sql怎么通过条件判断选取查询字段

[qq_34444180]

id f1 f2 f3 n1
1 A E I 2或3
2 B F J 2或3
3 C G K 2或3
4 D H L 2或3
如果n1=2就查询 f1,f2字段 显示 AE

如果n1=3就查询 f1,f3字段 显示 AI


这种语句该怎么个写法，求大神指导

[Ginnnnnnnn]

WITH A(id,f1,f2,f3,n1) AS(
SELECT 1,'A','E','I',2 UNION ALL          
SELECT 2,'B','F','J',2 UNION ALL 
SELECT 3,'C','G','K',3 UNION ALL 
SELECT 4,'D','H','L',3
)
SELECT *,
		f1 + CASE n1 WHEN 2 THEN f2 WHEN 3 THEN f3 END AS 组合
	FROM A

[xdashewan]

要么case when，要么用union all

[kingtiy]

select case when n1=2 then f1+f2
when n1=3 then f1=f3 end from tb
--试下看看?

[yooq_csdn]

select  f1
     ,case n1 when '2' then f2
                      when '3' then f3
       end

[道玄希言]

select * from ( select id, f1, f2 from 表 where n1 ='2' union all select id, f1, f3 as f2 from 表 where n1 ='3' ) as a order by a.id

[足球不是方的]

case when 呗

[-小蕾-]

WITH A(id,f1,f2,f3,n1) AS(
SELECT 1,'A','E','I',2 UNION ALL          
SELECT 2,'B','F','J',2 UNION ALL 
SELECT 3,'C','G','K',3 UNION ALL 
SELECT 4,'D','H','L',3
)
--方法1：
select *,f2 as 'newCol' from A where n1=2
union all
select *,f3 as 'newCol' from A where n1=3

--方法2：
--select *,case when n1=2 then f2 when n1=3 then f3 else '' end as 'newCol' from A

[xqchenxue2]

select case when n1=2 then f1+f2 when n1=3 then f1=f3 end from tb