We all know what rows and columns are in a relational database. Column names include Aadhar Number, Roll Number, Marks, and so on. we want to retrieve the data for a specific column, we may do so simply by selecting the column with the specified name. But what if we want to get to a certain table row? How can we access a specific row if it doesn't have a name? As a result, there must be some characteristic, attribute, or technique that allows us to uniquely identify a row from the database. This is where the keys come into play.
Some of the keys we use in SQL and databases are primary keys, unique keys, foreign keys, superkeys, alternate keys, composite keys, and so on. The distinction between Superkey Key and Candidate Key will be discussed in this blog. Before diving into the differences, it's important to first grasp the concept of database keys.
what are the keys?
A key is a necessary component of any relational database. In a database management system, a key is an attribute or group of attributes that help us distinguish a tuple in a table. Keys are also used to specify different integrity constraints and to construct associations between different database tables. In a relational database, a table represents a set of records or events for a specific relationship. Hundreds, if not thousands, of such records may now exist, some of which may be repeated. As a result, there should be a means to identify each record individually, ensuring that there are no duplicates. We can also be rid of this obstacle thanks to the keys.
What is a superkey, exactly?
In a relational database, a "superkey" is a single attribute or a group of attributes that can uniquely identify a row/tuple.
OR
A "superkey" is a single attribute or a combination of attributes that may be used to identify all other attributes in a unique way.
Consider the following fields in an employee table:
We can uniquely identify a row in the above-mentioned table by using Emp_Id, Emp_Email, Emp_Id and Emp_Name, Emp_Id and Emp_Email, and so on. As a result, each of them serves as a superkey for the employee table.
Set of all the super key is
- { Emp_Id }
- { Emp_Email }
- { Emp_Id , Emp_Name }
- { Emp_Id , Emp_Email }
- { Emp_Id, Emp_Department }
- { Emp_Email, Emp_Name }
- { Emp_Email, Emp_Department }
- {Emp_Id, Emp_Name, Emp_Email }
- {Emp_Id, Emp_Name, Emp_Department }
- { Emp_Name,Emp_Email,Emp_Department }
Superkey's Features
- Each row in a table is given a unique identifier.
- In a table, there may be several superkeys.
- It might be made up of a single or numerous columns.
- It serves as the foundation for selecting candidate keys.
What is the purpose of a Candidate Key?
A candidate key is said to be a superkey if any of its appropriate subsets is not a superkey in and of itself. It's just a stripped-down version of the superkey. A table can have a single candidate key or numerous candidate keys. The number of potential keys is determined by the relational table.
Now we'll revisit the employee database and see if we can extract Candidate keys from it.
Let's look at the superkeys {Emp_Id ,Emp_Department"; "Emp_Id" and "Emp_Department" are subsets of this superkey. Is any of the appropriate subsets a superkey in and of itself? Yes, because Emp_Id is a superkey in and of itself, it cannot be defined as a Candidate Key.
Let's look at the superkey Email_Id now. Because there is no valid subset of this superkey that is a superkey in and of itself, we can consider it a Candidate Key.
In a table, there may be several candidate keys, and they are all selected using the same process as before.
Set of all candidate keys:
- { Emp_Id }
- { Emp_Email }
The database administrator chooses one of the candidate keys as the primary key after locating all of them, and it serves as the primary means of identifying tuples. Prime attributes refer to the Candidate key's attributes.
Candidate key's Features
- It only has one-of-a-kind values.
- It identifies each row in a table in a unique way.
- It could have a variety of characteristics.
- To ensure uniqueness, it comprises only the bare minimum of fields.
Super key Vs Candidate key
Every Candidate key is Super key but every Super key can’t be Candidate key. Various Candidate keys can collectively select primary keys whereas, in Super key, a set of Super keys collectively selects candidate keys. In a relational database, there are fewer candidate keys as compared to super keys in the table.
For more references on super keys refer to Scaler Topics
Author: Arnav bhardwaj