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JOIN in SQL vs. UNION in SQL: What's the Difference?

Edited by Aimie Carlson || By Harlon Moss || Published on February 28, 2024
JOIN in SQL combines rows from two or more tables based on a related column. UNION in SQL combines results of two or more SELECT queries into a single result set.

Key Differences

JOIN in SQL is used to combine rows from two or more tables based on a related column, allowing for complex queries across multiple tables. UNION in SQL, on the other hand, is used to combine the results of two or more SELECT queries into a single result set, concatenating rows from these queries. While JOIN merges data horizontally, UNION does so vertically.
In SQL, a JOIN operation is pivotal in relational database management for fetching data that spans multiple tables, creating a relational link based on common fields. In contrast, the UNION operation is more about aggregating results from different SELECT queries, irrespective of relational links, as long as the data structure is similar. JOIN focuses on relational connections, UNION on aggregation of results.
JOIN clauses in SQL, including INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN, offer versatility in how tables are combined, catering to specific relational needs. Conversely, UNION in SQL, along with its variations UNION ALL, primarily focuses on combining datasets with identical columns, without regard to the relational aspects of the data. JOIN provides relational flexibility, UNION simplicity in aggregation.
When using JOIN in SQL, data integrity and relationships are essential as it relies on keys that link tables together. With UNION, the emphasis is on the uniformity of data structure across SELECT statements, as it combines rows from different queries into one dataset. JOIN emphasizes relational data integration, UNION focuses on uniform data structure.
Performance considerations differ between JOIN and UNION in SQL. JOIN operations can be resource-intensive, especially with large datasets and complex relationships. UNION operations, while seemingly simpler, also require attention to performance, particularly in data sorting and duplication removal. JOIN demands careful consideration of relationships and performance, UNION requires attention to data structure and duplication.

Comparison Chart


Combines rows from multiple tables based on a related column
Combines results of multiple SELECT queries into a single set

Data Relation

Requires a relational link between tables
Does not require relational links, only structurally similar data


Includes UNION and UNION ALL

Result Structure

Results in a single table with combined columns
Results in a table with the same columns, aggregated rows

Use Case

Used for complex queries across multiple tables
Used for simple aggregation of similar data from different sources

JOIN in SQL and UNION in SQL Definitions


Merges rows from two tables where there are matching values in a common column.
SELECT * FROM Orders JOIN Customers ON Orders.CustomerID = Customers.CustomerID;


Concatenates the results of multiple queries having the same number of columns.
SELECT city FROM customers UNION SELECT city FROM suppliers;


Allows querying data from multiple tables as if they were a single table.
SELECT product.name, order.amount FROM product JOIN order ON product.id = order.product_id;


Aggregates results from different SELECT queries, eliminating duplicate rows.
SELECT product FROM store1 UNION SELECT product FROM store2;


Creates a result set that combines data from multiple tables.
SELECT A.name, B.salary FROM tableA A JOIN tableB B ON A.emp_id = B.emp_id;


Combines the results of two or more SELECT statements into a single result set.
SELECT name FROM companyA UNION SELECT name FROM companyB;


Combines fields from different tables by using values common to each.
SELECT Employees.Name, Departments.Name FROM Employees JOIN Departments ON Employees.DeptID = Departments.ID;


Used to combine similar data from different tables or queries.
SELECT employee_name FROM departmentA UNION SELECT employee_name FROM departmentB;


Links tables based on a related column to form a new table.
SELECT name, order_date FROM customers JOIN orders ON customers.id = orders.customer_id;


Merges distinct results from multiple tables into one result.
SELECT name FROM alumni UNION SELECT name FROM current_students;


When would you use JOIN instead of UNION?

Use JOIN when you need to merge data from different tables based on a relationship.

What is JOIN in SQL?

JOIN in SQL is used to combine rows from multiple tables based on related columns.

Does JOIN in SQL require a common column?

Yes, JOIN operations typically require at least one common column in the tables.

What is UNION in SQL?

UNION in SQL combines the results of two or more SELECT queries into a single result set.

What's the difference between UNION and UNION ALL?

UNION removes duplicates, while UNION ALL includes all rows, duplicates included.

Can you JOIN more than two tables?

Yes, SQL allows joining multiple tables in a single query.

Can UNION combine data from unrelated tables?

Yes, as long as the data structure of the SELECT statements is the same.

Does UNION remove duplicate rows?

Yes, UNION removes duplicates, but UNION ALL keeps them.

What types of JOINs are there in SQL?

Common types include INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN.

Can JOIN be used for complex queries?

Yes, JOIN is ideal for complex queries across multiple related tables.

Is it necessary to have the same column names for UNION?

No, column names can differ, but the number and type of columns must match.

What's a common use case for JOIN?

JOIN is commonly used in reports that require data from multiple related tables.

Is UNION limited to identical columns?

Yes, the columns in UNION queries must be the same in number and compatible in data type.

Does UNION handle large datasets well?

UNION can be efficient but requires attention to data sorting and duplication.

How do you choose between JOIN and UNION?

It depends on whether you need to combine data based on relationships (JOIN) or aggregate similar data (UNION).

Is UNION a good choice for simple aggregations?

Yes, UNION is suitable for simple aggregation of data from different sources.

How does JOIN affect query performance?

JOIN can be resource-intensive, especially with large datasets and complex relationships.

Can you use aliases in JOIN operations?

Yes, aliases can be used to simplify complex JOIN queries.

Can UNION be used for data from the same table?

Yes, UNION can combine results from the same table in different SELECT queries.

When is UNION most useful?

UNION is most useful when needing to combine similar data from different tables or queries for a unified view.
About Author
Written by
Harlon Moss
Harlon is a seasoned quality moderator and accomplished content writer for Difference Wiki. An alumnus of the prestigious University of California, he earned his degree in Computer Science. Leveraging his academic background, Harlon brings a meticulous and informed perspective to his work, ensuring content accuracy and excellence.
Edited by
Aimie Carlson
Aimie Carlson, holding a master's degree in English literature, is a fervent English language enthusiast. She lends her writing talents to Difference Wiki, a prominent website that specializes in comparisons, offering readers insightful analyses that both captivate and inform.

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