10 Differences Between union and join

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In the world of databases, you often come across terms like union and join. While both are used to combine data from multiple tables, they serve different purposes. In this article, we will explore the differences between union and join, along with their uses and examples. So, let’s dive in!

What is/are union?

Union is an operation that combines the result sets of two or more select statements into a single result set. It eliminates duplicate rows and the order of the rows may vary. The columns in the select statements must have the same data type and be in the same order.

Examples of union:

Let’s consider two tables, “Employees” and “Customers,” and retrieve the names of employees and customers:

SELECT name FROM Employees
SELECT name FROM Customers;

This query will combine the names from both tables and remove duplicates, giving us a single result set with the names of employees and customers.

Uses of union:

Union is commonly used in scenarios where you want to merge data from multiple tables or perform set operations like combining the results of two separate queries in a single result set.

What is/are join?

Join is an operation used to combine rows from two or more tables based on a related column between them. It allows you to retrieve data from multiple tables based on their relationships.

Examples of join:

Consider two tables, “Orders” and “Customers,” and we want to retrieve all orders along with the customer details. We can achieve this using a simple join:

SELECT Orders.order_id, Customers.name
FROM Orders
JOIN Customers
ON Orders.customer_id = Customers.customer_id;

This query will combine the order_id from the “Orders” table with the customer name from the “Customers” table, based on the common customer_id column.

Uses of join:

Join is widely used to extract data from related tables, enabling us to perform complex queries that involve multiple tables. It allows us to establish connections between tables and retrieve data that is meaningful and relevant.

Differences Table

Difference Area Union Join
Definition An operation that combines result sets An operation that combines rows based on related columns
Result Set Eliminates duplicates Retains duplicates
Data Type and Order Columns must have the same data type and order Columns can have different and unrelated data types
Number of Tables Can combine results of two or more select statements Combines rows from two or more tables only
Common Column Not necessary Required to join tables
Order of Rows May vary Based on the join condition
Performance Faster processing Slower processing
Result Set Columns Columns come from select statements Columns come from joined tables
Filtering Cannot apply filtering directly in union Can apply filtering using WHERE clause
Output Single combined result set Results with joined rows


In summary, union and join are both useful operations in SQL for combining data from multiple tables, but they serve different purposes. Union allows you to combine result sets, removes duplicates, and works with multiple select statements. On the other hand, join is used to combine rows based on related columns, retains duplicates, and works specifically with tables that are connected. Understanding these differences will help you choose the right operation for your data retrieval needs.

People Also Ask:

Q: Can I use both union and join in the same query?

A: Yes, you can use both union and join in the same query. For example, you can join multiple tables and then use union to combine the results with another select statement.

Q: Can I perform a join without a common column?

A: No, a join operation requires a common column between the tables to establish a relationship. Without a common column, the join cannot be performed.

Q: Are union and join the only ways to combine data from multiple tables?

A: No, there are other ways like subqueries and cross joins. Union and join are commonly used, but depending on the requirements, you can explore other techniques as well.

Q: Can I apply a WHERE clause after using union or join?

A: Yes, you can apply a WHERE clause after using union or join to filter the combined data based on specific conditions.

Q: Is there any performance difference between union and join?

A: Yes, union usually performs faster as it involves combining result sets, while join involves comparing and matching rows between tables, which can be slower depending on the size of the tables.

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