Selecting data interval - mysql

Question

I have a table that has date and id column. How can I select id's of 7 days interval?
My data is:
date id
2013-07-01 11
2013-07-02 22
2013-07-03 33
2013-07-04 33
2013-07-05 44
2013-07-06 44
2013-07-07 45
2013-07-08 46
2013-07-09 47
2013-07-10 48
2013-07-11 48
2013-07-12 49
2013-07-13 50
2013-07-14 51
2013-07-15 52
2013-07-16 52
2013-07-17 53
2013-07-18 53
2013-07-19 54
What I want is:
date id
2013-07-01 11
2013-07-08 46
2013-07-15 52
Thanks

Answer 1

SELECT date,id FROM table1 GROUP BY WEEK(`date`, 1)
http://sqlfiddle.com/#!2/b128c/1

Answer 2

the group by does not do the trick with (any) sql - ok mysql does - others wont allow this
SELECT date,id FROM table1 GROUP BY WEEK(`date`, 1)
would result in something like error: id is not part of group by ....
if you group by a col1 and not by col2 the database does not know what value you want for col2.
MySql seems to assume "if i do not group by others, take the smallest or first in databsse order'
If - and only if - you want the first (!) result - or what ever MySql decides to use for your missing 2nd grouping element, you are ok.
explanation:
assume this:
CREATE TABLE table1
(`id` int, `val` int );
INSERT INTO table1
(`id`, `val`)
VALUES
(1,99), --'!!!!'
(1,2),
(1,3),
(1,4),
(1,2),
(2,1),
(1,1),
(2,2),
(3,1),
(4,1)
;
See - please, there a 6 rows with 'id'=1, two with id '2' others unique
select id, val FROM table1 GROUP BY id
evaluates to:
ID VAL
1 99
2 1
3 1
4 1
this is only - probably - what you want and if it is a date you have a chance(!) the it will be what you want
to get a valid (without database intepret) result set you will have to use:
select id, some_aggregation_function(val) from table1 group by id
where aggregation is min, max or so.
the is some thing like :
select id, val FROM table1 a
where (id,val)=(select id, min(val) from table1 b where a.id=b.id)
if you want the minimum ....