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436. FULL JOIN vs UNION

Understand the difference between combining related columns with FULL JOIN and combining result rows with UNION.

📘 Lesson 436 of 728 ⏱ 13 min 🎯 Intermediate

FULL OUTER JOIN combines columns from related rows across two tables, while UNION combines rows from separate SELECT statements with compatible column structures.

These operations solve different problems and should not be confused when designing SQL queries.

Use FULL OUTER JOIN to reconcile customer records between two systems, while UNION can combine customer lists from two separate sources.

SQL 
SELECT
    a.CustomerID,
    a.CustomerName,
    b.CustomerName AS OtherName
FROM CustomerSystemA a
FULL OUTER JOIN CustomerSystemB b
    ON a.CustomerID = b.CustomerID;

This distinction is important in data reconciliation, migration, reporting, and dataset consolidation.

  • FULL JOIN combines related columns.
  • UNION combines rows.
  • FULL JOIN requires a relationship condition.
  • UNION requires compatible SELECT structures.
  • Choose the operation based on the required result.
What is the difference between FULL JOIN and UNION?

FULL JOIN combines related records horizontally, while UNION combines result rows vertically.

Does UNION require a JOIN condition?

No. UNION combines compatible SELECT results and does not require a table relationship.

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