How can I avoid duplicate values in my result set? - mysql

Question

CREATE TABLE student
(
s_id int(10) NOT NULL AUTO_INCREMENT,
s_roll_no int(30),
s_name varchar(30),
s_gender varchar(4) not null,
class int(2) not null,
PRIMARY KEY (s_id)
);
CREATE TABLE attendance_date
(
date_today varchar(10),
PRIMARY KEY (date_today)
);
CREATE TABLE attendance_today
(
s_id int(10),
s_roll_no int(30),
s_name varchar(30),
s_gender varchar(4),
class int(2),
date_today varchar(10),
attendance_status varchar(2) not null default 'P'
);
delimiter $$
create trigger after_insertion_into_attendance_date
after insert on attendance_date
for each row
begin
insert into attendance_today(s_id, s_roll_no, s_name, s_gender, class, date_today)
select * from student cross join attendance_date order by date_today, s_id;
end$$
delimiter ;
INSERT INTO student
VALUES
(1,1,'Mridul Kumar','M',1),
(2,2,'Harish Paul','M',1),
(3,3,'Imtiaz Hossain','M',1);
INSERT INTO attendance_date
VALUES
('1st Jan'),
('2nd Jan');
now,
select * from attendance_today;
giving duplicates after every insertion, is there any way to avoid such duplicates inside trigger?
I'm not looking for
select distinct * from attendance_today;
after the trigger gets activated.

Answer 1

This will prevent duplicate values from being inserted
insert into attendance_today(s_id, s_roll_no, s_name, s_gender, class, date_today)
select *
from student cross join attendance_date ad
WHERE NOT EXISTS (
SELECT null FROM attendance_today at
WHERE ad.s_id = at.s_id and ad.date_today = at.date_today
)
order by date_today, s_id;

Answer 2

Try this:
begin
insert into attendance_today(s_id, s_roll_no, s_name, s_gender, class, date_today)
select * from student cross join attendance_date group by s_id order by date_today, s_id;
end$$