Get record in range of multiples of 5 - mysql

Question

i have a existing new week_table -
start_date end_date weekno ----------------------------------------------
1996-01-01 1996-01-05 1
1996-01-08 1996-01-12 2
1996-01-15 1996-01-19 3
1996-01-22 1996-01-26 4
''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''till
1998-12-21 1998-12-26 156
i am trying to extract records with a count of 5 weeks in group. I am looking at results like
start_date end_date weekno_start weekno_end ----------------------------------------------
1996-01-01 1996-02-02 1 5
1996-02-05 1996-03-08 6 10
1996-03-11 1996-04-12 11 16
i do get the results but the weekno numbers keep running over the maximum week no in the database. for records over weekno 156 i get rows with null value.
How can i avoid the records with null and limit the view to the maximum week no
my current code is-
SELECT (t1.weekno * 5) - 4 AS start_id
,t3.start_date
,t4.end_date
,(t1.weekno * 5) AS end_id
FROM weekcon_table t1
LEFT JOIN weekcon_table t2 ON (t2.weekno = t1.weekno * 5)
LEFT JOIN weekcon_table t3 ON (t3.weekno = (t1.weekno * 5) - 4)
LEFT JOIN weekcon_table t4 ON (t4.weekno = (t1.weekno * 5))

Answer 1

Have you tried something like this:
select min(weekno) as `start_id`,
min(start_date) as `start_date`,
max(end_date) as `end_date`,
min(weekno) as `weekno_start`,
max(weekno) as `weekno_end`
from weekcon_table
group by ((weekno - 1) DIV 5)
order by ((weekno - 1) DIV 5) asc
Here is the output:
start_id start_date end_date weekno_start weekno_end
1 01/01/1996 26/01/1996 1 5
6 04/03/1996 24/02/1996 6 10
11 01/04/1996 30/03/1996 11 15
16 06/05/1996 27/04/1996 16 20
21 03/06/1996 25/05/1996 21 23
Record Count: 5; Execution Time: 1ms View Execution Plan link

Answer 2

I create two tables and asign a rank_id
the first one is for star_date ... will be each row weekno % 5 = 1
second table is for end_date ... will be each row weekno % 5 = 0 and also include the last date of all weeks.
Then join by rank_id
Sql Fiddle Demo In the demo you can change the select fields for * if want see what is happening
SELECT ini_range.start_date,
end_range.end_date,
ini_range.weekno,
end_range.weekno
FROM
(
SELECT r.* ,
(SELECT count(distinct r2.weekno)
FROM
(
SELECT *
FROM t_week
WHERE weekno % 5 = 1
) r2
WHERE r2.weekno <= r.weekno
) as rank
FROM
(
SELECT *
FROM t_week
WHERE weekno % 5 = 1
) r
) ini_range
JOIN
(
SELECT r.* ,
(SELECT count(distinct r2.weekno)
FROM
(
SELECT *
FROM t_week
WHERE weekno % 5 = 0
or weekno = (SELECT max(weekno) FROM t_week)
) r2
WHERE r2.weekno <= r.weekno
) as rank
FROM
(
SELECT *
FROM t_week
WHERE weekno % 5 = 0
or weekno = (SELECT max(weekno) FROM t_week)
) r
) end_range
ON ini_range.rank = end_range.rank
OUTPUT
| start_date | end_date | weekno | weekno |
|------------|------------|--------|--------|
| 01/01/1996 | 03/02/1996 | 1 | 5 |
| 05/02/1996 | 09/03/1996 | 6 | 10 |
| 11/03/1996 | 13/04/1996 | 11 | 15 |
| 15/04/1996 | 18/05/1996 | 16 | 20 |
| 20/05/1996 | 08/06/1996 | 21 | 23 | <- 23 is last week
and group only have
3 week instead of 5

Answer 3

I found another solution
SQL Fiddle Demo
SELECT *
FROM t_week w_ini
JOIN t_week w_end
ON w_ini.weekno = w_end.weekno + 4
OR w_ini.weekno + 5 > w_end.weekno
WHERE
w_ini.weekno % 5 = 1
and w_ini.weekno < w_end.weekno
and(
w_end.weekno % 5 = 0 or
w_end.weekno = (SELECT max(weekno) FROM t_week)
)