现整理解法如下:
数据样本:
create table tx( id int primary key, c1 char(2), c2 char(2), c3 int );
insert into tx values (1 ,’A1′,’B1′,9), (2 ,’A2′,’B1′,7), (3 ,’A3′,’B1′,4), (4 ,’A4′,’B1′,2), (5 ,’A1′,’B2′,2), (6 ,’A2′,’B2′,9), (7 ,’A3′,’B2′,8), (8 ,’A4′,’B2′,5), (9 ,’A1′,’B3′,1), (10 ,’A2′,’B3′,8), (11 ,’