在同一查詢中多次調用CTE

Question

我有一個相當複雜的多個CTE查詢，但有一個主要的CTE，其他人都從中拉出，這是否會導致主CTE被多次執行？

Answer 1

你可以使用CROSS JOIN這樣的：爲cteFiveYear

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 

這將防止多次執行（實際執行計劃見Number of Executions屬性）。

例如：如果您使用執行數據庫AdventureWorks2008R2此查詢

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

那麼實際執行計劃將

Answer 2

看一看下面

DECLARE @Table TABLE(
     ID INT, 
     Val VARCHAR(50), 
     TypeID INT 
) 

DECLARE @TableTypes TABLE(
     TypeID INT, 
     TypeName VARCHAR(50) 
) 

;WITh Vals AS (
     SELECT * 
     FROm @Table 
     WHERE ID > 10 
) 
, UsingVals1 AS (
     SELECT v.*, 
       tt.TypeName 
     FROm Vals v INNER JOIN 
       @TableTypes tt ON v.TypeID = tt.TypeID 
) 
, UsingVals2 AS (
     SELECT v.*, 
       tt.TypeName 
     FROm Vals v INNER JOIN 
       @TableTypes tt ON v.TypeID = tt.TypeID 
     WHERE tt.TypeName LIKE '%%' 
) 
SELECT * 
FROM UsingVals1 
UNION 
SELECT * 
FROM UsingVals2 

，接着在SQL Fiddle DEMO

你會從該丘壑CTE部分執行兩次執行計劃通知。

或許也看看How many times the T-SQL inside the CTE is executed?