Mysql, Sprawdź zmianę wartości pola?

To powinno wystarczyć:

SELECT
  @s := @s + (@prev_size != a.size) `index`,
  id,
  @prev_size := a.size size,
  name
FROM a, (SELECT @s := 0, @prev_size := -1) s
ORDER BY a.size

Wyjście:

| INDEX | ID | SIZE | NAME |
|-------|----|------|------|
|     1 |  2 |  200 |  bcd |
|     1 |  5 |  200 |  aaa |
|     2 |  3 |  400 |  xyz |
|     3 |  1 |  500 |  abc |
|     3 |  4 |  500 |  bbb |

Fiddle tutaj .