sql - 在同一查询中多次调用 CTE

Question

我有一个相当复杂的查询，其中包含多个 CTE，但有 1 个主 CTE，其他所有 CTE 都从中提取，这是否会导致该主 CTE 被执行多次？

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 cteFiveYear 的多次执行(有关实际执行计划，请参阅执行次数属性)。

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