how to combine or join 2 MySQL queries together - mysql

Question

I have 2 following queries return the same columns:
first query is slightly modified one I found here in different post returning all day of the current month:
select null as id,
a.day, 0 as was_present,
0 as was_late,
0 as left_earlier,
null as student_id
from (
select
last_day(NOW())
- INTERVAL (a.a + (10 * b.a) + (100 * c.a)) DAY as day
from (
select 0 as a
union all select 1
union all select 2
union all select 3
union all select 4
union all select 5
union all select 6
union all select 7
union all select 8
union all select 9
) as a
cross join
(
select 0 as a
union all select 1
union all select 2
union all select 3
union all select 4
union all select 5
union all select 6
union all select 7
union all select 8
union all select 9
) as b
cross join
(
select 0 as a
union all select 1
union all select 2
union all select 3
union all select 4
union all select 5
union all select 6
union all select 7
union all select 8
union all select 9
) as c
) as a
where a.day between DATE_FORMAT(NOW() ,'%Y-%m-01') AND NOW()
order by a.day
while the second is returning attendance data from the real table like this:
select *
from student_attendance
where student_id = ?1
and (date between DATE_FORMAT(NOW() ,'%Y-%m-01') AND NOW())
Sample data from second query look like this:
2 2019-10-01 1 0 0 5
3 2019-10-02 1 0 0 5
4 2019-10-03 1 0 0 5
5 2019-10-04 1 0 0 5
Now my question is how to combine them together to return only unique days of the month (1 record per day) where the real record from second query replaces fake record in first query when it exist. I tried to use union but it return duplicates.
Again, using union doesn't work, here is what it returns:
2019-10-01 0 0 0
2 2019-10-01 1 0 0 5
2019-10-02 0 0 0
3 2019-10-02 1 0 0 5
4 2019-10-03 1 0 0 5
2019-10-03 0 0 0
2019-10-04 0 0 0
5 2019-10-04 1 0 0 5
2019-10-05 0 0 0

Answer 1

You could use a temporary table then do a LEFT JOIN to it:
CREATE TEMPORARY TABLE days
select a.day
from (
select last_day(NOW()) - INTERVAL (a.a + (10 * b.a) + (100 * c.a)) DAY as
day
from (select 0 as a union all select 1 union all select 2 union all select 3
union
all select 4 union all select 5 union all select 6 union all select 7 union
all
select 8 union all select 9) as a
cross join (select 0 as a union all select 1 union all select 2 union all
select 3
union all select 4 union all select 5 union all select 6 union all select 7
union all
select 8 union all select 9) as b
cross join (select 0 as a union all select 1 union all select 2 union all
select 3
union all select 4 union all select 5 union all select 6 union all select 7
union all
select 8 union all select 9) as c
) a where a.day between DATE_FORMAT(NOW() ,'%Y-%m-01') AND NOW() order by
a.day;
SELECT
id, days.`day`, was_present, was_late, left_earlier, student_id
FROM days
LEFT JOIN student_attendance sa ON days.`day` = sa.`day`;

Answer 2

try with the UNION between the second query and the rows of the first query with date not present in the second query, something like this:
select *
from student_attendance
where student_id = ?1 and (date between DATE_FORMAT(NOW() ,'%Y-%m-01') AND NOW())
UNION
select null as id,
a.day, 0 as was_present,
0 as was_late,
0 as left_earlier,
null as student_id
from (
select last_day(NOW()) - INTERVAL (a.a + (10 * b.a) + (100 * c.a)) DAY as day
from (
select 0 as a
union all select 1
union all select 2
union all select 3
union all select 4
union all select 5
union all select 6
union all select 7
union all select 8
union all select 9
) as a
cross join
(
select 0 as a
union all select 1
union all select 2
union all select 3
union all select 4
union all select 5
union all select 6
union all select 7
union all select 8
union all select 9
) as b
cross join
(
select 0 as a
union all select 1
union all select 2
union all select 3
union all select 4
union all select 5
union all select 6
union all select 7
union all select 8
union all select 9
) as c
) as a
where a.day between DATE_FORMAT(NOW() ,'%Y-%m-01') AND NOW()
and a.day NOT IN (
select date
from student_attendance
where student_id = ?1 and (date between DATE_FORMAT(NOW() ,'%Y-%m-01') AND NOW())
)
order by a.day