t-sql function to determine someone category - sql

Question

If I am given a date like 1999-07-08 15:49:00 what would be a good function to determine whether is an AM shift, PM shift or a NOC shift?
--AM: 06:45:00 - 14:44:59
--PM: 14:45:00 - 22:59:59
--NOC: 23:00:00 - 06:44:59
Here is my attempt but then I noticed a bug
ALTER FUNCTION [dbo].[DateToNocShift]
(
-- Add the parameters for the function here
#DummyDate DATETIME
)
RETURNS VARCHAR(10)
AS
BEGIN
-- Declare the return variable here
DECLARE #Shift VARCHAR(10)
DECLARE #DateValues TABLE
(
RawDate DATETIME,
HourNow int,
MinuteNow int,
TimeHourMinute FLOAT,
Shift VARCHAR(4)
)
INSERT INTO #DateValues
VALUES
(
#DummyDate,
DATEPART(hour,#DummyDate),
cast(DATEPART(minute,#DummyDate)as decimal),
ROUND(DATEPART(hour,#DummyDate) + cast(DATEPART(minute,#DummyDate)as decimal)/60,2),
null
)
UPDATE #DateValues
SET Shift = 'AM'
WHERE TimeHourMinute BETWEEN 6.75 AND 14.74 -- good estimate
UPDATE #DateValues
SET Shift = 'PM'
WHERE TimeHourMinute BETWEEN 14.75 AND 22.99
UPDATE #DateValues
SET Shift = 'NOC'
WHERE TimeHourMinute BETWEEN 23.00 AND 6.74
SELECT #Shift = Shift FROM #DateValues
RETURN #Shift

Answer 1

You don't need so much code, a single CASE statement will do
CREATE FUNCTION [dbo].[DateToNocShift]
(
-- Add the parameters for the function here
#DummyDate DATETIME
)
RETURNS VARCHAR(10)
AS
BEGIN
DECLARE #dateChar varchar(8)
set #dateChar = convert(varchar, #DummyDate, 108)
RETURN CASE
WHEN #dateChar >= '06:45:00' and #dateChar < '14:45:00' then 'AM'
WHEN #dateChar >= '14:45:00' and #dateChar < '23:00:00' then 'PM'
ELSE 'NOC'
END -- CASE
END

Answer 2

Another approach would be to create a table with a row in it for every minute of the day, 1440 rows.
CREATE TABLE ShiftCheck
(
id int,
MyTime datetime not null,
ShiftNumber int not null,
ShiftName char(3) not null
CONSTRAINT [PK_ShiftCheck] PRIMARY KEY CLUSTERED
(
[id] ASC
)
)
populate that table with a statement similar to this
; WITH DateIntervalsCTE AS
(
SELECT 0 i, cast('1/1/1900' as datetime) AS Date
UNION ALL
SELECT i + 1, DATEADD(MINUTE, i,cast('1/1/1900' as datetime) )
FROM DateIntervalsCTE
WHERE DATEADD(MINUTE, i, cast('1/1/1900' as datetime) ) < cast('1/2/1900' as datetime)
)
SELECT
ROW_NUMBER() over (order by Date),
Date,
CASE
WHEN convert(varchar, Date, 108) >= '06:45:00' and convert(varchar, Date, 108) < '14:45:00' then 1
WHEN convert(varchar, Date, 108) >= '14:45:00' and convert(varchar, Date, 108) < '23:00:00' then 2
ELSE 3
END,
CASE
WHEN convert(varchar, Date, 108) >= '06:45:00' and convert(varchar, Date, 108) < '14:45:00' then 'AM'
WHEN convert(varchar, Date, 108) >= '14:45:00' and convert(varchar, Date, 108) < '23:00:00' then 'PM'
ELSE 'NOC'
END
FROM DateIntervalsCTE
OPTION (MAXRECURSION 32767);
Then, you just need to join to the ShiftCheck table on a time. There is only a need to calculate what time a shift is in one time, to populate the table.
Scalar Valued Functions, like the one listed in your question are executed for every row in a given query. For example
Select *,[dbo].[DateToNocShift](ShiftDate)
from myTable
the function will be executed for every row in myTable which at a certain point (Saturday morning while you are sleeping) will get very slow. In conclusion, this will eventually become a performance problem and eventually someone will want to see the word 1st, 2nd, 3rd for the shift name. This solution will solve both of those as well as force a look-up instead of a calculation.
*If that is not accurate enough, put a row for every second of the day (24*60*60 = 86400 rows, still not that big for sql server)
*I took the generate sql from here