Difference Between Superkey And Candidate Key in SQL Explained

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We know that in Relational Database, we have rows and columns. Columns have particular names like Aadhar Number, Roll no, Marks, etc. So if we want to access the data of a particular column, we can easily do so by selecting the column with the given name. But what if we want to access a particular row of a table? Rows don't have any specific name, then how can we access a particular row? So, there must be some property, attribute, or method through which we can uniquely identify a particular row from the database. Here is when keys come into the picture.

There are many keys that we use in SQL and databases, like Primary key, Unique key, Foreign key, Superkey, Alternate Key, Composite Key, etc. Before jumping directly to the difference between superkey and candidate key, let's first understand about keys in the database.

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What are keys in DBMS?

Keys are an essential element of any relational database. A key in a DBMS refers to an attribute or set of attributes that help us to distinguish a tuple in a table. Keys are also used to establish relationships between different database tables and to define different integrity constraints.

A table in a relational database represents a set of records or events of a particular relation. There may now be hundreds upon hundreds of such records, some of which may be repeated. So, there should be a way to identify each record separately so there are no duplicates. The keys allow us to be free from this difficulty.

Here in this article, we will discuss the difference between superkey key and candidate key. 

What is a Superkey?

A superkey is a single attribute or a set of attributes that can uniquely identify a row/tuple in a relational database.

Let's consider an Employee Table with the following fields: 

In the above-mentioned table, we can identify uniquely identify a row using Emp_Id, Emp_Email, Emp_Id and Emp_Name, Emp_Id And Emp_Email, and so on. So they all act as a superkey for the above-mentioned Employee Table.

Properties Of Superkey

• It uniquely identifies each row in a table.
• There may be multiple superkeys in a table.
• It may be formed from a single column or multiple columns.
• It forms the basis for selecting the candidate key.

What is a Candidate Key?

Candidate key is the superkey whose any of the proper subset is not a superkey in itself.

Candidate key is basically the minimal version of the superkey. There may be a single candidate key in a table or multiple candidate keys in the table. The count of candidate keys depends on the relational table we have.

Now, we will consider the above-mentioned Employee table once again and will try to find candidate keys from it.

Now let's consider the superkey { Emp_Id, Emp_Department }. Subsets of this superkey are { }, {Emp_Id}, {Emp_Department}, { Emp_Id, Emp_Department } . Now is any of the proper subsets a superkey in itself? Yes, here {Emp_Id} is a superkey in itself, so it can't be considered as a candidate key according to the definition.

Let's take the superkey {Email_Id}. Proper Subset of this superkey is { } only. So, there is no proper subset that is a superkey in itself, and hence, we can consider it as a candidate key.

There may be numerous candidate keys in a table, and the selection of candidate keys takes place using the same method as explained above.

The attributes of the Candidate key are called prime attributes. After determining all the candidate keys, one of the candidate keys is chosen as the primary key by the Database Administrator. It acts as a primary means to identify tuples. 

Properties of Candidate key

• It contains only unique values.
• It uniquely identifies each row in a table.
• It may have multiple attributes.
• It contains minimum fields to ensure uniqueness.

Difference between a Superkey and Candidate key

Following are the mail differences between a superkey and a candidate key. 

To sum it up, the key difference between both is 'Every Candidate key is a Superkey but every Superkey is not a Candidate key'.

FAQs 

1. How are superkeys and candidate keys related? 

 Superkeys and candidate keys are related in that every candidate key is a superkey, but not every superkey is a candidate key. Candidate keys are a subset of superkeys that are chosen for their uniqueness and minimality. A superkey may include attributes that are not necessary for uniqueness, while a candidate key is specifically chosen to avoid such unnecessary attributes. When designing a database schema, identifying candidate keys is important to ensure efficient data storage and integrity.

2. Why is a superkey called so? 

A superkey is called so because of its ability to ensure the uniqueness of rows within a table and the flexibility it offers in uniquely identifying rows.

3. Can a candidate key be composed of multiple columns?

Yes, a candidate key can indeed be composed of multiple columns. In fact, a candidate key can consist of any combination of one or more attributes (columns) within a relational database table. Candidate keys are sets of attributes that uniquely identify individual rows in the table. The key requirement for a candidate key is that it must be both unique and irreducible, meaning no subset of its attributes can provide the same unique property.

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