JOIN vs UNION in SQL: Key Differences & When to Use Each
JOIN merges rows horizontally from two or more tables based on matching columns; UNION stacks rows vertically from multiple result sets after they share identical columns.
Picture two spreadsheets: one lists customers, another their orders. JOIN lets you glue them side-by-side so every order sits next to the buyer. UNION feels like copying rows from each sheet into a single list. Because both “combine” data, people often confuse the direction and purpose.
Key Differences
JOIN keeps column count the same but adds more rows when matches exist. UNION keeps column names identical but adds more rows while keeping column count unchanged. Performance differs too: JOINs usually cost more CPU; UNION may cost more memory for deduplication.
Which One Should You Choose?
Need richer, side-by-side detail? Use JOIN. Need to merge lists—like daily sales from two regions—into one clean table? Use UNION. Always check data types and column order before UNION; ensure foreign keys for JOIN.
Can UNION add columns?
No. UNION only stacks rows; all SELECTs must share the same number and order of columns.
Is JOIN slower than UNION?
Not always. Indexed JOINs can outperform large UNIONs that require deduplication.