What are keys in DBMS?

Databases are used to store massive amounts of information which is stored across multiple tables. Each table might be running into thousands of rows. Needless to say, there will be many duplicate rows with redundant information. How do we deal with that? How do we manage records so that we are storing only unique data? And, how do we relate the multiple tables that are present in the database?

SQL keys are the answer to all these queries.

An SQL key is either a single column (or attribute) or a group of columns that can uniquely identify rows (or tuples) in a table.

SQL keys ensure that there are no rows with duplicate information. Not only that, but they also help in establishing a relationship between multiple tables in the database. Therefore, it becomes imperative to learn about the different keys in SQL.

What is a Super key in SQL?

Super key is a single key or a group of multiple keys that can uniquely identify tuples in a table.

Super Key can contain multiple attributes that might not be able to independently identify tuples in a table, but when grouped with certain keys, they can identify tuples uniquely.

Let me take an example to clarify the above statement. Have a look at the following table.

Consider that Id attribute is unique to every employee. In that case, we can say that the Id attribute can uniquely identify the tuples of this table. So, Id is a Super key of this table. Note that we can have other Super Keys too in this table.

For instance — (Id, Name), (Id, Email), (Id, Name, Email), etc. can all be Super keys as they can all uniquely identify the tuples of the table. This is so because of the presence of the Id attribute which is able to uniquely identify the tuples. The other attributes in the keys are unnecessary. Nevertheless, they can still identify tuples.

What is a Candidate key?

Candidate key is a single key or a group of multiple keys that uniquely identify rows in a table.

A Candidate key is a subset of Super keys and is devoid of any unnecessary attributes that are not important for uniquely identifying tuples.

The value for the Candidate key is unique and non-null for all tuples. And every table has to have at least one Candidate key. But there can be more than one Candidate Key too.

For example, in the example that we took earlier, both Id and Email can act as a Candidate for the table as they contain unique and non-null values.

On the other hand, we cannot use the attributes like City or Gender to retrieve tuples from the table as they have no unique values.

Whereas on querying the table on the Id attribute will help us to retrieve unique tuples.

Primary Key in SQL

Primary key is the Candidate key selected by the database administrator to uniquely identify tuples in a table.

Out of all the Candidate keys that can be possible for a table, there can be only one key that will be used to retrieve unique tuples from the table. This Candidate key is called the Primary Key.

There can be only one Primary key for a table. Depending on how the Candidate Key is constructed the primary key can be a single attribute or a group of attributes. But the important point to remember is that the Primary key should be a unique and non-null attribute(s).

There can be two ways to create a Primary key for the table. The first way is to alter an already created to add the Primary key constraint on an attribute. This is shown below:

Now if I try to add a new row with duplicate Id value, it will give me an error message.

The second way of adding a Primary key is during the creation of the table itself. All you have to do is add the Primary Key constraint at the end after defining all the attributes in the table.

To define a Primary Key constraint on multiple attributes, you can list all the attributes in the parenthesis as shown below.

But remember that these attributes should be defined as non-null values otherwise the whole purpose of using the Primary key to identify tuples uniquely gets defeated.

Alternate or Secondary keys in SQL

Alternate keys are those candidate keys which are not the Primary key.

There can be only one Primary key for a table. Therefore all the remaining Candidate keys are known as Alternate or Secondary keys. They can also uniquely identify tuples in a table, but the database administrator chose a different key as the Primary key.

If we look at the Employee table once again, since I have chosen Id as the Primary key, the other Candidate Key (Email), becomes the Alternate key for the table.

Foreign key in SQL

Foreign key is an attribute which is a Primary key in its parent table, but is included as an attribute in another host table.

A Foreign key generates a relationship between the parent table and the host table. For example, in addition to the Employee table containing the personal details of the employees, we might have another table Department containing information related to the department of the employee.

The Primary key in this table is the Department Id. We can add this attribute to the Employee by making it the Foreign key in the table. We can either do this when we are creating the table or we can alter the table later to add the Foreign Key constraint. Here I have altered the table, but creating Foreign Key during table creation is similar to that for Primary Key.

Here, Dep_Id is now the Foreign Key in table Employee while it is a Primary Key in the Department table.

The Foreign key allows you to create a relationship between two tables in the database. Each of these tables describes data related to a particular field (employee and department here). Using the Foreign key we can easily retrieve data from both the tables.

Note: To operate on Foreign keys, you need to know about Joins.

Using Foreign keys makes it easier to update the database when required. This is so because we only have to make the necessary changes in limited rows. For example, if the Marketing department shifts from Kolkata to Pune, instead of updating it for all the relevant rows in the Employee table, we can simply update the location in the Department table. This ensures that there are only a few places to update and less risk of having different data in different places.

What are Composite keys?

A Composite key is a Candidate key or Primary key that consists of more than one attribute.

Sometimes it is possible that no single attribute will have the property to uniquely identify tuples in a table. In such cases, we can use a group of attributes to guarantee uniqueness. Combining these attributes will uniquely identify tuples in the table.

Consider the following table:

Here, neither of the attributes contains unique values to identify the tuples. Therefore, we can combine two or more attributes to create a key that can uniquely identify the tuples. For example, we can group Transaction_Id and Product_Id to create a key that can uniquely identify the tuples. These are called composite keys

A particular candidate key is called as a primary key over the other candidate keys. It becomes easier to deal with a database when we can have a single distinct key for a particular relation instead of having more than one key.

source: https://www.analyticsvidhya.com/blog/2020/07/difference-between-sql-keys-primary-key-super-key-candidate-key-foreign-key/