select 客户,'字段A' as 字段类型,字段A as 值 from tb
union all
select 客户,'字段B' as 字段类型,字段B as 值 from tb
union all
select 客户,'字段C' as 字段类型,字段C as 值 from tb

------解决方案--------------------
SQL code

declare @T table([客户] varchar(1),[字段A] int,[字段B] int,[字段C] int)
insert @T
select 'A',10,20,30 union all
select 'B',40,50,60 union all
select 'B',70,80,90

select [客户],'[字段A]' as 字段类型,[字段A] as 值 from @T 
union all
select [客户],'[字段B]' as 字段类型,[字段B]  from @T 
union all
select [客户],'[字段C]' as 字段类型,[字段C]  from @T 
order by 1,3

/*
客户   字段类型    值
---- ------- -----------
A    [字段A]   10
A    [字段B]   20
A    [字段C]   30
B    [字段A]   40
B    [字段B]   50
B    [字段C]   60
B    [字段A]   70
B    [字段B]   80
B    [字段C]   90
*/