Xóa các bản sao

SELECT
          min(a) as a
        , b
    FROM 
    (
        select a = 1, b = 30
        union all 
        select a = 2, b = 50
        union all 
        select a = 3, b = 50
        union all 
        select a = 4, b = 50
        union all 
        select a = 5, b = 60
    ) t
GROUP BY b    
ORDER BY a