What is a candidate key in SQL Server?

Asked By: Mikelats Ospital | Last Updated: 21st January, 2020
Category: technology and computing databases
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Candidate Key – A candidate key is a column or a set of columns that can qualify as a primary key in the database. There can be multiple candidate keys in a database relation and each candidate can work as a primary key for the table.

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Besides, what is candidate key in SQL Server with example?

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.

Similarly, what is difference between primary and candidate key? Difference between Primary Key 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.

Also Know, what is the candidate key in database?

A candidate key is a column, or set of columns, in a table that can uniquely identify any database record without referring to any other data. Each table may have one or more candidate keys, but one candidate key is unique, and it is called the primary key.

What is a key in SQL Server?

A key is a single or combination of multiple fields in a table. It is used to fetch or retrieve records/data-rows from data table according to the condition/requirement. Keys are also used to create a relationship among different database tables or views.

37 Related Question Answers Found

Is Super key a primary key?

A Super Key is a set of one or more attributes that are taken collectively and can identify all other attributes uniquely. As our Super key. Each super key is able to uniquely identify each tuple (record). OR A key which is used to uniquely identify each record is known as primary key.

What are different kinds of keys?

Summary. Seven Types of DBMS keys are Super, Primary, Candidate, Alternate, Foreign, Compound, Composite, and Surrogate Key. A super key is a group of single or multiple keys which identifies rows in a table.

Can a candidate key be null?

Primary key can't contain Null value, but Candidate key value can have one NULL value. If a column (part of candidate keys) contain one null value, then that column will remain as part of candidate key, but not be considered as primary 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 candidate key and super key?


Super Key is an attribute (or set of attributes) that is used to uniquely identifies all attributes in a relation. Candidate Key is a proper subset of a super key. In a relation, number of super keys are more than number of candidate keys.

What is primary key and example?

A primary key is either an existing table column or a column that is specifically generated by the database according to a defined sequence. For example, students are routinely assigned unique identification (ID) numbers, and all adults receive government-assigned and uniquely-identifiable Social Security numbers.

Can a foreign key be a candidate key?

4 Answers. In the relational model of data, a foreign key must reference a candidate key. In almost all SQL dbms, a foreign key must reference a candidate key. However, the system does not enforce a requirement that the referenced columns be UNIQUE or be declared NOT NULL.

What is 1nf 2nf 3nf?

Types of Normal Forms
A relation is in 1NF if it contains an atomic value. 2NF. A relation will be in 2NF if it is in 1NF and all non-key attributes are fully functional dependent on the primary key. 3NF. A relation will be in 3NF if it is in 2NF and no transition dependency exists.

How candidate keys are different from foreign keys?

Foreign Key: FK is a key in one table (child) that uniquely identifies a row of another table (parent). A FK is not-unique in the child table. Candidate Key: A candidate key can be uniquely used to identify a database record without any extraneous data. They are Not Null and unique.

What is foreign key and primary key?


Primary key uniquely identify a record in the table. Foreign key is a field in the table that is primary key in another table. Primary Key can't accept null values. Foreign key can accept multiple null value.

What do you mean by normalization?

Normalization is a systematic approach of decomposing tables to eliminate data redundancy(repetition) and undesirable characteristics like Insertion, Update and Deletion Anomalies. It is a multi-step process that puts data into tabular form, removing duplicated data from the relation tables.

How do you identify a primary key?

Primary Keys
The primary key consists of one or more columns whose data contained within is used to uniquely identify each row in the table. You can think of them as an address. If the rows in a table were mailboxes, then the primary key would be the listing of street addresses.

What is alternative key?

An alternate key is a key associated with one or more columns whose values uniquely identify every row in the table, but which is not the primary key. For example, where the primary key for a table may be the employee id, the alternate key might combine the first, middle, and last names of the employee.

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.

Why candidate key is required?


Candidate key means exactly the same thing as primary key. All candidate keys serve the same purpose: to ensure data integrity by preventing duplicate data. It's perfectly sensible for a table to have more than one candidate key enforced in it if that's necessary to ensure some data is not duplicated.

Is a foreign key a candidate key?

Candidate Key - The candidate keys in a table are defined as the set of keys that is minimal and can uniquely identify any data row in the table. Foreign Key - A foreign key is an attribute value in a table that acts as the primary key in another another.

Can a relation have more than one key?

The primary key of a relational table uniquely identifies each record in the table. So, in order to keep the uniqueness of each record, you cant have more than one primary key for the table. Primary keys may consist of a single attribute or multiple attributes in combination.