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

Key differences between JOIN and UNION

In the field of database query, JOIN and UNION are two basic operations. While they both combine data from multiple tables or queries, the underlying mechanisms and results are quite different.

JOIN operation

JOIN, as the name suggests, joins tables based on specified conditions and creates derived rows that meet these conditions. This operation is similar to forming a Cartesian product of the input tables and then applying a filter to select the desired combination. For example, consider the following query:

JOIN
(SELECT * FROM Customers) AS cust
JOIN (SELECT * FROM Products) AS prod
ON (cust.id = prod.customer_id)

This query will merge customer and product data based on a shared "customer_id", effectively cross-referencing these tables.

UNION operation

In contrast, UNION operates by appending the results of multiple queries one after the other. Unlike JOIN, it does not perform any table merging or filtering. The main goal of UNION is to combine data from different sources into a single unified result set. An example of UNION is as follows:

UNION
(SELECT * FROM Sales)
UNION
(SELECT * FROM Returns)

This query will create a dataset containing sales and return records while retaining its original data integrity.

Summary of main differences

The main differences between JOIN and UNION are summarized below:

• JOIN: Join tables based on conditions and create derived rows based on matching records.
• UNION: Appends the results of multiple queries one after the other without performing any filtering or merging.
• Purpose: JOIN is used to merge data with a specific relationship, while UNION is used to merge unrelated data.

Understanding these differences is critical to effectively leveraging the power of SQL and manipulating data to meet your specific needs.

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