在同一个查询中多次调用 CTE

小编典典

在同一个查询中多次调用 CTE

sql

我有一个相当复杂的查询，其中包含多个 CTE，但有 1 个主要 CTE，其他人都从中提取，这是否会导致该主要 CTE 被多次执行？

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2021-05-30

共1个答案

小编典典

您可以使用 CROSS JOIN 因此：

SELECT 
    AVG(CASE WHEN instructorID = @instructorID THEN score END) InstructorSemesterAverage,
    STDEVP(CASE WHEN instructorID = @instructorID THEN score END) InstructorSemesterSTDeviation,
    AVG(CASE WHEN subjectCode = @subjectCode THEN score END) DepartmentSemesterAverage, 
    STDEVP(CASE WHEN subjectCode = @subjectCode THEN score END) DepartmentSemesterSTDeviation, 
    AVG(CASE WHEN bannerCRN=@CRN AND Q.year = @year AND semester = @semester THEN score END) ClassScore,
    STDEVP(CASE WHEN bannerCRN=@CRN AND Q.year = @year AND semester = @semester THEN score END) ClassSTDeviation,
    (SELECT DecTile FROM cteNtile WHERE instructorID = @instructorID)*10 DecTile,
    X.DepartmentClassFiveYearAverage AS DepartmentClassFiveYearAverage,
    X.DepartmentClassFiveYearSTDeviation AS DepartmentClassFiveYearSTDeviation,
    X.InstructorClassFiveYearAverage AS InstructorClassFiveYearAverage,
    X.InstructorClassFiveYearSTDeviation AS InstructorClassFiveYearSTDeviation
FROM 
    cteMain Q CROSS JOIN cteFiveYear X

这将防止多次执行（实际执行计划见Number of Executions属性）cteFiveYear。

示例：如果您执行此查询

SELECT  h.ProductID,h.StandardCost,
        x.AvgPrice
FROM    Production.ProductCostHistory h
CROSS JOIN (
    SELECT  AVG(p.ListPrice) AvgPrice
    FROM    Production.Product p
) x

使用AdventureWorks2008R2数据库那么实际的执行计划将是

2021-05-30