ROW_NUMBER vs DENSE_RANK vs RANK — the actual difference

Three rows tied at second place:

• ROW_NUMBER → 1, 2, 3, 4 (always unique, picks one of the ties arbitrarily)
• RANK → 1, 2, 2, 4 (ties get the same rank, then a gap)
• DENSE_RANK → 1, 2, 2, 3 (ties same rank, no gap)

Use ROW_NUMBER when you want to pick exactly one row per group (e.g. 'first paid order'). RANK/DENSE_RANK when ties are real and matter.