[求助] 判断查询怎么写?

[yangbiao_8]

table有两个字段,sz和sm,当sz='1'或'2'或'3'的时候两个字段拼接,否则等于sz字段.
下面语句能实现,怎么改为一个select实现.
select concat(sz,'+',sm) as sz from table where sz in ('1','2','3')
union sz from table where sz not in ('1','2','3')
求一条判断查询语句

[ACMAIN_CHM]

select case sz 
	when '1' then concat(sz,'+',sm)
	when '2' then concat(sz,'+',sm)
	when '3' then concat(sz,'+',sm)
	else sz 
	END	as sz 
from table

[yangbiao_8]

谢谢!case语句没学习好