Quick Answer: Why Do We Use Unique Keys?

Is foreign key unique?

Short answer: Yes, it can be NULL or duplicate.

I want to explain why a foreign key might need to be null or might need to be unique or not unique.

First remember a Foreign key simply requires that the value in that field must exist first in a different table (the parent table).

That is all an FK is by definition..

Can 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.

Is candidate a key?

Candidate Key: The minimal set of attribute which can uniquely identify a tuple is known as candidate key. For Example, STUD_NO in STUDENT relation. The value of Candidate Key is unique and non-null for every tuple. There can be more than one candidate key in a relation.

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 table have both primary key and unique key?

Yes. The only limit on a table is, it can have one Primary Key per table. You can have many Unique columns/keys on the same table, you can even have multiple keys to be unique together at the same time.

What is difference between primary and candidate key?

Primary Key is a unique and non-null key which identify a record uniquely in table. … A table can have only one primary key. Candidate key is also a unique key to identify a record uniquely in a table but a table can have multiple candidate keys.

Can a primary key be unique?

A Primary key is a unique key. Each table must have at most ONE primary key but it can have multiple unique key. A primary key is used to uniquely identify a table row. A primary key cannot be NULL since NULL is not a value.

What is the use of unique key?

A unique key is a set of one or more than one fields/columns of a table that uniquely identify a record in a database table. You can say that it is little like primary key but it can accept only one null value and it cannot have duplicate values.

Why do primary keys need to be unique?

The PRIMARY KEY constraint uniquely identifies each record in a table. Primary keys must contain UNIQUE values, and cannot contain NULL values. A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).

Are candidate keys unique?

Candidate key is a set of columns which can uniquely identify the values in a table and can act as a unique key. One of these candidate keys will become the primary key and the rest will become alternate keys. Unique key as the name suggests , is used to uniquely identify a value in a table. For example Emp_id etc.

Why is it important for each record to have a unique primary key?

A primary key is a table column that serves a special purpose. Each database table needs a primary key because it ensures row-level accessibility. … The values that compose a primary key column are unique; no two values are the same. Each table has one and only one primary key, which can consist of one or many columns.

What is difference between primary and unique key?

Multiple unique keys can present in a table. NULL values are allowed in case of a unique key. These can also be used as foreign keys for another table….Difference between Primary Key and Unique Key.Primary KeyUnique KeyOnly one primary key can be present in a tableMultiple Unique Keys can be present in a table4 more rows•Feb 3, 2020