在 WHERE 语句中使用子查询的别名

Question

我正在尝试在 WHERE 语句中使用在 SELECT 中创建的别名。它不起作用，我在另一个 SO 问题中读到了原因。

如何在不重复子查询的情况下完成这项工作？

SELECT p.PatientID, p.PatientType, p.AccountNumber,
    p.FirstName + ' ' + p.LastName PatientFullName,
    p.CreatedDate, DATEDIFF(hour, p.CreatedDate, GETDATE()) TotalTime,
    (SELECT AVG(BGValue)
        FROM BloodGlucose
        WHERE PatientID = p.PatientID) AvgBG
FROM Patients p
WHERE AvgBG > 60;

这有效：

SELECT p.PatientID, p.PatientType, p.AccountNumber,
    p.FirstName + ' ' + p.LastName PatientFullName, p.CreatedDate,
    DATEDIFF(hour, p.CreatedDate, GETDATE()) TotalTime,
    (SELECT AVG(BGValue) FROM BloodGlucose WHERE PatientID = p.PatientID) AvgBG
FROM Patients p
WHERE (SELECT AVG(BGValue) FROM BloodGlucose WHERE PatientID = p.PatientID) > 60;

但是我不想重复那个子查询。而且我怀疑它的性能很差。

Answer 1

尝试使用派生表。

SELECT p.PatientID, p.PatientType, p.AccountNumber,
    p.FirstName + ' ' + p.LastName PatientFullName,
    p.CreatedDate, DATEDIFF(hour, p.CreatedDate, GETDATE()) TotalTime,
    bg.AvgBG 
FROM Patients p 
JOIN (SELECT PatientID, AVG(BGValue) AvgBG
    FROM BloodGlucose group by PatientID) BG
ON BG.PatientID = p.PatientID 
WHERE AvgBG > 60;

派生表按集合工作，相关子查询逐行工作，这就是我的速度更快的原因。

Answer 2

WHERE 子句中的别名只能来自FROM 子句。这是重写查询的一种方法：

SELECT p.PatientID, p.PatientType, p.AccountNumber,
       p.FirstName + ' ' + p.LastName as PatientFullName, p.CreatedDate,
       DATEDIFF(hour, p.CreatedDate, GETDATE()) TotalTime,
       av.AvgBG
FROM Patients p join
     (SELECT PatientId, AVG(BGValue) as AvgBG
      FROM BloodGlucose
      group by PatientID
     ) av
     on p.PatientId = av.PatientId
WHERE av.AvgBG > 60;

Answer 3

SELECT p.PatientID, p.PatientType, p.AccountNumber, p.FirstName + ' ' + p.LastName PatientFullName, p.CreatedDate, DATEDIFF(hour, p.CreatedDate, GETDATE()) TotalTime, bg.AvgBG  
FROM Patients p  
outer apply (SELECT PatientID, AVG(BGValue) AvgBG   
FROM BloodGlucose where PatientID = p.PatientID  
 group by PatientID ) BG 
 WHERE AvgBG > 60; 

这也应该工作得很快

Answer 4

SELECT p.PatientID, p.PatientType, p.AccountNumber, p.FirstName p.LastName PatientFullName, p.CreatedDate,
DATEDIFF(hour, p.CreatedDate, GETDATE()) TotalTime, a.AvgBG
FROM Patients p,
(SELECT PatientID, AVG(BGValue) AvgBG
  FROM BloodGlucose
group by PatientID
having AVG(BGValue) > 60) a
where p.PatientID = a.PatientID

Answer 5

您可以尝试在标准 SQL 中使用 WITH 子句。

WITH Subquery AS
(
    Your Code
)

然后是你的代码

SELECT ....
Subquery As sq
From Patients p
WHERE(subquery)>60

Answer 6

使用通用表表达式：

;WITH CTE (PatientID, PatientType, AccountNumber, PatientFullName, CreatedDate, TotalTime, AvgBG) AS (
    SELECT p.PatientID, p.PatientType, p.AccountNumber, p.FirstName + ' ' + p.LastName PatientFullName, p.CreatedDate,
    DATEDIFF(hour, p.CreatedDate, GETDATE()) TotalTime,
    (SELECT AVG(BGValue) FROM BloodGlucose WHERE PatientID = p.PatientID) AvgBG
    FROM Patients p
)

SELECT PatientID, PatientType, AccountNumber, PatientFullName, CreatedDate, TotalTime, AvgBG
FROM CTE
WHERE AvgBG > 60