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JOIN vs. UNION: Which Database Operator Should You Use?

Linda Hamilton
Release: 2025-01-15 07:06:44
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JOIN vs. UNION: Which Database Operator Should You Use?

JOIN and UNION in database operations

In database operations, the two operators JOIN and UNION play completely different roles. Understanding their differences is critical for effective data manipulation.

JOIN operator

The JOIN operator combines rows from two or more tables based on common or related columns. It forms the Cartesian product of individual table rows and filters out the appropriate subset based on the specified join conditions. This allows cross-referencing data between tables and extracting insights from their relationships.

Example:

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<code class="language-sql">SELECT * FROM table1 JOIN table2 ON table1.id = table2.id;</code>

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This query retrieves all columns in rows in table1 and table2 where all id columns match.

UNION operator

In contrast, the UNION operator appends the result rows of one query to another query. It combines the results of multiple queries into a single rowset without establishing any relationship between them. The results of the UNION operator do not exhibit any cross-join properties.

Example:

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<code class="language-sql">SELECT * FROM table1 UNION SELECT * FROM table2;</code>

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This query will produce a table containing all rows from table1 and table2, arranged one after the other, regardless of any specific join conditions.

Main differences

  • JOIN combines rows based on relationships, while UNION appends rows.
  • JOIN filters rows based on specified criteria, while UNION does not filter.
  • The result of a JOIN is a new table containing cross-joined rows, while a UNION produces a single table containing additional rows.

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