SQL查询选择每种类型之一

假设我有这张表:

id colorName
1 red
2 blue
3 red
4 blue

如何选择每种颜色的一个代表？
结果:
1红
2蓝

不是随机代表,而是......

select color, min(id)
from   mytable
group by color;

select distinct colorname from mytable

在MS SQL Server和Oracle:

SELECT  id, colorName
FROM    (
        SELECT  id, colorName,
                ROW_NUMBER() OVER (PARTITION BY colorName ORDER BY id) AS rn
        FROM    colors
        ) q
WHERE   rn = 1