Last n weekdays in sql where clausel

We are using MySQL as our database to store messages with timestamps. Is it possible to create a query that returns messages of the last n weekdays?

I.e. if n is 4 and today is Tuesday, I want messages from this weeks Monday, last weeks Friday, last weeks Thursday and last weeks Wednesday .

Solution

If you want to do this directly with mysql it would be a little complicated. As Vatev recommended you should calculate date star date in advance, but if you really want to do this, you'll probably need following functions:

ADD_DATE, with INTERVAL -N WEEKS
FLOOR, in C int/int would do just fine
MOD, a % b :)
WEEKDAY

First of all you need should count how many weeks you should go back, that's easy... For you one week = 5 days, that means

weeks = FLOOR(days / 5)

We've taken care of weeks, so we'll now have to work with the rest:

rest = days MOD 5

Now we have two cases, weekend has occurred or no, for the case that there wasn't weekend days are good. We have to add 2 days to skip it. The weekend occurred if (WEEKDAY(now) - rest) < 0

rest = IF( (WEEKDAY(now) - rest) < 0, rest + 2, rest)

And now we can build it to one par (let's assume you have {days} and {rest} pre-calculated):

WHERE date >= ADD_DATE(
    ADD_DATE (
        {now},
        INTERVAL -IF( (WEEKDAY({now}) - {rest}) < 0, {rest} + 2, {rest}) DAYS, 
    ),
    INTERVAL -FLOOR({days} / 5) WEEKS
)