What is Candidate Key in DBMS And Difference Between Keys in SQL

Summary: In this article, you will learn what is candidate key in DBMS and also learn the difference between keys in SQL. The candidate key in DBMS is a single key or a group of multiple keys that uniquely determine or identify rows in a table. Let’s understand with an example in detail.

What is Candidate Key in DBMS

Basically, a candidate key is a part of the super key only. We can describe or we can say that a candidate key is a combination of attributes of a table that can uniquely identify/determine other attributes of the table. A candidate key in dbms is also called a minimal super key.

Furthermore, as we know, the candidate key is chosen or selected from the set of super keys. Among the super keys set, the key which doesn’t contain any redundant attribute is selected as the candidate key.

Candidate Key in DBMS With Example

The value for the Candidate key’s in SQL is unique and non-null for all tuples. and each table has to have at least one(1) Candidate key. however, there can be over one Candidate Key too.

For instance, in the example that we took earlier, both Stu_ID and Stu_Email can act as a Candidate for the table as they contain unique and non-null values.
Candidate Key in DBMS

Properties of Candidate Key

  • The candidate key can uniquely identify or determine all the other attributes of a table.
  • The candidate key mustn’t have redundant attributes.
  • Basically, The candidate keys are the subset of the super key. Hence, a candidate key is a super key however, vice versa is not correct.
  • The attributes of a candidate key can hold null values.
  • A Primary key(PK) is determined or identified from the set of candidate keys by the Data Base Administrator.
  • The candidate key helps in determining the prime & non-prime attributes of a table.
  • Also, the candidate key ensures or guarantees the integrity of the data by preventing duplicate data.
  • Always, the candidate key must be distinctive or unique.

Difference Between Keys in SQL

  • SQL keys are basically used for the purpose of uniquely identifying rows in a table.
  • SQL keys can either be a single column or a set of columns.
  • A super key is a single key or a set of multiple keys that can uniquely identify or determine tuples in a table.
  • Furthermore, a super key can contain redundant attributes that might not be necessary for identifying tuples.
  • Basically, candidate keys are a sub-set of Super keys. which contain only those attributes which are required/needed to uniquely identify tuples.
  • All Candidate keys are Super keys. But the vice-versa isn’t true.
  • The primary key is a Candidate key chosen to uniquely identify/determine tuples in the table.
  • Primary key values always must be unique and non-null.
  • There can be multiple Super keys & Candidate keys in a table, but remember that there can be only one Primary key in a table.
  • Alternate keys are those Candidate keys that weren’t chosen or selected to be the Primary key of the table.
  • A composite key is also known as a Candidate key that consists of more than one attribute.
  • The foreign key is an attribute that is a Primary key(PK) in its parent table. however, is enclosed or included as an attribute in the host table.
  • Foreign keys(FK) may accept non-unique and null values.

Read More:Difference Between Primary Key and Foreign Key

Super Keys Vs Candidate Keys

  • The use of super keys and candidate keys is most complicated.
  • Essentially, a super key is composed of a group of attributes (and by proxy, typically a group of table columns) that identify or determine a unique record.
  • The candidate key also determines a unique record but is composed of one(1) individual field or column
  • You will typically see the candidate key referred to as a “minimal” of a super key.
  • For instance, consider a manufacturing facility with two terminals, wherever only one product can be built at a given terminal at a specific time.
  • As the database logs during the creation of products on a table, referencing time & terminal, the super key, the combination or merger of time and terminal, will be unique for every individual record. but, the individual columns’ time and terminal will have redundancies.
  • If the database engineer attaches a candidate key to the table composed of discretionary alphanumerical input, that candidate key will do the identical thing that the time and terminal super key attribute combination does, with just one(1) column.
  • With all of this in mind, the utilization of a candidate key can identify how records are accessed & how the subjects of those records are identified.

Candidate Keys Vs Primary Keys

  • The relationship between the candidate key and the primary key is easier to explain — basically, a relational database can have more than one candidate key, but only one of these candidate keys will be the primary key(PK) that will be primarily utilized to identify or determine every record uniquely.
  • The best way to define candidate keys is with an instance: A bank’s database is being designed. To uniquely describe each customer’s account number, a combination of the customer’s birthdate & a sequential number for every of his or her accounts can be used.
  • So, Mr. Aditya Ahukla’s checking account can be numbered 117345-1, and his savings account 117345-2. A candidate key has simply been created.

    This can raise problems.

    What if more than one person with an identical birth date wants to open an account with the bank?

    Because of such potential pitfalls, an often-used option is to create a unique candidate key.
    Therefore, in that situation, the bank’s database can issue unique account numbers for customers which are guaranteed to prevent issues just highlighted.
    For good measure, these bank account numbers can have few built-in logic.

  • For instance, checking accounts can begin with a “B,” followed by the year and month of creation, and within that month, a sequential number. So, Aditya Shukla’s checking account can now be B-200706-22.
  • So if a database table has two or more unique identifiers, thus the database administrator will choose one of those to be the PK-primary key