Calling CTE multiple times in same query

Question

I have a fairly complicated query with multiple CTEs but 1 main CTE that the others all pull from, does this cause that main CTE to be executed multiple times?

Answer 1

You could use CROSS JOIN thus:

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

This will prevent multiple executions (for actual execution plan see Number of Executions property) for cteFiveYear.

Example: If you execute this query

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

using AdventureWorks2008R2 database then the actual execution plan will be

Answer 2

Have a look at the below

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

And then at this SQL Fiddle DEMO

You will notice from the execution plan that the Vals CTE section is executed twice.

Maybe also have a look at How many times the T-SQL inside the CTE is executed?