如何获得给定日期的前一个星期五和下个星期四？

Question

I am trying to query the previous thursday as well as the next friday given a certain date. If the date passed is friday then PrevFriday should be the current date. Similarily if the date passed is thursday then nextThursday should be the date that was passed.

I have seen some other solutions on this site that include the use of WEEKDAY() offset by some interval but this only works when the date is static. As I am going to be passing a column WorkDate through this later I need this to work for every date and not just specific dates.

I currently have a query that works ONLY for today and not a range of dates.

1.

SELECT '2019-05-16' + INTERVAL -3 - weekday('2019-05-16') DAY AS PrevFriday,
       '2019-05-16' + INTERVAL 3 - weekday('2019-05-16') DAY AS NextThursday

This code outputs the expected result but if we change the date we are passing to tomorrow:

2.

SELECT '2019-05-17' + INTERVAL -3 - weekday('2019-05-17') DAY AS PrevFriday,
       '2019-05-17' + INTERVAL 3 - weekday('2019-05-17') DAY AS NextThursday

The EXPECTED output for the first piece of code:

PrevFriday -> 2019-05-10 , NextThursday -> 2019-05-16

The EXPECTED output for the second piece of code:

PrevFriday -> 2019-05-17 , NextThursday -> 2019-05-23

But the ACTUAL results are:

PrevFriday -> 2019-05-10 , NextThursday -> 2019-05-16

Answer 1

Try something like this:

select t.*
    , t.dt + interval (7 + 3 - weekday(t.dt)) % 7 day as NextThursday
    , t.dt - interval (7 - 4 + weekday(t.dt)) % 7 day as PrevFriday
from test t

The main idea is to use the modulo operator % 7 , so we never add or subtract more than 6 days.

Test Data:

create table test(
  d char(2),
  dt date
);
insert into test(d, dt) values
('mo', '2019-05-13'),
('tu', '2019-05-14'),
('we', '2019-05-15'),
('th', '2019-05-16'),
('fr', '2019-05-17'),
('su', '2019-05-18'),
('so', '2019-05-19');

Result:

d  | dt         | NextThursday | PrevFriday
---|------------|--------------|-----------
mo | 2019-05-13 |   2019-05-16 | 2019-05-10
tu | 2019-05-14 |   2019-05-16 | 2019-05-10
we | 2019-05-15 |   2019-05-16 | 2019-05-10
th | 2019-05-16 |   2019-05-16 | 2019-05-10
fr | 2019-05-17 |   2019-05-23 | 2019-05-17
su | 2019-05-18 |   2019-05-23 | 2019-05-17
so | 2019-05-19 |   2019-05-23 | 2019-05-17

db-fiddle demo

Answer 2

IF WEEKDAY(DateVar) = 4 then --Friday logic
    PrevFriday = DateVar
    NextThursday =  DATE_ADD(DateVar, INTERVAL 6 DAY);
ELSEIF WEEKDAY(DateVar) = 3 THEN --Thursday logic
    PrevFriday = DATE_ADD(DateVar, INTERVAL -6 DAY)
    NextThursday = DateVar;
ELSE
    -- logic to ind the Prev Friday and Next Thursday;