SQL：獲取具有兩個特定事件的日期

Question

Table: 
    Logs 
Fields:  
    LogDate datetime, 
    ErrId int 

錯誤9和5有時發生在同一日期。 我如何找到這些日期？ 當然的任務可以對任何對ErrIds的不只是9和5

我結束了以下SQL語句：

select distinct l_1.LogDate 
from logs l_1 
where exists (select * from logs l_2 where l_1.LogDate = l_2.LogDate and l_2.ErrId = 9) 
    and exists (select * from logs l_3 where l_1.LogDate = l_3.LogDate and l_3.ErrId = 5) 

的問題：是有任務的更好的解決方案？

Answer 1

你可以使用一個自連接是這樣的：

select  distinct l_1.LogDate 
from  logs l_1 
inner join logs l_2 on l_1.LogDate = l_2.LogDate 
where  l_1.ErrId = 9 
and  l_2.ErrId = 5 

注意，通常最好是使用group by而不是distinct。試試看：

select  l_1.LogDate 
from  logs l_1 
inner join logs l_2 on l_1.LogDate = l_2.LogDate 
where  l_1.ErrId = 9 
and  l_2.ErrId = 5 
group by l_1.LogDate 

Answer 2

也許最簡單的方法是簡單地group by與having：

select l.LogDate 
from logs l 
group by l.log_date 
having sum(case when l.ErrId = 9 then 1 else 0 end) > 0 and 
     sum(case when l.ErrId = 5 then 1 else 0 end) > 0; 

如果你只是在尋找一個固定的值，你也可以寫爲：

select l.LogDate 
from logs l 
where l.ErrId in (5, 9) 
group by l.log_date 
having count(distinct l.ErrId) = 2; 

這兩個應該比用加入一起做group by/distinct更快。