在同一查询中多次调用CTE

Question

我有一个相当复杂的查询,有多个CTE,但有一个主要CTE,其他人都从中拉出来,这是否会导致主CTE多次执行？

Answer 1

您可以这样使用 CROSS JOIN：

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 的多次执行（有关实际执行计划，请参阅Number of Executions属性）cteFiveYear。

示例：如果执行此查询

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