week ending

Hi All,

I just finished writing a query that groups data based on the week number.

SELECT EXTRACT(week FROM col_a) AS week_number,       sum(col_b) AS col_b_total  FROM foo WHERE foobar GROUP BY
EXTRACT(weekFROM col_a) ORDER BY EXTRACT(week FROM col_a);
 

I would like to generate the starting date or ending date based on this 
number.  IOW instead of telling the user "week number" which they won't 
understand I would like to provide either Friday's date for "week 
ending" or Monday's date for "week beginning".

SELECT <something> AS week_ending,       sum(col_b) AS col_b_total  FROM foo WHERE foobar GROUP BY EXTRACT(week FROM
col_a)ORDER BY EXTRACT(week FROM col_a);
 

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Kind Regards,
Keith