INNER JOIN vs. UNION in SQL – Understanding the Key Differences

Introduction

Structured Query Language (SQL) is widely used for managing and querying databases. When working with multiple tables, we often need to combine data in meaningful ways. Two common techniques for achieving this are INNER JOIN and UNION.

At first glance, both INNER JOIN and UNION seem similar since they allow us to combine data from different tables. However, they serve different purposes and function differently.

In this article, we will explore the differences between INNER JOIN and UNION, when to use each, and their practical applications.

What is INNER JOIN in SQL?

Definition

INNER JOIN is used to retrieve records from two or more tables based on a common column. It returns only the matching rows from the participating tables.

Syntax of INNER JOIN

SELECT t1.column_name, t2.column_name  

FROM table1 t1  

INNER JOIN table2 t2  

ON t1.common_column = t2.common_column;

Example of INNER JOIN

Let’s say we have two tables:

Customers Table:

Orders Table:

Now, if we use an INNER JOIN to retrieve the customer names and their corresponding orders:

SELECT Customers.Name, Orders.Order_Amount  

FROM Customers  

INNER JOIN Orders  

ON Customers.Customer_ID = Orders.Customer_ID;

Output:

The row with Customer_ID = 3 (Charlie) is missing because there is no matching record in the Orders table.

Similarly, the row with Customer_ID = 4 in Orders is missing because it does not exist in the Customers table.

INNER JOIN ensures that only the rows that exist in both tables are included in the result.

When to Use INNER JOIN?

When you need to combine data based on a relationship.

When you want to exclude unmatched records.

When working with relational databases where foreign keys define table relationships.

What is UNION in SQL?

Definition

UNION is used to combine the results of two or more SELECT statements into a single result set. It removes duplicates by default.

Syntax of UNION

SELECT column_name FROM table1  

UNION  

SELECT column_name FROM table2;

Example of UNION

Let’s consider two different tables:

Employees Table:

Contractors Table:

Now, let’s use UNION to get a list of all unique people and their departments:

SELECT Name, Department FROM Employees  

UNION  

SELECT Name, Department FROM Contractors;

Output:

Alice appears only once, even though she is present in both tables.

(UNION automatically removes duplicate rows)

What if You Want to Include Duplicates?

You can use UNION ALL, which keeps duplicate values:

SELECT Name, Department FROM Employees  

UNION ALL  

SELECT Name, Department FROM Contractors;

This will return all rows, including duplicates.

When to Use UNION?

When two tables have similar structure and you want to combine their results.

When you don’t need to join tables based on common columns.

When removing duplicates is important (UNION) or keeping them (UNION ALL).

INNER JOIN vs. UNION – Key Differences

1. Performance Issues:

INNER JOIN can slow down queries if tables are large and not indexed properly.

UNION can be slow because SQL removes duplicates, which requires extra computation.

2. Handling NULL Values:

INNER JOIN excludes rows with NULLs in the common column.

UNION keeps NULL values as long as they are not duplicates.

3. Data Type Mismatch:

UNION requires columns to have matching data types, or an error will occur.

INNER JOIN allows different data types but may require type conversion (CAST or CONVERT).

4. Indexing Considerations:

Proper indexing speeds up INNER JOIN queries.

UNION queries may require temporary tables to process large datasets efficiently.

Conclusion

INNER JOIN and UNION both serve the purpose of combining data but are used in completely different scenarios.

Use INNER JOIN when working with relational tables and needing to retrieve only matching data.

Use UNION when dealing with separate datasets and needing to combine all results.

Understanding the differences will help you write optimized queries, improve database performance, and avoid unnecessary complexity in SQL operations.

Would you like to see real-world examples with performance comparisons? Let me know in the comments!

Final Thoughts

I hope this article helps you understand INNER JOIN and UNION better. If you're preparing for SQL interviews or working on database optimization, mastering these concepts will be highly beneficial!