How to create date based on year, week number and day in bigquery

Question

How to form date from the year, week number and day value in Bigquery query.

For example- Year =2019, week number =37 day=Wednesday.

Output: (2019-09-18)

Year =2019, week number =36 day=Tuesday.

Output: (2019-09-10)

Here month will be calculated from the week and date will be based on when the given day falls in that week.

Note: Bigquery starts a week from 0, so if check above value for comparison in another system it could be 38 and 37

Answer 1

Below is for BigQuery Standard SQL

#standardSQL
WITH `project.dataset.table` AS (
  SELECT 2019 year, 37 week, 'Wednesday' day UNION ALL
  SELECT 2019, 36, 'Tuesday' 
)
SELECT *, 
  (
    SELECT DATE_ADD(DATE_ADD(DATE(year, 1, 1), INTERVAL week WEEK), INTERVAL hit DAY)         
    FROM UNNEST(GENERATE_ARRAY(0, 6)) hit
    WHERE FORMAT_DATE('%A', DATE_ADD(DATE_ADD(DATE(year, 1, 1), INTERVAL week WEEK), INTERVAL hit DAY)) = day
  ) AS result
FROM `project.dataset.table`  

with result as

Row year    week    day         result   
1   2019    37      Wednesday   2019-09-18   
2   2019    36      Tuesday     2019-09-10   

Answer 2

Shouldn't it be week-1 like this:

WITH tbl AS (
  SELECT 2020 year, 21 week, 'Friday' day
)
SELECT *, 
  (
    SELECT DATE_ADD(DATE_ADD(DATE(year, 1, 1), INTERVAL week-1 WEEK), INTERVAL hit DAY)         
    FROM UNNEST(GENERATE_ARRAY(0, 6)) hit
    WHERE FORMAT_DATE('%A', DATE_ADD(DATE_ADD(DATE(year, 1, 1), INTERVAL week-1 WEEK), INTERVAL hit DAY)) = day
  ) AS result
FROM tbl

Otherwise it returns the date of the next week