Convert year, week into date based on year's first day in mySQL

Question

I have sum of all week sales into one object and object need to display in charts and all chart need to have date in x-axis so, Here I am calculating date from year+week based on Monday

SELECT STR_TO_DATE(CONCAT('20131',' Monday'), '%X%V %W');

I want it based on first day on year

SELECT DAYNAME(MAKEDATE(2013,1))

All togather

SELECT STR_TO_DATE(CONCAT(CONCAT(2013, 1), DAYNAME(MAKEDATE(2013,1))), '%X%V %W')

return '2013-01-08' which is bad value I need '2013-01-01'

SELECT STR_TO_DATE(CONCAT(CONCAT(2017, 1), DAYNAME(MAKEDATE(2017,1))), '%X%V %W')

here it return '2017-01-01' which is good

My problem is in some years it went to next week which is bad I am not sure where and what I am doing wrong ... So Please any kind of help...

Problem:

In some year like 2013 when I convert year+ week to date I get date in one extra week like 2013year+ 1week gives 2013-01-08 which is in second week instead of first

but

In some year like 2017 when I convert year+ week to date I get date in right week like 2017year+ 1week gives 2017-01-01

Why is this behavior?

Answer 1

MySQL supports intervals which seems to be the best feature for creating a date from a year and week number. See the following query as an example which calculates the first week of 2013 and the second week of 2013.

SELECT DATE_ADD(makedate(`year`, 1), INTERVAL (`week` - 1) WEEK) as weekdate
FROM (
   (SELECT 2013 as `year`, 1 as `week`) UNION 
   (SELECT 2013 as `year`, 2 as `week`)
) t1

The output is two rows

weekdate
2013-01-01
2013-01-08

The following also works but is far less easy to read

SELECT MAKEDATE(`year`, 1 + ((`week` - 1) * 7)) as weekdate 
FROM (
    (SELECT 2013 as `year`, 1 as `week`) UNION 
    (SELECT 2013 as `year`, 2 as `week`)
) t1