擺脫不需要的sql子查詢的最佳方法？

Question

我有一個稱爲註冊記憶具有以下字段表：

• 標識
• DateStarted（不爲空）
• DateCompleted（可爲空的）

我有一個條形圖，其顯示了編號的註冊按日期開始和完成。 我的查詢是這樣的：

; 
WITH Initial(DateStarted, StartCount) 
as (
    select Datestarted, COUNT(*) 
    FROM Registrations 
    GROUP BY DateStarted  
) 
select I.DateStarted, I.StartCount, COUNT(DISTINCT R.RegistrationId) as CompleteCount 
    from Initial I 
     inner join Registrations R 
      ON (I.DateStarted = R.DateCompleted) 
    GROUP BY I.DateStarted, I.StartCount 

它返回一個表，看起來像：

DateStarted StartCount CompleteCount 
2009-08-01 1033  903 
2009-08-02 540   498 

查詢只是有這些代碼味道的問題之一。什麼是更好的方式來做到這一點？

Answer 1

編輯：那麼爲什麼不會在下面的工作？如果您希望將計數設置爲零而不是空值，則可以在最後一個選擇語句中的計數週圍引發coalesce（）語句。它還包括已完成（或在下面的示例中結束）註冊日期，即使該日期尚未開始註冊。

---

我假設下面的表結構（大致）。

create table temp 
(
    id int, 
    start_date datetime, 
    end_date datetime 
) 

insert into temp values (1, '8/1/2009', '8/1/2009') 
insert into temp values (2, '8/1/2009', '8/2/2009') 
insert into temp values (3, '8/1/2009', null) 
insert into temp values (4, '8/2/2009', '8/2/2009') 
insert into temp values (5, '8/2/2009', '8/3/2009') 
insert into temp values (6, '8/2/2009', '8/4/2009') 
insert into temp values (7, '8/4/2009', null) 

然後你可以做下面的事情來得到你想要的。

with start_helper as 
(
    select start_date, count(*) as count from temp group by start_date 
), 

end_helper as 
(
    select end_date, count(*) as count from temp group by end_date 
) 

select coalesce(a.start_date, b.end_date) as date, a.count as start_count, b.count as end_count 
from start_helper a full outer join end_helper b on a.start_date = b.end_date 
where coalesce(a.start_date, b.end_date) is not null 

我認爲完全外部聯接是必要的，因爲創紀錄的今天可以完成的，昨天開始，但我們可能還沒有開始今天的新紀錄，所以你會從結果失去了一天。

Answer 2

我沒有看到問題。我看到使用了一個通用表格表達式。

---

你沒有提供表的DDL，所以我不打算重現這一點。不過，我認爲你可以直接用SELECT代替Initial。

Answer 3

副手，我覺得這個做的：

SELECT 
    DateStarted 
    , COUNT(*) as StartCount 
    , SUM(CASE 
     WHEN DateCompleted = DateStated THEN 1 
     ELSE 0 END 
     ) as CompleteCount 

FROM Registration 

GROUP BY DateStarted 

---

OK，顯然我錯了之前所擁有的要求。鑑於CompleteCounts獨立於起始日期的，那麼這是我會怎麼做：

;WITH StartDays AS 
(
    SELECT DateStarted 
    , Count(*) AS CompleteCount 
    FROM Registration 
    GROUP BY DateStarted 
) 
, CompleteDays AS 
(
    SELECT DateCompleted 
    , Count(*) AS StartCount 
    FROM Registration 
    GROUP BY DateCompleted 
) 
SELECT 
    DateStarted 
    , COALESCE(StartCount, 0) AS StartCount 
    , COALESCE(CompleteCount, 0) AS CompleteCount 

FROM StartDays 
FULL OUTER JOIN CompleteDays ON DateStarted = DateCompleted 

這實際上是非常接近你有什麼。

Answer 4

我相信下面是在功能上是否有什麼相同：

select DS.DateStarted 
    , count(distinct DS.RegistrationId) as StartCount 
    , count(distinct DC.RegistrationId) as CompleteCount 
from Registrations DS 
inner join Registrations DC on DS.DateStarted = DC.DateCompleted 
group by Ds.DateStarted 

我有點在結果的列DateStarted的名稱混淆。它看起來只是一些事情的開始和一些事情的結束。計數是當天開始和完成的數量或註冊。

內部連接將丟棄任何有0個開始或0個完成的日期。要獲得所有：

select coalesce(DS.DateStarted, DC.DateCompleted) as "Date" 
    , count(distinct DS.RegistrationId) as StartCount 
    , count(distinct DC.RegistrationId) as CompleteCount 
from Registrations DS 
full outer join Registrations DC on DS.DateStarted = DC.DateCompleted 
group by Ds.DateStarted, DC.DateCompleted 

如果你想包括日期既不是DateStarted也不DateCompleted，用0和0計數，則需要日期的來源，我認爲這將是更清晰的使用兩個相關select子句中的子查詢，而不是連接和計數不同：

select DateSource."Date" 
    , (select count(*) 
     from Registrations 
     where DateStarted = DateSource."Date") as StartCount 
    , (select count (*) 
     from Registrations 
     where DateCompleted = DateSource."Datge") as CompleteCount 
from DateSource -- implementation of date source left as exercise 
where DateSource.Date between @LowDate and @HighDate