What is a Secondary Key in DBMS?

A secondary key in a database is a means of searching and sorting information, even though it is not the primary key as a unique identifier. It can help people find information more efficiently by searching on a more common field of search, such as email, phone number, or city. By using a secondary key, access to data can be faster and easier. In this blog, you are going to learn what a secondary key is, how it works, and where it is used in DBMS.

Table of Contents:

• What is a Secondary Key in DBMS?
• Characteristics of Secondary Keys in DBMS
• Role of Secondary Keys in Database Normalization
• How Secondary Keys Are Handled in SQL
  1. Creating a Secondary Key (Index)
  2. Using Secondary Keys in Queries
• Can a Secondary Key Also Be a Candidate Key?
• Difference Between Primary Key and Secondary Key in DBMS
• Best Practices for Using Secondary Keys
• Common Secondary Key Mistakes and How to Avoid Them
• Common Use Cases of Secondary Keys in a DBMS
• Conclusion

What is a Secondary Key in DBMS?

A secondary key in DBMS is a field (or fields) in a table that isn’t the primary key, but is still very helpful when users are looking to search, sort, or filter data by its common fields. The primary key is used to identify each record uniquely. The secondary key provides efficient pathways for users to locate records based on common referencing fields. Secondary keys make queries more efficient and are generally indexed to make searching faster. Secondary keys enhance the usability of large databases that use a user to search, filter, or group columns of records. Consider a student database; If you wanted to look up the email address or phone number of a student, you wouldn’t use those records as unique identifiers, as they would be helpful to locate the record for that student. Therefore, they can be treated as secondary keys.

Let us understand with the help of an example:

CREATE TABLE Students (
    StudentID INT PRIMARY KEY,
    Name VARCHAR(100),
    Email VARCHAR(100),
    PhoneNumber VARCHAR(15)
);

Explanation: Here, in this table, StudentID is the primary key that uniquely identifies each student. Email is not the primary key, but most users often use it to search for students. Therefore, it can be considered a secondary key.

Master Database Management: Organize, Secure, and Scale Your Data

Learn to design, manage, and optimize modern databases with real-world tools and industry best practices.

Explore Program

Characteristics of Secondary Keys in DBMS

• Not necessarily unique: Secondary keys can contain duplicate values, unlike primary keys.
• Used for searching: They are often used in queries with WHERE, ORDER BY, or GROUP BY.
• Often indexed: They are often indexed to optimize data retrieval
• No uniqueness enforced: Contains duplicates and NULL values (unless you create a unique index).

Role of Secondary Keys in Database Normalization

Normalization of a database refers to the process of organizing data in a way that minimizes redundant data. 

We use secondary keys to: 

• Join related tables on common fields.
• Speed up searches for the columns people use most often.
• Listen and filter (by email, department, city ).

In our example, the Email Column will help in searching for a student, even though Email is not the primary key.

How Secondary Keys Are Handled in SQL

Let’s look at how Secondary Keys in SQL are handled

1. Creating a Secondary Key (Index)

You can create an index on the column to define the secondary key.

CREATE INDEX idx_employee_department
ON employees(department);

Explanation: Here, this will help you to create a secondary index on the department column in the employees table.

2. Using Secondary Keys in Queries

The SQL engine quickly searches or filters the data if the column is indexed.

SELECT * FROM employees
WHERE department = 'Sales';

Explanation:  A secondary key, which is indexed, helps the database to find the matching rows faster without needing to scan the whole table.

Can a Secondary Key Also Be a Candidate Key?

Yes, a secondary key can also be a candidate key if it can identify every row in a table, just like a primary key.

For our example with the Students table:

• If all of the students have unique email addresses, the Email column is a candidate key.
• This means that Email could be used as the primary key, but has not been chosen as such.

Because StudentID has been elected as the primary key, the Email field can stay as a secondary key or candidate key, unless we want to replace the primary key with it.

A table can have many candidate keys, but only one candidate key can be elected as the primary key. The others can either be used as secondary keys or kept as candidate keys. This gives us flexibility in how we are able to organize and find data.

Difference Between Primary Key and Secondary Key in DBMS

Best Practices for Using Secondary Keys

• Use for Search Optimization: Secondary keys are best suited to columns that are often used in WHERE, JOIN, or ORDER BY clauses. Indexing those columns will drastically speed up your queries.
• Avoid Excessive Indexing: It is easy to get carried away by creating secondary keys on many columns. Excessive indexing can make inserting and updating slower and consume more storage.
• Find Columns with High Selectivity: Look for columns with a broad range of distinct values (e.g., email, phone number) for optimal performance when using as a secondary key.
• Use Consistent Naming Conventions: Use clear and different names for additional secondary key indexes in your schema (e.g., idx_customers_city).
• Combine Keys Appropriately: When using composite secondary keys, ensure that the order of the columns reflects typical querying patterns.

Common Secondary Key Mistakes and How to Avoid Them

• Using Low-Selectivity Columns as Secondary Keys: Choose columns with a lot of unique values for better filtering performance. 
• Forgetting to Index Secondary Key Columns:  Always create indexes on secondary keys that are used in WHERE, JOIN, or ORDER BY clauses frequently. 
• Treating Secondary Keys Like Primary Keys: Realising that Secondary Keys can have duplicate values, so they cannot uniquely identify records. 
• Adding Too Many Secondary Keys: Limit secondary keys to only those columns that are important, as too many secondary keys will make performance poor and increase storage. 
• Not Updating Indexes After Changes to the Schema: After making major changes to the schema or making changes that can influence query patterns, you should reevaluate and update secondary key indexes. 

Learn Database Management for Free

Master the fundamentals of data storage, SQL, and database design—at zero cost. Start building your data career today!

Explore Program

Common Use Cases of Secondary Keys in a DBMS

1. Filtering and Searching

Example: Searching employees by department or city.

Secondary keys allow for efficient search queries, enabling the database to access the returned rows quickly.

2. Sorting and Reporting

Example: Creating a report of students sorted by grade or age.

Secondary keys improve the performance of report data access and methods of organizing it for very large datasets.

3. Joining Tables

Example: Joining on product_name, which is a non-unique field, to another table in the process of analytics.

Secondary keys make it more efficient, as there are fewer full table scans.

4. Supporting UI Queries

Example: Quickly filtering in a dashboard, dropdowns, etc. (filtering orders by region or status).

They support a better user experience by speeding access to filtering, and lookup on interactive queries.

Conclusion

Although secondary keys aren’t unique like primary keys, they enhance the performance of a database. Secondary keys provide quick search, filter, sort, and join functionality based on indexed columns. When done correctly, secondary keys speed up queries and provide a better user experience. Be careful not to use too many secondary keys or use them on the wrong columns. If used properly, secondary keys can enhance your database design.

You can learn more about SQL in the SQL Course and also explore SQL Interview Questions prepared by industry experts.

What is a Secondary Key in DBMS-FAQs