JOIN vs. UNION: When Should I Use Each Database Operation?

JOIN and UNION: Differences in Database Operations

In the field of database operations, JOIN and UNION are two completely different data processing mechanisms. Both involve combining information from multiple data sources, but the paths they take and the end results are very different.

JOIN: Create Cartesian product

For each row in one table, JOIN attempts to pair it with every row in the other table that meets the specified criteria. This process creates a huge Cartesian product, where each combination forms a result row in the output. Filters or constraints can be applied to filter this Cartesian product and return only the required matches.

UNION: append line

In contrast, UNION takes two or more queries and appends the result rows one after the other. This operation does not attempt to establish relationships between rows or create new rows. It simply concatenates the query results, potentially eliminating duplicate rows if the UNION ALL variant is used.

Example: Explain the difference

To illustrate the difference more clearly, let us consider the following table:

<code>表 1：
+-----+--------+
| ID   | Name   |
|-----+--------|
| 1    | Alice  |
| 2    | Bob    |
+-----+--------+

表 2：
+-----+------------+
| ID   | Occupation |
|-----+------------|
| 1    | Developer  |
| 3    | Manager   |
+-----+------------+</code>

Using the ID column as the join condition, a JOIN between these two tables will produce the following results:

<code>+-----+--------+------------+
| ID   | Name   | Occupation |
|-----+--------+------------|
| 1    | Alice  | Developer  |
| 2    | Bob    | NULL      |
+-----+--------+------------+</code>

This result contains a combination of rows from both tables where the ID values ​​match. Conversely, a UNION of the same two queries will produce:

<code>+-----+--------+
| ID   | Name   |
|-----+--------|
| 1    | Alice  |
| 2    | Bob    |
| 1    | Developer  |
| 3    | Manager   |
+-----+--------+</code>

Here, the rows from both queries are simply appended, no matching or relationship is established.

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