Today let me explain you about how to make an Index on #temp tables. #Temp tables are much like SQL tables that are defined and stored in TempDB. Difference between them and a permanent table is they are not allowed to have foreign keys.

One of the feature of temp table (#temp) is that we can add a clustered or non clustered index. Also, #temp tables allow for the auto-generated columns (fields) to be created. This can help the optimizer when determining the number of records. Below is an example of creating both a clustered and non-clustered index on a temp table.
-- creating temp table - #employees
CREATE TABLE #Employees 
( 
ID INT IDENTITY(1,1), 
EmployeeID INT, 
EmployeeName VARCHAR(50) 
) 

INSERT INTO #Employees 
( 
EmployeeID, 
EmployeeName 
) 
SELECT 
EmployeeID = e.EmployeeID 
,EmployeeName = e.EmployeeName 
FROM dbo.Employees e 
 
-- creating clustered index 
CREATE CLUSTERED INDEX IDX_C_Employees_EmployeeID ON #Employees(EmployeeID)
 
-- creating non-clustured index 
CREATE INDEX IDX_Users_UserName ON #Employees(EmployeeName) 

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