SQL 选择过滤器

Question

我有这个代码，可以按天输出取消行程的数量：

select request_at, count(*) as cancelled 
from trip
where status = 'cancelled_by_client' 
   or status = 'cancelled_by_driver'
group by request_at order by request_at

它可以工作，但是，当没有取消的行程时，它不会输出任何内容。

我希望它输出“数据”和0作为取消值。

如何更改我的代码来执行此操作？

Answer 1

通过使用

Union ALL

和 CTE 表，您可以实现您想要的，如下所示：

;WITH query AS
(
select request_at, count(*) as cancelled 
from trip
where status = 'cancelled_by_client' 
   or status = 'cancelled_by_driver'
group by request_at order by request_at
)

Select * from query
UNION ALL
Select 'data' as request_at, 0 as cancelled 
where (Select count(*) from query) = 0 --if no rows in the query then attach this default values