syntax error in calculation of attendance % in mysql - mysql

Question

SELECT name, DISTINCT studentid, count(attendance)
from attendance a,students s
where attendance = 'p'and s.studentid=a.studentid
having count(attendance)<3/4*sum(attendance);
I have 2 tables attendance and students from which i wanna choose the name of the student(from student table) and attendance (from attendance table) where studentid is the foreign key of those students whose attendance<75%. i save attendance as p and a for present and absent respectively.

Answer 1

You could try something like this:
Data prep
create table attendance (studentid int, attendance char(1));
insert into attendance values (1,'p'),(1,'a'),(2,'p'),(2,'p'),(2,'a'),(3,'p');
Data
select * from students;
+-----------+------+
| studentid | name |
+-----------+------+
| 1 | John |
| 2 | Matt |
| 3 | Mary |
+-----------+------+
select * from attendance;
+-----------+------------+
| studentid | attendance |
+-----------+------------+
| 1 | p |
| 1 | a |
| 2 | p |
| 2 | p |
| 2 | a |
| 3 | p |
+-----------+------------+
Query
select s.*, a.total, a.p_present
from students s
inner join (
select studentid, count(*) as total, sum(case attendance when 'p' then 1 else 0 end) * 100/count(*) as p_present
from attendance
group by studentid
) a on s.studentid = a.studentid
where a.p_present < 75 ;
Result
+-----------+------+-------+-----------+
| studentid | name | total | p_present |
+-----------+------+-------+-----------+
| 1 | John | 2 | 50.0000 |
| 2 | Matt | 3 | 66.6667 |
+-----------+------+-------+-----------+
p_present is percent present. Note that John and Matt had 50% and 66.6% attendance, respectively.
Explanation
In order to get total records, we'd do something like this:
select studentid, count(*)
from attendance
group by studentid;
In order to get total times each student was present, we'd do:
select studentid, sum(case attendance when 'p' then 1 else 0 end)
from attendance
group by studentid;
% present is going to be the # of times the student was present divided by the total. So, that's what I did in the subquery.
Once the data about the student was available, join that result with student's information and extract the information desired from both tables.