get list of student with attendance min15days in a month and come for continuous 4 months in a year - sql

Question

I need a query to get the list of students attended there class for atleast 15 days in a month for continuous 4 months.
table maybe like
studentid monthyear attendance
1 Apr2018 16
1 May2018 23
1 Jun2018 18
1 Jul2018 16
1 Aug2018 25
2 Apr2018 2
2 May2018 15
and so on...
Db fiddle

Answer 1

Try this query:
select #rn := 0;
select studentid from (
select studentid, month(dt) - (#rn := #rn + 1) grp from (
select * ,
str_to_date(concat('01 ', insert(monthyear, 4, 0, ' ')), '%d %M %Y') dt
from tbl
where attendance >= 15 --only those records, where attenadnce is at least 15
) a where year(dt) = 2018 --particular year
order by studentid,dt
) a group by studentid,grp having count(*) >= 4
Demo - I exapnded your data with some more cases :)
The idea is simple - if student has attended for some consecutive months, consecutive months would increment by one, just like row number, so I used difference between months and row numbers - for consecutive months, the difference should be constant, so it's enought to group by that difference and take those groups, where count is >= 4 :)
UPDATE
For SQL Server:
select studentid from (
select studentid, month(dt) - row_number() over (order by studentid, dt) grp from (
select * ,
cast(concat('01 ', stuff(monthyear, 4, 0, ' ')) as date) dt
from tbl
where attendance >= 15 --only those records, where attenadnce is at least 15
) a where year(dt) = 2018 --particular year
) a group by studentid, grp having count(*) >= 4
SQL Server demo

Answer 2

In general, a simple selft join that would catch the difference of months would suffice
In this case, a conversion of the column monthyear is required in the join command itself
The query, without the conversion :
SELECT t1.studentid, count(*) as cnt
FROM
table t1
INNER JOIN table t2 ON t1.studentid = t2.studentid AND
t2.attendance >= 15
AND t1.monthyear BETWEEN t2.monthyear AND (t2.monthyear - 3)
WHERE
t1.attendance >= 15
GROUP BY
studentid
HAVING
count(*) >=4
The conversion is as follows:
STR_TO_DATE(
CONCAT(SUBSTR(t1.monthyear,1, LENGTH(t1.monthyear) - 4),' ', RIGHT(t1.monthyear, 4), %M %Y)
so the query should be:
SELECT t1.studentid, count(*) as cnt
FROM
table t1
INNER JOIN table t2 ON t1.studentid = t2.studentid AND
t2.attendance >= 15
AND STR_TO_DATE(
CONCAT(SUBSTR(t1.monthyear,1, LENGTH(t1.monthyear) - 4),' ', RIGHT(t1.monthyear, 4), %M %Y) BETWEEN STR_TO_DATE(
CONCAT(SUBSTR(t2.monthyear,1, LENGTH(t2.monthyear) - 4),' ', RIGHT(t2.monthyear, 4), %M %Y) AND DATE_SUB(STR_TO_DATE(
CONCAT(SUBSTR(t2.monthyear,1, LENGTH(t2.monthyear) - 4),' ', RIGHT(t2.monthyear, 4), %M %Y), INTERVAL 3 MONTH)
WHERE
t1.attendance >= 15
GROUP BY
studentid
HAVING
count(*) >=4

Answer 3

I think this is the simplest method:
select distinct studentid
from (select t.*, cast(monthyear as date) as my,
lag(cast(monthyear as date), 3) over (partition by studentid order by cast(monthyear as date)) as prev_my
from tbl t
where attendance >= 15
) t
where prev_my = dateadd(month, -3, my);
Here is a db<>fiddle.
The logic is pretty simple:
Only consider rows that satisfy the attendance criterion.
Use LAG() to look at the 3rd record in past.
If all months meet the attendance criterion, then this will be exactly 3 months before.
The select distinct is because you want students, not the specific periods.