Using count for two different values in the same table - mysql

Question

In the following query I am trying to fetch data from tables: company, classes_by_company, and person. I have a foreign key in all tables with the name company_id. In the classes_by_company table a class can have a class_status that can be either active or inactive. The same goes for table person with its corresponding person_status. I am able to count the total number of classes and the total number of persons, find the working query here: SQFIDDLE. In the query below I am trying to incorporate the previous query but now also count the total number of class_status=ACTIVE and person_status=ACTIVE. I am getting error Not unique table/alias: 'c'. How can I can count the total number and the total number of active classes and person? SQLFIDDLE
SELECT a.id, a.company_id, a.status, c.class_status, p.person_status,
c.total_count AS classes_per_company,
p.total_count AS employees_per_company,
c.active_count AS active_classes,
p.active_count AS active_instructors
FROM company a
LEFT JOIN (SELECT company_id, COUNT(*) as total_count
FROM classes_by_company
GROUP BY company_id) c
ON a.company_id = c.company_id
LEFT JOIN (SELECT company_id, COUNT(*) as active_count
FROM classes_by_company
WHERE class_status = 'ACTIVE'
GROUP BY company_id) c
ON a.company_id = c.company_id
LEFT JOIN (SELECT company_id, COUNT(*) as total_count
FROM person
GROUP BY company_id) p
ON a.company_id = p.company_id
LEFT JOIN (SELECT company_id, COUNT(*) as active_count
FROM person
WHERE person_status = 'ACTIVE'
GROUP BY company_id) p
ON a.company_id = c.company_id
Table Structure:
CREATE TABLE company
(
id int auto_increment primary key,
company_id int,
status varchar(20)
);
CREATE TABLE classes_by_company
(
id int auto_increment primary key,
company_id int,
class_name varchar(20),
class_status varchar(20)
);
CREATE TABLE person
(
id int auto_increment primary key,
employee_id int,
company_id int,
person_name varchar(20),
person_status varchar(20)
);

Answer 1

Your query structure is fine, you just need to give different aliases to each subquery that you join with.
You also failed to return class_status and person_status from the c and p subqueries.
SELECT a.id, a.company_id, a.status, c.class_status, p.person_status,
c.total_count AS classes_per_company,
p.total_count AS employees_per_company,
ca.active_count AS active_classes,
pa.active_count AS active_instructors
FROM company a
LEFT JOIN (SELECT company_id, class_status, COUNT(*) as total_count
FROM classes_by_company
GROUP BY company_id) c
ON a.company_id = c.company_id
LEFT JOIN (SELECT company_id, COUNT(*) as active_count
FROM classes_by_company
WHERE class_status = 'ACTIVE'
GROUP BY company_id) ca
ON a.company_id = ca.company_id
LEFT JOIN (SELECT company_id, person_status, COUNT(*) as total_count
FROM person
GROUP BY company_id) p
ON a.company_id = p.company_id
LEFT JOIN (SELECT company_id, COUNT(*) as active_count
FROM person
WHERE person_status = 'ACTIVE'
GROUP BY company_id) pa
ON a.company_id = pa.company_id
I changed the aliases to ca for active classes, and pa for active person.
SQLFIDDLE
You can also reduce the number of joins by counting the active rows in the same query as the total rows:
SELECT a.id, a.company_id, a.status, c.class_status, p.person_status,
c.total_count AS classes_per_company,
p.total_count AS employees_per_company,
c.active_count AS active_classes,
p.active_count AS active_instructors
FROM company a
LEFT JOIN (SELECT company_id, class_status, COUNT(*) as total_count, SUM(class_status = 'ACTIVE') as active_count
FROM classes_by_company
GROUP BY company_id) c
ON a.company_id = c.company_id
LEFT JOIN (SELECT company_id, person_status, COUNT(*) as total_count, SUM(person_status = 'ACTIVE') as active_count
FROM person
GROUP BY company_id) p
ON a.company_id = p.company_id
SQLFIDDLE

Answer 2

Very small change...table alias's (or subqueries) cannot be the same
SELECT a.id, a.company_id, a.status, c.class_status, p.person_status,
c.total_count AS classes_per_company,
p.total_count AS employees_per_company,
ca.active_count AS active_classes,
pa.active_count AS active_instructors
FROM company a
LEFT JOIN (SELECT company_id, COUNT(*) as total_count
FROM classes_by_company
GROUP BY company_id) c
ON a.company_id = c.company_id
LEFT JOIN (SELECT company_id, COUNT(*) as active_count
FROM classes_by_company
WHERE class_status = 'ACTIVE'
GROUP BY company_id) ca
ON a.company_id = c.company_id
LEFT JOIN (SELECT company_id, COUNT(*) as total_count
FROM person
GROUP BY company_id) p
ON a.company_id = p.company_id
LEFT JOIN (SELECT company_id, COUNT(*) as active_count
FROM person
WHERE person_status = 'ACTIVE'
GROUP BY company_id) pa
ON a.company_id = c.company_id