DATE FOR FIRST DAY OF THE WEEK

Question

I am new to Oracle and am looking for a procedure that will calculate the first day of the week given today's date.
Given 1/23/2000 I need to find Sunday 1/21/2000.

Answer 1

Hi there,
I don't think there's a built-in Oracle procedure for what you want. However...
In SQL (at least, Oracle's SQL), you have the function TO_CHAR and TO_DATE that allow you to convert a date to a string and vice-versa. They work pretty much in a similar way, that is :
TO_<something>( <data_to_convert>, <format> )
eg.: To have the day of the week for the current date, you can do
SELECT TO_CHAR( SYSDATE, 'D' ) FROM DUAL;
Wednesday, January 24th, 2001 would then return 4.
To have the day of the year, you would do
TO_CHAR( SYSDATE, 'DDD' )
Then, to have the day of the year of the first day of the week, you then could subtract the day of the week of your date from the day of the year of your date, and then add one. To be able to do math stuff of char value, you must use TO_NUMBER.
In one ugly line, you could do this by doing the following :
select to_date( (to_char(sysdate,'YYYY') &#0124; &#0124; to_char( to_number( to_char(sysdate,'DDD')) - to_number( to_char(sysdate,'D')) + 1)),'YYYYDDD') from dual;
Eurk.
Or you could write a function that would return the date of the first day of the week of a specific date, like :
create or replace
FUNCTION FDOW (P_DATE DATE) RETURN DATE
IS
R_FDOW DATE;
BEGIN
R_FDOW := to_date( (to_char(P_DATE,'YYYY') &#0124; &#0124; to_char( to_number( to_char(P_DATE,'DDD')) - to_number( to_char(P_DATE,'D')) + 1)),'YYYYDDD');
return R_FDOW;
END;
(FDOW : First Day Of Week - not really original, I know)
Then, you can call it wherever you want:
SELECT FDOW(SYSDATE) FROM DUAL;
Have fun!
Frederic Denis