Count from multiple tables with group - mysql

Question

I have two tables. The first tables registers admissions.
The second one has exits, as shown below:
Table 1: Admissions
+----------+---------------------+---------+
| entry_id | join_date | name |
+----------+---------------------+---------+
| 26 | 2017-01-01 00:00:00 | James |
| 29 | 2017-01-01 00:00:00 | Jan |
| 27 | 2017-01-01 00:00:00 | Chris |
| 28 | 2017-01-01 00:00:00 | Mary |
| 22 | 2017-01-02 00:00:00 | Anna |
| 21 | 2017-01-02 00:00:00 | Andy |
| 24 | 2017-01-02 00:00:00 | Bob |
| 20 | 2017-01-04 00:00:00 | Alice |
| 23 | 2017-01-04 00:00:00 | Chris |
| 25 | 2017-01-04 00:00:00 | Happy |
+----------+---------------------+---------+
Table 2: Exits
+----------+---------------------+----------+
| entry_id | exit_date | name |
+----------+---------------------+----------+
| 322 | 2017-01-01 00:00:00 | Kay |
| 344 | 2017-01-01 00:00:00 | Agnes |
| 920 | 2017-01-02 00:00:00 | Andre |
| 728 | 2017-01-02 00:00:00 | Mark |
| 583 | 2017-01-03 00:00:00 | Alsta |
| 726 | 2017-01-03 00:00:00 | Bull |
| 816 | 2017-01-03 00:00:00 | Jane |
| 274 | 2017-01-04 00:00:00 | Jack |
| 723 | 2017-01-04 00:00:00 | Anna |
| 716 | 2017-01-04 00:00:00 | Bill |
+----------+---------------------+----------+
I am looking for a solution to know the number of admissions, the number of exits and the balance, grouped by date.
I am looking for this >
+---------------------+--------+--------+-----------+
| date | joins | exist | net |
+---------------------+--------+--------+-----------+
| 2017-01-01 00:00:00 | 4 | 2 | 2 |
| 2017-01-02 00:00:00 | 3 | 2 | 1 |
| 2017-01-03 00:00:00 | 0 | 3 | -3 |
| 2017-01-04 00:00:00 | 3 | 3 | 0 |
+---------------------+--------+--------+-----------+
Notes: There may be days when admissions occur, but no exits are registered and vice versa.

Answer 1

Here you go:
SELECT
d,
SUM(CASE WHEN t = 'j' THEN 1 ELSE 0 END) as joins,
SUM(CASE WHEN t = 'x' THEN 1 ELSE 0 END) as exits,
SUM(CASE WHEN t = 'j' THEN 1 ELSE 0 END) - SUM(CASE WHEN t = 'x' THEN 1 ELSE 0 END) as net
FROM
(SELECT join_date as d, 'j' as t FROM admissions) j
UNION ALL
(SELECT exit_date as d, 'x' as t FROM exits) x
GROUP BY d
We concatenate the data using a UNION ALL and make a note of its type- join or exit, with a simple char we can compare later
We group this up by d, giving one date per line, and we sum the result of conditionally looking at whether its a 'j'oin or an e'x'it. If the line is a j, then a 1 is added to the column tracking the joins total for that day, and so on
The only thing this doesn't give you, is days where there are no joins or exits.. (For example 2018-12-25, 0, 0, 0 because xmas day was closed and noone did anything on that day).. But you didn't say you wanted those.
If you do want lines with a date, and 0 exits, 0 joins, 0 net, then we have to we have to work some additional magic, and it's a bit more of a headache/makes it harder to understand (so i left it out)

Answer 2

Following will do:
select
CASE WHEN join_date is not null THEN join_date
WHEN exit_date is not null THEN exit_date END as date,
entry.cnt as joins,
exit.cnt as exits,
(extry.cnt - exit.cnt) as net
FROM
(select join_date, COALESCE(count(*), 0) as cnt from Admissions group by join_date) entry
FULL OUTER JOIN
(select exit_date, COALESCE(count(*), 0) as cnt from Exits group by exit_date) exit
ON
entry.join_date=exit.exit_date
;

Answer 3

I didn't find the answer. Here's an answer from one of my friends, below is a MySQL version:
select aa.date, IFNULL(aa.joins, 0) joins, IFNULL(bb.exits,0) exits, (IFNULL(aa.joins,0) - IFNULL(bb.exits,0)) net
from
(
select join_date date, count(name) joins
from Admissions
group by join_date
) aa
left join
(
select exit_date date, count(name) exits
from Exits
group by exit_date
) bb on aa.date = bb.date
UNION
select bb.date, IFNULL(aa.joins, 0) joins, IFNULL(bb.exits,0) exits, (IFNULL(aa.joins,0) - IFNULL(bb.exits,0)) net
from
(
select join_date date, count(name) joins
from Admissions
group by join_date
) aa
right join
(
select exit_date date, count(name) exits
from Exits
group by exit_date
) bb on aa.date = bb.date order by date;