Optimization of SQL Server Query - sql-server

Question

My problem is that I created a query that takes too long to execute.
City | Department | Employee | Attendance Date | Attendance Status
------------------------------------------------------------------------
C1 | Dept 1 | Emp 1 | 2016-01-01 | ABSENT
C1 | Dept 1 | Emp 2 | 2016-01-01 | LATE
C1 | Dept 2 | Emp 3 | 2016-01-01 | VACANCY
So I want to create a view that contains same data and adds a column that contains the total number of employees (that serves me later in a SSRS project to determine the percentage of each status).
So I created a function that makes simple select filtering by department and date.
and this is the query that uses the function:
SELECT City, Department, Employee, [Attendence Date], [Attendance Status], [Get Department Employees By Date](Department, [Attendence Date]) AS TOTAL
FROM attendenceTable
This is the function:
CREATE FUNCTION [dbo].[Get Department Employees By Date]
(
#deptID int = null,
#date datetime = null
)
RETURNS nvarchar(max)
AS
BEGIN
declare #result int = 0;
select #result = count(*) from attendenceTable where DEPT_ID = #deptID and ATT_DATE_G = #date;
RETURN #result;
END
The problem is that query takes too long (I mean very long time) to execute.
Any Suggestion of optimization?

Answer 1

Your function is a scalar function, which is run once for every row in the result set (~600,000) times, and is a known performance killer. It can be rewritten into an inline table-valued function, or if the logic is not required elsewhere, a simple group, count & join would suffice:
WITH EmployeesPerDeptPerDate
AS ( SELECT DEPT_ID ,
ATT_DATE_G ,
COUNT(DISTINCT Employee) AS EmployeeCount
FROM attendenceTable
GROUP BY DEPT_ID ,
ATT_DATE_G
)
SELECT A.City ,
A.Department ,
A.Employee ,
A.[Attendence Date] ,
A.[Attendance Status] ,
ISNULL(B.EmployeeCount, 0) AS EmployeeCount
FROM attendenceTable AS A
LEFT OUTER JOIN EmployeesPerDeptPerDate AS B ON A.DEPT_ID = B.DEPT_ID
AND A.ATT_DATE_G = B.ATT_DATE_G;