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

SQL JOIN and UNION: A Detailed Comparison

SQL provides two powerful operators, JOIN and UNION, for manipulating data across multiple tables. This article clarifies their distinct functionalities with illustrative examples.

JOIN and UNION: How They Differ

JOIN merges rows from different tables based on a shared column or condition, generating a combined result set containing fields from both tables. Conversely, UNION vertically stacks the results of multiple SELECT statements, producing a single result set encompassing all rows from each query.

Understanding the JOIN Operator

JOIN (including variations like INNER JOIN and LEFT JOIN) initially creates a Cartesian product of the involved tables. Subsequently, it filters this product based on the defined join conditions. This allows for integrated data analysis from diverse sources.

JOIN Example

SELECT * 
FROM table1
INNER JOIN table2 ON table1.id = table2.id;

This INNER JOIN returns only rows where the id column matches in both table1 and table2.

Understanding the UNION Operator

UNION (including UNION ALL) combines results from multiple SELECT statements into one unified result set. Unlike JOIN, UNION doesn't filter or compare rows; it simply concatenates them.

UNION Example

SELECT * FROM table1
UNION
SELECT * FROM table2;

This UNION statement returns all rows from both table1 and table2, eliminating duplicate rows. UNION ALL would retain duplicates.

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