Error building a SQL query while trying to get order by a day for a period of week - sql

Question

I have an ASP.NET Core application, through controller endpoint I pass #by and #period string values to the SQL query.
#by takes one of the following values: day, week
#period takes one of the following values: week, month, year
When the #period is month or year, then #by is a week, else it's a day.
I have the following working query when the #period is a month or a year:
SELECT
l.region_id AS region_id,
'Region ' + r.region_desc AS region_name,
MIN(DATEADD(D, -(DATEPART(WEEKDAY, s.pos_date) - 1), s.pos_date)) AS date_pos,
CONVERT(VARCHAR(20), MIN(DATEADD(D, -(DATEPART(WEEKDAY, s.pos_date) - 1), s.pos_date)), 107) AS display_date_pos
FROM
incent_summary s
INNER JOIN
location l ON s.store_num = l.store_num
INNER JOIN
region r ON l.region_id = r.region_id
WHERE
s.pos_date >= DATEADD(day, #period , CONVERT(date, GETDATE()))
AND s.pos_date <= GETDATE()
GROUP BY
DATEPART (#by, s.pos_date),
l.region_id, r.region_desc
ORDER BY
DATEPART (#by, pos_date),
l.region_id, r.region_desc
The issue is when the #period is a week, #by is day, and the statement
MIN(DATEADD(D, -(DATEPART(WEEKDAY, s.pos_date) - 1), s.pos_date)) AS date_pos
returns the same day for all the 7 days.
Sample output when #period = year and #by = week:
region_id region_name date_pos display_date_pos
---------------------------------------------------------------------
34 Region 43 2019-12-29 00:00:00.000 Dec 29, 2019
50 Region 22 2019-12-29 00:00:00.000 Dec 29, 2019
34 Region 43 2020-01-05 00:00:00.000 Jan 05, 2020
50 Region 22 2020-01-05 00:00:00.000 Jan 05, 2020
34 Region 43 2020-01-12 00:00:00.000 Jan 12, 2020
50 Region 22 2020-01-12 00:00:00.000 Jan 12, 2020
34 Region 43 2020-01-19 00:00:00.000 Jan 19, 2020
50 Region 22 2020-01-19 00:00:00.000 Jan 19, 2020
34 Region 43 2020-01-26 00:00:00.000 Jan 26, 2020
50 Region 22 2020-01-26 00:00:00.000 Jan 26, 2020
Sample output when #period = week and #by = day:
region_id region_name date_pos display_date_pos
--------------------------------------------------------------------
34 Region 43 2020-07-12 00:00:00.000 Jul 12, 2020
50 Region 22 2020-07-12 00:00:00.000 Jul 12, 2020
34 Region 43 2020-07-12 00:00:00.000 Jul 12, 2020
50 Region 22 2020-07-12 00:00:00.000 Jul 12, 2020
34 Region 43 2020-07-19 00:00:00.000 Jul 19, 2020
50 Region 22 2020-07-19 00:00:00.000 Jul 19, 2020
34 Region 43 2020-07-19 00:00:00.000 Jul 19, 2020
50 Region 22 2020-07-19 00:00:00.000 Jul 19, 2020
34 Region 43 2020-07-19 00:00:00.000 Jul 19, 2020
50 Region 22 2020-07-19 00:00:00.000 Jul 19, 2020
How can I fix this?

Answer 1

SELECT
DATEADD(D, -(DATEPART(WEEKDAY, s.pos_date) - 1), s.pos_date)
Will always return the first day of the week because the logic is: "subtract from my date the number of days from sunday and add 1."
Sunday: 1 - 1 + 1 = 1 = Sunday
Monday: 2 - 2 + 1 = 1 = Sunday
.
.
.
Saturday: 7 - 7 + 1 = Sunday
That's fine when you want the first Sunday of the year/month/whatever. But the first sunday of every week is always... sunday. But in this case you really just need to take the MIN(s.pos_date) if #period is week.
There's probably some crazy way to do this in a single statement using quaternions or something else super mathy, but it's easiest to just use a case statement:
MIN
(
CASE
WHEN '#by' = 'day' THEN s.pos_date
ELSE DATEADD(D, -(DATEPART(WEEKDAY, s.pos_date) - 1), s.pos_date)
END
)
I'm not a C# programmer so I can't tell you the exact way to make sure the string DAY is passed to the query as "DAY" but I'm sure you can handle that part.
ALSO IMPORTANT The datepart "day" is day of month, so if you're going to possibly have a span greater than one month (but under a year), use dayofyear.