Question

如果第一个表为空，如何使查询从另一个表中进行选择。

这是我的C＃程序中的当前查询，我正在使用SQL Server。

private const string sql = @'SELECT TOP (1) (LOCALTIME) from dbo.[devicemessage] order by (localtime) desc;

因此，此代码的作用是从在localtime desc中排列的前1行获取localtime列中的值；最新时间。

我想做的是这样的：

If dbo.[devicemessage] is empty select top (1) (localtime) from dbo.[devicemessagehistory] order by (localtime) desc,
if table dbo.[devicemessage] not empty, get from dbo.[devicemessage].

就像在C＃中使用它一样，是否可以在一个查询字符串中完成它。

Answer 1

选择isnull()或coalesce()将得到您的max() devicemessage LOCALTIME（如果有），如果没有，将选择您的devicemessagehistory。

如果您只有两个条件，isnull()就足够了，但是如果您有多个条件，我建议您使用coalesce()。

select isnull(
    (select max(LOCALTIME) from [devicemessage]),
        (select max(LOCALTIME) from [devicemessagehistory]))

select coalesce(
    (select max(LOCALTIME) from [devicemessage]),
        (select max(LOCALTIME) from [devicemessagehistory]))

使用order by

select isnull(
    (select TOP 1 localtime from dbo.[devicemessage] order by localtime desc),
        (select TOP 1 localtime from dbo.[devicemessagehistory] order by localtime desc))

Answer 2

只需将两个表与not exists条件结合在一起即可。这样，您就可以从同一查询的2个表中进行选择，但如果第一个表中没有行，则仅从第二个表中给出结果：

select top 1 localtime
from (
  select localtime
  from dbo.[devicemessage]
  union all
  select localtime
  from dbo.[devicemessagehistory]
  where not exists (select 1 from dbo.[devicemessage])
) X
order by localtime desc;

如果原始表为空，则从另一个表中选择

2 个答案: