Find date in a specific week - sql

Question

I would like to select an item where the date is in a specific week in the year.
For example :
SELECT item
FROM table
WHERE my_date in week 44
Where week 44 is from monday 31/10 to sunday 06/11.
I know to get the week number with
to_number(to_char(to_date(my_date,'MM/DD/YYYY'),'IW'))
How can I do that ?
Thanks.

Answer 1

Try:
SELECT item
FROM table
WHERE to_char( my_date, 'IW ) = '44'
See this link for details:
https://docs.oracle.com/cd/B19306_01/server.102/b14200/sql_elements004.htm#i34924
IW ---> Week of year (1-52 or 1-53) based on the ISO standard.

Answer 2

Working with ISO Weeks is not trivial because week 1 can start in previous year and first days in January may be counted as week 52 or 53.
So, providing just week number without a year can be ambiguous (for week number 52, 53, 1).
The best function I found in order to get the first day of an ISO-Week is
NEXT_DAY(TO_DATE( yearNo || '0104', 'YYYYMMDD' ) - INTERVAL '7' DAY, 'MONDAY') + ( weekNo - 1 ) * 7
So, for your need it would be
SELECT item
FROM table
WHERE my_date
between NEXT_DAY(TO_DATE( yearNo || '0104', 'YYYYMMDD' ) - INTERVAL '7' DAY, 'MONDAY') + ( weekNo - 1 ) * 7
AND 6 + (NEXT_DAY(TO_DATE( yearNo || '0104', 'YYYYMMDD' ) - INTERVAL '7' DAY, 'MONDAY') + ( weekNo - 1 ) * 7)
Actually the "full error proven" way would be this one:
FUNCTION ISOWeekDate(weekNo INTEGER, yearNo INTEGER) RETURN DATE DETERMINISTIC IS
res DATE;
BEGIN
IF weekNo > 53 OR weekNo < 1 THEN
RAISE VALUE_ERROR;
END IF;
res := NEXT_DAY(TO_DATE( yearNo || '0104', 'YYYYMMDD' ) - INTERVAL '7' DAY, 'MONDAY') + ( weekNo - 1 ) * 7;
IF TO_CHAR(res, 'fmIYYY') = yearNo THEN
RETURN res;
ELSE
RAISE VALUE_ERROR;
END IF;
END ISOWeekDate;

Answer 3

If the data type of the my_date column is, in fact, VARCHAR2 (which it shouldn't be, but it often is), then you do need to convert it to date first, and then back to char (string) in a different format. Then, if 44 is not necessarily a number but it may be the result of a calculation, or a subquery, or a value in another table (and if it is of NUMBER data type), then you need to do exactly what you were doing. What was the problem with it?
select item
from your_table
where to_number(to_char(to_date(my_date,'MM/DD/YYYY'),'IW')) = 44;
Perhaps you want to know if this can be done more efficiently? So, for example, can you instead write the where condition as
my_date between (something) and (something else)
which would allow the use of an index on my_date? The answer is NO, if my_date is in VARCHAR2 format - you would still have to wrap it within to_date() so you would lose the index.
But, you may build an index function on to_date(my_date) which perhaps would help you in other queries as well. So then the question of making your present query more efficient is meaningful. Now you run into the complication of ISO week not being well defined (you are not also saying which YEAR), so the assumption is that you want the date to be in ISO week 44 of whatever year it is in. If the date is a pure date (time component equal to 00:00:00), you could do this:
where to_date(my_date, 'MM/DD/YYYY') between
trunc(sysdate, 'iw') + 7 * (44 - to_number(to_char(sysdate, 'iw'))) and
trunc(sysdate, 'iw') + 7 * (44 - to_number(to_char(sysdate, 'iw'))) + 6
There are a lot of computation on the right-hand side, but they are done just once for all the rows in your table; they will take essentially no time at all.