casting custom string to date in SQL

Question

I have a column with strings such as 2015-W02, meaning the second week of 2015. I would like to convert it to a date (e.g., corresponding to the Saturday of that week). It does not seem that CONVERT nor PARSE support this style. Any idea how to do that?

Answer 1

If that week number identifies the ISO week (= week starts on Monday, first week belongs to the year where the "bigger" part falls into), you can use to_date()

select to_date('2015-W02', 'iyyy-"W"iw')

Note that this returns the Monday of that week, as this is how the ISO week is defined.

Answer 2

To deal with dates I would suggest a calendar table, you can find some scripts to generate your own calendar tables on google.

In this case I've used a CTE to generate a series of dates for 2015 and below sample data:

create table t (id int, dt text);
insert into t values (1, '2015-W02'), (2, '2015-W08'), (3, '2015-W21');

with days as
(
    select dt, 
           extract(dow from dt) dow, 
           extract(week from dt) wk,
           extract(year from dt) yr
    from  generate_series('2015-01-01'::timestamp, 
                          '2015-12-31'::timestamp, 
                          '1 day'::interval) dt
)
select
  *
from 
  t
join
  days 
  on substring(t.dt, 1, 4)::int = days.yr
  and substring(t.dt, 7, 2)::int = days.wk
where
  days.dow = 6; --saturdays only

id | dt       | dt                  | dow | wk | yr  
-: | :------- | :------------------ | :-- | :- | :---
 1 | 2015-W02 | 2015-01-10 00:00:00 | 6   | 2  | 2015
 2 | 2015-W08 | 2015-02-21 00:00:00 | 6   | 8  | 2015
 3 | 2015-W21 | 2015-05-23 00:00:00 | 6   | 21 | 2015

db<>fiddle here