以下代码在MYSQL中测试通过，MSSQL应该能跑通，未测试。

复制代码 代码如下:

#创建表如下

create temporary table tmp (a int, b int ,c int);

insert into tmp VALUES (1,10,1),(10,10,2),(10,100,2);

#mysql执行

select sum(case when c = '1' then A else B end) from tmp

#oracle执行

select sum(decode(c,'1',a,b)) from tmp

#普通联合查询

select sum(d) from

(

select a as d from tmp where c=1

union

select b as d from tmp where c=2

)