What happens to row counts in a LEFT JOIN?

Question

Accepted Answer

3,550. The 950 orders that have items contribute one row per item — 3,500 rows in total. The 50 orders with no items each contribute exactly one row, with NULLs on the right side. 3,500 + 50 = 3,550. The common wrong answer is 3,500, which comes from forgetting that a LEFT JOIN preserves unmatched left rows. The other common error in practice is assuming a LEFT JOIN cannot increase the row count above the left table's — it can and usually does, which is why counting rows after a join is a habit worth keeping.