JOIN in SQL vs. UNION in SQL — What's the Difference?
By Tayyaba Rehman — Published on January 3, 2024
JOIN combines rows from two or more tables based on a related column, UNION combines result sets of two or more SELECT statements.
Difference Between JOIN in SQL and UNION in SQL
Table of Contents
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Key Differences
JOIN in SQL is used to retrieve data that appears in two or more tables by linking them through a common field, effectively combining columns from each table. UNION, on the other hand, is used to combine the results of two or more SELECT statements, stacking them on top of one another, thus combining rows.
A JOIN operation in SQL can be of different types like INNER JOIN, LEFT JOIN, RIGHT JOIN, or FULL JOIN, depending on the set of records you want to retrieve. Whereas UNION simply concatenates the result sets of two queries, but it eliminates duplicate rows in the process. The UNION ALL command can be used if the duplicates need to be retained.
When using JOIN, the size of the result set can increase since it merges data horizontally by adding columns. In contrast, UNION merges data vertically by adding rows, which could potentially increase the number of rows in the result set. JOIN is about correlating data from different tables, whereas UNION is about aggregating results from separate queries.
A JOIN clause is often used when there's a logical relationship between different tables, like an order and the customer who made it. UNION is typically used when you have distinct, separate queries that you wish to combine into a single result, such as sales figures from different years.
With JOIN, one can filter the joined data using a WHERE clause, combining the ON clause, which dictates how the tables should be linked. With UNION, each SELECT statement within the UNION must have the same number of columns in the result sets with similar data types.
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Comparison Chart
Operation Type
Combines columns from two or more tables
Combines rows from two or more SELECT statements
Result Structure
Adds columns to the result set
Adds rows to the result set
Duplicate Handling
Can result in duplicates if not managed
Automatically removes duplicates (unless UNION ALL)
Column Requirements
Number and type of columns can differ
Number and type of columns must be the same
Relationship Between Data
Requires a logical association between the tables
No relationship needed between the result sets
Compare with Definitions
JOIN in SQL
Used with an ON clause to define the join condition.
SELECT * FROM books JOIN authors ON books.author_id = authors.id;
UNION in SQL
Combines the result sets of multiple SELECT statements.
SELECT column_name FROM table1 UNION SELECT column_name FROM table2;
JOIN in SQL
Merges rows from two or more tables.
SELECT * FROM table1 JOIN table2 ON table1.id = table2.id;
UNION in SQL
Eliminates duplicate rows between the various result sets.
SELECT city FROM customers UNION SELECT city FROM suppliers;
JOIN in SQL
Can specify types like INNER, LEFT, RIGHT, and FULL.
SELECT * FROM employees LEFT JOIN departments ON employees.department_id = departments.id;
UNION in SQL
UNION ALL selects all values including duplicates.
SELECT employee_name FROM sales UNION ALL SELECT manager_name FROM management;
JOIN in SQL
Allows for complex queries by joining multiple tables.
SELECT * FROM users JOIN posts ON users.id = posts.user_id JOIN comments ON posts.id = comments.post_id;
UNION in SQL
Each SELECT statement within the UNION must have the same number of columns.
SELECT name, age FROM actors UNION SELECT name, age FROM directors;
JOIN in SQL
Combines columns from related tables.
SELECT name, order_date FROM customers INNER JOIN orders ON customers.id = orders.customer_id;
UNION in SQL
Columns must also have similar data types for the UNION to work.
SELECT first_name FROM employees UNION SELECT customer_name FROM customers;
Common Curiosities
Do all databases support all types of JOINs?
Most relational databases support the basic JOIN types, but specific support may vary.
Is there a performance difference between JOIN and UNION?
Yes, performance can vary; JOINs are generally faster than UNIONs, but this depends on the dataset size and indexing.
Can UNION be used with different tables?
Yes, as long as the selected columns have the same data types.
How many tables can be joined in a single SQL query?
There's technically no limit, but performance degrades with many joins, and the query can become difficult to manage.
When would you use UNION ALL instead of UNION?
Use UNION ALL when you need to include all duplicates, not just distinct rows.
Can you use aliases with UNION?
Yes, column aliases can be used in the SELECT statements involved in a UNION.
Can ORDER BY be used with UNION?
Yes, but it must be placed at the end of the UNION statement, not on individual SELECT statements.
Does JOIN order affect the result set?
The order of joins doesn't typically affect the result set but can affect performance.
What types of JOINs are there in SQL?
There are several types, including INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN.
Can the WHERE clause be used with UNION?
Yes, each SELECT statement in a UNION can have its own WHERE clause.
What happens to NULL values in JOIN operations?
JOINs can exclude rows with NULL values in the join column unless specifically handled.
What is the difference between CROSS JOIN and UNION?
CROSS JOIN creates a Cartesian product of two tables, while UNION combines the results of two queries.
Are JOINs or UNIONs better for combining data from multiple tables?
It depends on whether you need to combine rows or columns and the relationship between the data.
Is it possible to join a table to itself?
Yes, this is called a self-join and it's done by giving the table different aliases.
How does UNION impact NULL values?
UNION treats NULL values as equal for the purposes of eliminating duplicates.
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Written by
Tayyaba RehmanTayyaba Rehman is a distinguished writer, currently serving as a primary contributor to askdifference.com. As a researcher in semantics and etymology, Tayyaba's passion for the complexity of languages and their distinctions has found a perfect home on the platform. Tayyaba delves into the intricacies of language, distinguishing between commonly confused words and phrases, thereby providing clarity for readers worldwide.
