如何解决使用公共相关子查询有效地拉不同的列
| 我需要从子查询中拉出多个列，该子查询还需要一个WHERE过滤器来引用FROM表的列。我对此有两个问题： 除了下面的我，还有其他解决此问题的方法吗？ 是否需要另一种解决方案，或者该解决方案是否足够有效？ 例： 在下面的示例中，我正在编写一个视图以呈现测试分数，特别是发现可能需要解决或重试的失败。 我不能简单地使用JOIN，因为我需要首先过滤我的实际子查询（请注意，我的\“ examinee \\”排名第一，按得分或日期降序排列） 我的目标是避免重复编写（和执行）基本上相同的子查询。SELECT ExamineeID,LastName,FirstName,Email,(SELECT COUNT(examineeTestID)
FROM exam.ExamineeTest tests
WHERE E.ExamineeID = ExamineeID AND TestRevisionID = 3 AND TestID = 2) Attempts,(SELECT TOP 1 ExamineeTestID
FROM exam.ExamineeTest T
WHERE E.ExamineeID = ExamineeID AND TestRevisionID = 3 AND TestID = 2
ORDER BY Score DESC) bestExamineeTestID,(SELECT TOP 1 Score
FROM exam.ExamineeTest T
WHERE E.ExamineeID = ExamineeID AND TestRevisionID = 3 AND TestID = 2
ORDER BY Score DESC) bestScore,(SELECT TOP 1 DateDue
FROM exam.ExamineeTest T
WHERE E.ExamineeID = ExamineeID AND TestRevisionID = 3 AND TestID = 2
ORDER BY Score DESC) bestDateDue,(SELECT TOP 1 TimeCommitted
FROM exam.ExamineeTest T
WHERE E.ExamineeID = ExamineeID AND TestRevisionID = 3 AND TestID = 2
ORDER BY Score DESC) bestTimeCommitted,(SELECT TOP 1 ExamineeTestID
FROM exam.ExamineeTest T
WHERE E.ExamineeID = ExamineeID AND TestRevisionID = 3 AND TestID = 2
ORDER BY DateDue DESC) currentExamineeTestID,(SELECT TOP 1 Score
FROM exam.ExamineeTest T
WHERE E.ExamineeID = ExamineeID AND TestRevisionID = 3 AND TestID = 2
ORDER BY DateDue DESC) currentScore,(SELECT TOP 1 DateDue
FROM exam.ExamineeTest T
WHERE E.ExamineeID = ExamineeID AND TestRevisionID = 3 AND TestID = 2
ORDER BY DateDue DESC) currentDateDue,(SELECT TOP 1 TimeCommitted
FROM exam.ExamineeTest T
WHERE E.ExamineeID = ExamineeID AND TestRevisionID = 3 AND TestID = 2
ORDER BY DateDue DESC) currentTimeCommitted
FROM exam.Examinee E
解决方法
首先要回答第二个问题，是的，一种更好的方法是有序的，因为您正在使用的查询难以理解，难以维护，即使现在的性能可以接受，查询它也是一种耻辱。如果您的应用程序增长到可观的大小，那么多次添加同一张表（不需要添加时）的性能可能并不总是可以接受的。
为了回答您的第一个问题，我为您提供了几种方法。除非另有说明，否则这些假定使用SQL 2005或更高版本。
请注意，您不需要BestExamineeID和CurrentExamineeID，因为除非未进行任何测试且它们为NULL，否则它们将始终与ExamineeID相同，您可以从其他列中将其识别为NULL。
您可以将OUTER / CROSS APPLY视为一个运算符，该运算符使您可以将相关子查询从WHERE子句移至JOIN子句。它们可以具有对先前命名表的外部引用，并且可以返回多个列。这使您每个逻辑查询仅执行一次该工作，而不是每个列一次。
SELECT
ExamineeID,LastName,FirstName,Email,B.Attempts,BestScore = B.Score,BestDateDue = B.DateDue,BestTimeCommitted = B.TimeCommitted,CurrentScore = C.Score,CurrentDateDue = C.DateDue,CurrentTimeCommitted = C.TimeCommitted
FROM
exam.Examinee E
OUTER APPLY ( -- change to CROSS APPLY if you only want examinees who\'ve tested
SELECT TOP 1
Score,DateDue,TimeCommitted,Attempts = Count(*) OVER ()
FROM exam.ExamineeTest T
WHERE
E.ExamineeID = T.ExamineeID
AND T.TestRevisionID = 3
AND T.TestID = 2
ORDER BY Score DESC
) B
OUTER APPLY ( -- change to CROSS APPLY if you only want examinees who\'ve tested
SELECT TOP 1
Score,TimeCommitted
FROM exam.ExamineeTest T
WHERE
E.ExamineeID = T.ExamineeID
AND T.TestRevisionID = 3
AND T.TestID = 2
ORDER BY DateDue DESC
) C
Count(*) OVER ()
OUTER APPLY
exam.Examinee
WITH Data AS (
SELECT
*,Count(*) OVER (PARTITION BY ExamineeID) Cnt,Row_Number() OVER (PARTITION BY ExamineeID ORDER BY Score DESC) ScoreOrder,Row_Number() OVER (PARTITION BY ExamineeID ORDER BY DateDue DESC) DueOrder
FROM
exam.ExamineeTest
),Vals AS (
SELECT
ExamineeID,Max(Cnt) Attempts,Max(CASE WHEN ScoreOrder = 1 THEN Score ELSE NULL END) BestScore,Max(CASE WHEN ScoreOrder = 1 THEN DateDue ELSE NULL END) BestDateDue,Max(CASE WHEN ScoreOrder = 1 THEN TimeCommitted ELSE NULL END) BestTimeCommitted,Max(CASE WHEN DueOrder = 1 THEN Score ELSE NULL END) BestScore,Max(CASE WHEN DueOrder = 1 THEN DateDue ELSE NULL END) BestDateDue,Max(CASE WHEN DueOrder = 1 THEN TimeCommitted ELSE NULL END) BestTimeCommitted
FROM Data
GROUP BY
ExamineeID
)
SELECT
E.ExamineeID,E.LastName,E.FirstName,E.Email,V.Attempts,V.BestScore,V.BestDateDue,V.BestTimeCommitted,V.CurrentScore,V.CurrentDateDue,V.CurrentTimeCommitted
FROM
exam.Examinee E
LEFT JOIN Vals V ON E.ExamineeID = V.ExamineeID
-- change join to INNER if you only want examinees who\'ve tested
SELECT
E.ExamineeID,Y.Attempts,Y.BestScore,Y.BestDateDue,Y.BestTimeCommitted,Y.CurrentScore,Y.CurrentDateDue,Y.CurrentTimeCommitted
FROM
exam.Examinee E
LEFT JOIN ( -- change to inner if you only want examinees who\'ve tested
SELECT
X.ExamineeID,X.Cnt Attempts,Max(CASE Y.Which WHEN 1 THEN T.Score ELSE NULL END) BestScore,Max(CASE Y.Which WHEN 1 THEN T.DateDue ELSE NULL END) BestDateDue,Max(CASE Y.Which WHEN 1 THEN T.TimeCommitted ELSE NULL END) BestTimeCommitted,Max(CASE Y.Which WHEN 2 THEN T.Score ELSE NULL END) CurrentScore,Max(CASE Y.Which WHEN 2 THEN T.DateDue ELSE NULL END) CurrentDateDue,Max(CASE Y.Which WHEN 2 THEN T.TimeCommitted ELSE NULL END) CurrentTimeCommitted
FROM
(
SELECT ExamineeID,Max(Score) MaxScore,Max(DueDate) MaxDueDate,Count(*) Cnt
FROM exam.ExamineeTest
WHERE
TestRevisionID = 3
AND TestID = 2
GROUP BY ExamineeID
) X
CROSS JOIN (SELECT 1 UNION ALL SELECT 2) Y (Which)
INNER JOIN exam.ExamineeTest T
ON X.ExamineeID = T.ExamineeID
AND (
(Y.Which = 1 AND X.MaxScore = T.MaxScore)
OR (Y.Which = 2 AND X.MaxDueDate = T.MaxDueDate)
)
WHERE
T.TestRevisionID = 3
AND T.TestID = 2
GROUP BY
X.ExamineeID,X.Cnt
) Y ON E.ExamineeID = Y.ExamineeID
TOP (1) ORDER BY Score DESC
TOP (1) ORDER BY DateDue DESC
create function dbo.topexaminee_byscore(@ExamineeID int)
returns table
as
return (
SELECT top (1)
ExamineeTestID as bestExamineeTestID,Score as bestScore,DateDue as bestDateDue,TimeCommitted as bestTimeCommitted
FROM exam.ExamineeTest
WHERE (ExamineeID = @ExamineeID) AND (TestRevisionID = 3) AND (TestID = 2)
ORDER BY Score DESC
)
select bestExamineeTestID,bestScore,bestDateDue,bestTimeCommitted
from (
SELECT
ExamineeTestID as bestExamineeTestID,TimeCommitted as bestTimeCommitted,row_number() over (partition by ExamineeID order by Score DESC) as takeme
FROM exam.ExamineeTest
WHERE (TestRevisionID = 3) AND (TestID = 2)
) as foo
where foo.takeme = 1
ORDER BY DateDue DESC
CTE
OUTER APPLY
;WITH testScores AS
(
SELECT ExamineeID,ExamineeTestID,Score,TimeCommitted
FROM exam.ExamineeTest
WHERE TestRevisionID = 3 AND TestID = 2
)
SELECT ExamineeID,total.Attempts,bestTest.*,currentTest.*
FROM exam.Examinee
LEFT OUTER JOIN
(
SELECT ExamineeID,COUNT(ExamineeTestID) AS Attempts
FROM testScores
GROUP BY ExamineeID
) AS total ON exam.Examinee.ExamineeID = total.ExamineeID
OUTER APPLY
(
SELECT TOP 1 ExamineeTestID,TimeCommitted
FROM testScores
WHERE exam.Examinee.ExamineeID = t.ExamineeID
ORDER BY Score DESC
) AS bestTest (bestExamineeTestID,bestTimeCommitted)
OUTER APPLY
(
SELECT TOP 1 ExamineeTestID,TimeCommitted
FROM testScores
WHERE exam.Examinee.ExamineeID = t.ExamineeID
ORDER BY DateDue DESC
) AS currentTest (currentExamineeTestID,currentScore,currentDateDue,currentTimeCommitted)
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