一个SQL的写法——关于分组加顺序号(分数形式的序号)

CREATE TABLE #Tmp (a Char(1),b int IDENTITY (1,1))

INSERT INTO #Tmp VALUES ('a') 
INSERT INTO #Tmp VALUES ('a') 
INSERT INTO #Tmp VALUES ('b') 
INSERT INTO #Tmp VALUES ('b') 
INSERT INTO #Tmp VALUES ('b') 
INSERT INTO #Tmp VALUES ('c') 
INSERT INTO #Tmp VALUES ('c')

SELECT* FROM #Tmp 
DROP TABLE #tmp
select t.a, t.b, (Ltrim(str((SELECT COUNT(*) FROM #Tmp WHERE a = t.a AND b <= t.b))) + '/' + Ltrim(str((SELECT COUNT(*) FROM #Tmp WHERE a = t.a)))) AS SN FROM #Tmp t ORDER BY t.a