你要小數點~
要先一開始就轉換~才會得到~
select  生產線
,(
	case when text=OK 
	then 1 
	else 0 
	end 
) as OK數量
,(
	case when text=NG 
	then 1 
	else 0 
	end 
) as NG數量
,(
	Convert(decimal(18, 2),Convert(decimal(18, 2), OK數量) / (Convert(decimal(18, 2),NG數量) + Convert(decimal(18, 2),OK數量))) 
) as 成功率
FROM
select 生產線,OK數量,NG數量,OK數量/(NG數量+OK數量)成功率
from (select  生產線,count(生產線) OK數量,0 NG數量
from TABLE
WHERE text=OK
group by 生產線
union all
select  生產線, 0 OK數量,count(生產線) NG數量
from TABLE
WHERE text=NG
group by 生產線
)
group by 生產線