JOIN vs. UNION: What's the Difference in SQL Data Manipulation?
Jan 15, 2025 am 08:30 AMSQL 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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