

-null. Therefore if the child table has a foreign key referencing the parent’s primary key, it must match a non-null, unique value, and therefore references exactly one row in the parent table. In this case you can’t make a child row that references multiple parent rows.

Is foreign key can be primary key?

Yes,

it is legal to have a primary key being a foreign key

. This is a rare construct, but it applies for: a 1:1 relation. The two tables cannot be merged in one because of different permissions and privileges only apply at table level (as of 2017, such a database would be odd).

Can foreign key be null?

By

default there are no constraints on the foreign key

, foreign key can be null and duplicate. while creating a table / altering the table, if you add any constrain of uniqueness or not null then only it will not allow the null/ duplicate values.

Can primary key be null?

A primary key defines the set of columns that uniquely identifies rows in a table. When you create a primary key constraint,

none of the columns included in the primary key can have NULL

constraints; that is, they must not permit NULL values. … NULL values are not allowed.

What is a foreign key example?

A foreign key is a set of attributes in a table that refers to the primary key of another table. … 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.

What is difference between primary key and foreign key?

A primary key is used to ensure data in the specific column is 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. … Only one primary key is allowed in a table.

Can foreign keys be NULL mysql?


NULLs in foreign keys are perfectly acceptable

. Dealing with NULLs in foreign keys is tricky but that does not mean that you change such columns to NOT NULL and insert dummy (“N/A”, “Unknown”, “No Value” etc) records in your reference tables.

Why foreign key is bad?

The obvious problem with the lack of foreign keys is that a

database can’t enforce referential integrity

and if it wasn’t taken care of properly at the higher level then this might lead to inconsistent data (child rows without corresponding parent rows).

What is a bad primary key?

Primary Key Example

Imagine you have a STUDENTS table that contains a record for each student at a university. … Other poor choices for primary keys include

ZIP code, email address, and employer

, all of which can change or represent many people. The identifier used as a primary key must be unique.

Can a table have two primary keys?


You cannot use more than 1 primary key in the table

. for that you have composite key which is combination of multiple fields. It needs to be a composite key. Yes, we can have more than one column as primary key to solve some business requirements.

Why foreign keys are not redundant?

Essentially, primary and foreign keys are used as a way to constrain or link related data in a database. This ensures that

data remains consistent and that the database contains no redundant data

. For example, if you delete a table (or even a row in a table) that other tables rely on, the redundant data is removed.

Can a varchar be a primary key?

It

is perfectly acceptable to

use a varchar column as the primary key.

Maria Kunar
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Maria Kunar
Maria is a cultural enthusiast and expert on holiday traditions. With a focus on the cultural significance of celebrations, Maria has written several blogs on the history of holidays and has been featured in various cultural publications. Maria's knowledge of traditions will help you appreciate the meaning behind celebrations.