Merging 2 Table and GROUP BY date - mysql

Question

I need to merge multiple table group by the count base on date's day.
Below are my table structure :
#table1
id date
1 2015-07-01 00:00:00
2 2015-07-02 00:00:00
3 2015-07-03 00:00:00
#table2
id date
1 2015-07-02 00:00:00
2 2015-07-02 00:00:00
3 2015-07-02 00:00:00
4 2015-07-10 00:00:00
What I wanted to achieve :
#query result
date t1_count t2_count
2015-07-01 1 NULL
2015-07-02 1 3
2015-07-03 1 NULL
2015-07-10 NULL 1
Below are my query that refer to this link:
SELECT left(A.date,10) AS `day`
, COUNT(A.ID) AS `a_count`
, COUNT(B.ID) AS `b_count`
FROM table1 A
LEFT JOIN table2 B
ON LEFT(A.date,10) = LEFT(B.date,10)
GROUP BY LEFT(A.date,10)
UNION
SELECT left(B.date,10) AS `day`
, COUNT(A.ID) AS `a_count`
, COUNT(B.ID) AS `b_count`
FROM table1 A
RIGHT JOIN table2 B
ON LEFT(A.date,10) = LEFT(B.date,10)
GROUP BY LEFT(A.date,10);
but the result was
#query result
date t1_count t2_count
2015-07-01 1 0
2015-07-02 3 3
2015-07-03 1 0
2015-07-10 0 1
I'd try to modified and search other solution like UNION ALL, LEFT JOIN, etc, but I'd no luck to solve this problem.

Answer 1

You can do this using union all and group by:
select date, sum(istable1) as numtable1, sum(istable2) as numtable2
from ((select date(date) as date, 1 as istable1, NULL as istable2
from table1
) union all
(select date(date) as date, NULL as istable1, 1 as istable2
from table2
)
) t
group by date
order by 1;
Under some circumstances, it can be faster to aggregate the data in the subqueries as well:
select date, sum(numtable1) as numtable1, sum(numtable2) as numtable2
from ((select date(date) as date, count(*) as numtable1, NULL as numtable2
from table1
group by date(date)
) union all
(select date(date) as date, NULL as numtable1, count(*) as numtable2
from table2
group by date(date)
)
) t
group by date
order by 1;
If you want 0 instead of NULL in the desired results, use 0 instead of NULL in the subqueries.