CODEWITHSUNDEEP
mysql
FAYSS
FAYSS

Reputation: 55

Select every other day of week using SQL (MySQL)

The goal is to return a set of dates that correspond to the day of week requested.

For example: every other Monday between 2018-08-13 and 2018-12-31.

The following statement returns all dates and works great

select * from 
(select adddate('2010-01-01',t4*10000 + t3*1000 + t2*100 + t1*10 + t0) DATES from
 (select 0 t0 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t0,
 (select 0 t1 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t1,
 (select 0 t2 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t2,
 (select 0 t3 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t3,
 (select 0 t4 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t4) v
where DATES between '2018-08-13' and '2018-12-31'
and dayname(DATES) ='MONDAY'

So to return every other MONDAY I added the following

HAVING DATES % 2 = 0

I assumed that by taking MOD of dates it would, in this case return every MONDAY that met this criteria. Well it does not work. I've tried all sorts of combos but not getting it.

Any ideas?

Upvotes: 3

Views: 866

Answers (2)

Dhruv Saxena
Dhruv Saxena

Reputation: 1346

This can be made to work with a few little modifications as follows:

SELECT * FROM
( 
SELECT ADDDARE('1970-01-01', t4*10000 + t3*1000 + t2*100 + t1*10 + t0) selected_date FROM
    (SELECT 0 t0 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t0,
    (SELECT 0 t1 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t1,
    (SELECT 0 t2 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t2,
    (SELECT 0 t3 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t3,
    (SELECT 0 t4 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t4
) v
WHERE 
    selected_date between '2018-08-08' AND '2018-10-19'
    AND DAYOFWEEK(selected_date) = 2
    AND WEEK(selected_date) % 2 = WEEK('2018-08-08') % 2

The idea is that it will include the Monday of the week that the selection period begins from. So, it doesn't matter whether the starting week is odd or even, you'd always get alternate Mondays without having to worry about specifying 0 or 1 for the result of modulo operation.

Upvotes: 0

Sentinel
Sentinel

Reputation: 6449

Dates are a complex data structure and the details of how they are stored and manipulated vary between DBMSs. Most often you can't treat them as numeric data types as you are trying to do here. Instead you should calculate the number of days between the date of interest and a fixed date then you can take the modulus of that number: for example:

select * from 
(select adddate('2010-01-01',t4*10000 + t3*1000 + t2*100 + t1*10 + t0) DATES from
 (select 0 t0 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t0,
 (select 0 t1 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t1,
 (select 0 t2 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t2,
 (select 0 t3 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t3,
 (select 0 t4 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t4) v
where DATES between '2018-08-13' and '2019-01-31'
and dayname(DATES) ='MONDAY'
and datediff(dates,'2018-08-13') % 2 = 0
order by dates

Alternatively since every other Monday is 14 days apart you could use modulus 14 and drop the dayname(DATES) ='MONDAY' predicate:

select * from 
(select adddate('2010-01-01',t4*10000 + t3*1000 + t2*100 + t1*10 + t0) DATES from
 (select 0 t0 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t0,
 (select 0 t1 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t1,
 (select 0 t2 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t2,
 (select 0 t3 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t3,
 (select 0 t4 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t4) v
where DATES between '2018-08-13' and '2019-01-31'
and datediff(dates,'2018-08-13') % 14 = 0
order by dates

Upvotes: 1

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