MySQL Set 0 if null (SUM CASE) doesn't work - mysql

Question

I want to count of condition from all employees of each area, which conditions has 2 parameters, WELL AND UNWELL. Like this,
conditions | area | count_conditions
Well AREA1 1
UNWELL AREA1 0
Well AREA2 5
UNWELL AREA2 1
...
This is the closest so far.
SELECT a.conditions, k.area,
SUM(CASE WHEN a.conditions IS NOT NULL THEN 1 ELSE 0 END) AS count_conditions
FROM tb_attended a
INNER JOIN tb_employees k ON a.nrp = k.nrp
AND a.date = '2020-07-20'
GROUP BY k.area, a.conditions
My code above is running well, but if there is no entry of UNWELL OR WELL in some area, then that conditions does not appear. Like this.
conditions | area | count_conditions
Well AREA1 1
Well AREA2 5
UNWELL AREA2 1
...
This is example data that I use,
SQL Fiddle
Any suggest?
Thank you.

Answer 1

First you need a CROSS join of the distinct areas to the distinct conditions and then LEFT joins of the tables:
SELECT t1.condition, t2.area,
COUNT(k.nrp) AS count_conditions
FROM (SELECT DISTINCT `condition` FROM tb_attended) t1
CROSS JOIN (SELECT DISTINCT Area FROM tb_employees) t2
LEFT JOIN tb_attended a ON a.condition = t1.condition
LEFT JOIN tb_employees k ON k.area = t2.area AND a.nrp = k.nrp AND a.date = '2020-07-20'
GROUP BY t1.condition, t2.area
ORDER BY t2.area, t1.condition DESC
See the demo.

Answer 2

Use left join
SELECT a.conditions, k.area,
SUM(CASE WHEN a.conditions IS NOT NULL THEN 1 ELSE 0 END) AS count_conditions
FROM tb_attended a
left JOIN tb_employees k
ON a.nrp = k.nrp
AND a.date = '2020-07-20'
GROUP BY k.area, a.conditions

Answer 3

You can use LEFT JOIN without redundant conditional statement, with SUM() aggregation :
SELECT a.conditions, k.area,
SUM(a.conditions IS NOT NULL) AS count_conditions
FROM tb_attended a
LEFT JOIN tb_employees k
ON a.nrp = k.nrp
AND a.date = '2020-07-20'
GROUP BY k.area, a.conditions

Answer 4

I think you want a left join:
SELECT a.conditions, k.area, COUNT(a.conditions) AS count_conditions
FROM tb_employees k LEFT JOIN
tb_attended a
ON a.nrp = k.nrp AND a.date = '2020-07-20'
GROUP BY k.area, a.conditions;
EDIT:
I see. You want to get all rows, even those with no matches. Use a CROSS JOIN to generate the combinations of areas and conditions that you want. Then use LEFT JOIN to match to the existing data:
SELECT c.condition, e.area, COUNT(a.nrp) AS count_conditions
FROM (SELECT DISTINCT a.condition
FROM tb_attended a
) c CROSS JOIN
(SELECT DISTINCT area
FROM tb_employees
) e LEFT JOIN
tb_employees k
ON k.area = e.area LEFT JOIN
tb_attended a
ON a.nrp = k.nrp AND
a.condition = c.condition AND
a.date = '2020-07-20'
GROUP BY c.condition, e.area;
Here is a db<>fiddle.