一例

--ｻﾝﾌﾟﾙﾃﾞｰﾀの準備
CREATE TEMPORARY TABLE `ﾃｰﾌﾞﾙ名`(name text, income int);
INSERT INTO `ﾃｰﾌﾞﾙ名` VALUES ('C社',100),('C社',30),('C社',50),('B社',70),('A社',60),('A社',50);

--求めるｸｴﾘ
SET @n:=0;
SELECT *, @n:= @n + 1 AS rank
FROM (
 SELECT name , avg(income) AS income
 FROM `ﾃｰﾌﾞﾙ名`
 GROUP BY name
 ORDER BY income DESC
) AS t
;

結果

ｿｰﾄ順変更ﾊﾞｰｼﾞｮﾝ

SET @n:=0;
SELECT *
FROM (
 SELECT *, @n:= @n + 1 AS rank
 FROM (
 SELECT name , avg(income) AS income
 FROM `ﾃｰﾌﾞﾙ名`
 GROUP BY name
 ORDER BY income DESC
 ) AS t
) AS tt
ORDER BY name
;

結果