你可以试着用 SUM 以及
group by a 以及 b 子查询中的列，然后是self join 测试DLL

create table t(
   a int,
   b varchar(5),
   c int
);

insert into t values (1,'p',2);
insert into t values (1,'p',3);
insert into t values (2,'q',4);
insert into t values (1,'q',2);
insert into t values (2,'r',4);
insert into t values (2,'r',2);
insert into t values (1,'p',2);

查询

SELECT t1.*,t2.d 
FROM T t1 INNER JOIN (
    SELECT a,b,SUM(c) d
    FROM T
    GROUP BY a,b
) t2 on t1.a = t2.a and t1.b=t2.b

[结果]：

| a | b | c | d |
|---|---|---|---|
| 1 | p | 2 | 7 |
| 1 | p | 3 | 7 |
| 2 | q | 4 | 4 |
| 1 | q | 2 | 2 |
| 2 | r | 4 | 6 |
| 2 | r | 2 | 6 |
| 1 | p | 2 | 7 |

sqlfiddle公司

使用 case when 如果生成的值是常量，则创建另一列

select a,b,c, case when a=1  and b='p'  then 7
    when a=2 and b='q' then 4
    when a=2 and b='r' then 6
    when a=1 and b='q' then 2 else '' end as d from
    your_table

但在我看来，你需要a，b的群和作为第四列，所以你可以做如下的事情

select t1.a,t1.b,t1.c,t2.d from yourtable t1
inner join
(select a,b ,sum(c) as d from your table group by a,b) as t2
on t1.a=t2.a and t1.b=t2.b

如果支持sum的窗口函数

select a, b, c, sum(c) over (partition by a, b) as d
from yourtable