SQL specific time with a moving date - sql

Question

I'm using MS SQL and I'd like to have a query that is basically
SELECT * FROM Data Where TimeStamp <= 06:30:00 Last Saturday
Anyone have anything functional to achieve this?

Answer 1

You can get that time for last Saturday using 30 minutes and 6*60 for minutes in 6 hours (you can get this a few ways but I hard coded it using that:
DATEADD(MINUTE,(30 + (6 * 60) ), DATEADD(WEEK, DATEDIFF(WEEK, -1, CURRENT_TIMESTAMP), -2))
Then you can use that in the where clause:
SELECT
*
FROM dbo.Data AS d
WHERE
d.TimeStamp <= DATEADD(MINUTE,(30 + (6 * 60) ), DATEADD(WEEK, DATEDIFF(WEEK, -1, CURRENT_TIMESTAMP), -2))
UPDATE: TLDR; Break this down using a sample query: (hopefully it helps)
DECLARE #MyDatetime AS DATETIME = '2021-09-12 03:02:05.257' --CURRENT_TIMESTAMP;
SELECT
#MyDatetime AS FromWhen,
/* all these are The int difference between the startdate and enddate, expressed in the boundary set by datepart (WEEK)) */
DATEDIFF(WEEK, 0, #MyDatetime) AS MyWeek0,
DATEDIFF(WEEK, -1, #MyDatetime) AS MyWeek1,
DATEDIFF(WEEK, -2, #MyDatetime) AS MyWeek2,
DATEDIFF(WEEK, -3, #MyDatetime) AS MyWeek3,
DATEDIFF(WEEK, -4, #MyDatetime) AS MyWeek4,
DATEDIFF(WEEK, -5, #MyDatetime) AS MyWeek5,
DATEDIFF(WEEK, -6, #MyDatetime) AS MyWeek6,
DATEDIFF(WEEK, -7, #MyDatetime) AS MyWeek7,
DATEDIFF(WEEK, -8, #MyDatetime) AS MyWeek8,
DATEDIFF(WEEK, -1, #MyDatetime) AS AWeekBack, -- get the week a week from RightNow DATEDIFF ( datepart , startdate , enddate )
DATEADD(WEEK, DATEDIFF(WEEK, -1, #MyDatetime), -2) AS Saturday0000, -- Get the Saturday date from that (-2 is saturday, -1 sunday, ...-7 monday)
DATEADD(MINUTE,(30 + (6 * 60) ), DATEADD(WEEK, DATEDIFF(WEEK, -1, #MyDatetime), -2)) AS Saturday0630 -- now add the minutes for 6:30 AM to that saturday date
Values for a given set from that query:
FromWhen MyWeek0 MyWeek1 MyWeek2 MyWeek3 MyWeek4 MyWeek5 MyWeek6 MyWeek7 MyWeek8 AWeekBack Saturday0000 Saturday0630
2021-09-12 03:02:05.257 6350 6350 6351 6351 6351 6351 6351 6351 6351 6350 2021-09-11 00:00:00.000 2021-09-11 06:30:00.000
Broken down:
Go back a week (-1) from NOW
DATEDIFF always uses Sunday as the first day of the week to ensure the function operates in a deterministic way this -2 is Saturday.
DATEDIFF(WEEK, -1, CURRENT_TIMESTAMP)
For example this is one query I use often get get those few thousand rows from a 120 second range on a given DATETIME column:
SELECT *
FROM SomeTable AS st
WHERE
st.DateColumn >= DATEADD(SECOND, -60, #SomeDateTime)
AND st.DateColumn < DATEADD(SECOND, 60, #SomeDateTime)

Answer 2

With a calendar table, this is quite logical. Get me the highest row from the table that is a Saturday before today.
DECLARE #d smalldatetime;
SELECT #d = DATEADD(MINUTE, 390, MAX(TheDate))
FROM dbo.CalendarTable
WHERE TheDayName = 'Saturday'
AND TheDate < CONVERT(date, GETDATE());
SELECT ... FROM dbo.Data WHERE [TimeStamp]/*shudder*/ <= #d;
Without a calendar table, you can do this in a slightly less intuitive way, by picking a known Saturday in the past, and adding the number of weeks that have passed since that date, less 1:
DECLARE #KnownSat smalldatetime = '20110101';
SELECT #d = DATEADD(MINUTE, 390, DATEADD
(
WEEK,
DATEDIFF(WEEK, #KnownSat, GETDATE()) - 1,
#KnownSat
));
SELECT ... FROM dbo.Data WHERE [TimeStamp]/*shudder*/ <= #d;
Note that both solutions will return the previous Saturday when run on a Saturday.