INNER JOIN with aggregate functions in my SELECT - sql

Question

I'm trying to join a new column to my current query that uses aggregate functions. I create this column with a new query that also uses an aggregate function from a different table but I'm not sure if a JOIN will work for me since I need to join it to its respective row.
TABLE A (employees that are enrolled or were enrolled in a project)
ID
DEPARTMENT
ENROLLED
PROJECT
1
MARKETING
Yes
ARQ
2
MARKETING
Yes
TC
3
MARKETING
No
ARQ
4
MARKETING
No
TC
5
FINANCE
Yes
ARQ
6
FINANCE
Yes
TC
7
FINANCE
No
ARQ
8
FINANCE
Yes
TC
This table has more departments and more projects, but I simplified.
TABLE B (relation with departments and employees)
ID
DEPARTMENT
TOTAL_EMPLOYEES
1
MARKETING
2
2
MARKETING
3
3
FINANCE
4
4
FINANCE
8
In my first query I was asked to achieve the following result - using only table A:
(employees enrolled) (employees not enrolled)
DEPARTMENT
ARQ_E
TC_E
TOTAL_ENROLLED
ARQ_N
TC_N
TOTAL_NOT_ENROLLED
TOTAL
MARKETING
1
1
2
1
1
2
4
FINANCE
1
1
2
1
1
2
4
Using the following query:
SELECT tableA.department,
sum(case when enrolled = 'Yes' and tableA.project = 'ARQ' then 1 else 0 end) as ARQ_E,
sum(case when enrolled = 'Yes' and tableA.project = 'TC' then 1 else 0 end) as TC_E,
sum(case when enrolled = 'Yes' then 1 else 0 end) as TOTAL_ENROLLED,
sum(case when enrolled != 'Yes' and tableA.project = 'ARQ' then 1 else 0 end) as ARQ_N,
sum(case when enrolled != 'Yes' and tableA.project = 'TC' then 1 else 0 end) as TC_N,
sum(case when enrolled != 'Yes' then 1 else 0 end) as TOTAL_NOT_ENROLLED,
count (*) AS Total
FROM tableA
GROUP BY tableA.department;
My second query gets departments and their total employees from table B:
DEPARTMENT
TOTAL_EMPLOYEES
MARKETING
5
FINANCE
12
Using the following query:
SELECT tableB.department,
sum(tableB.total_employees) AS TOTAL_EMPLOYEES
FROM tableB
GROUP BY tableB.department;
I need to add the column TOTAL_EMPLOYEES to my first query, next to TOTAL will be TOTAL_EMPLOYEES. But it has to be placed with its respective department row. I need this to compare this 2 columns and see how many employees were not assigned to any project.
This is my expected result.
(employees enrolled) (employees not enrolled)
DEPARTMENT
ARQ_E
TC_E
TOTAL_ENROLLED
ARQ_N
TC_N
TOTAL_NOT_ENROLLED
TOTAL
T_EMPL
MARKETING
1
1
2
1
1
2
4
5
FINANCE
1
1
2
1
1
2
4
12
I have tried to achieve this using the following query:
SELECT tableA.department,
sum(case when enrolled = 'Yes' and tableA.project = 'ARQ' then 1 else 0 end) as ARQ_E,
sum(case when enrolled = 'Yes' and tableA.project = 'TC' then 1 else 0 end) as TC_E,
sum(case when enrolled = 'Yes' then 1 else 0 end) as TOTAL_ENROLLED,
sum(case when enrolled != 'Yes' and tableA.project = 'ARQ' then 1 else 0 end) as ARQ_N,
sum(case when enrolled != 'Yes' and tableA.project = 'TC' then 1 else 0 end) as TC_N,
sum(case when enrolled != 'Yes' then 1 else 0 end) as TOTAL_NOT_ENROLLED,
count (*) AS Total,
sum (tableB.total_employees) AS T_EMPL
FROM tableA
JOIN tableB
ON tableA.department = tableB.department
GROUP BY tableA.department;
But the numbers I get in my query are completely wrong since the JOINS repeat my rows and my SUMS duplicate.
I don't know if I really need to use a join or a subquery to place my sum(tableB.department) in its respective row.
I'm using PostgreSQL but since I'm using Standard 92 any SQL solution will help.

Answer 1

Your main issue stemmed from inadvertently multiplying rows with the join,
and has already been addressed. See:
Two SQL LEFT JOINS produce incorrect result
But use the standard SQL aggregate FILTER clause. It's shorter, cleaner, and noticeably faster. See:
Aggregate columns with additional (distinct) filters
For absolute performance, is SUM faster or COUNT?
SELECT *
FROM (
SELECT department
, count(*) FILTER (WHERE enrolled AND project = 'ARQ') AS arq_e
, count(*) FILTER (WHERE enrolled AND project = 'TC') AS tc_e
, count(*) FILTER (WHERE enrolled) AS total_enrolled
, count(*) FILTER (WHERE NOT enrolled AND project = 'ARQ') AS arq_n
, count(*) FILTER (WHERE NOT enrolled AND project = 'TC') AS tc_n
, count(*) FILTER (WHERE NOT enrolled) AS total_not_enrolled
, count(*) AS total
FROM tableA a
GROUP BY 1
) a
LEFT JOIN ( -- !
SELECT department
, sum(total_employees) AS total_employees
FROM tableB b
GROUP BY 1
) b USING (department);
enrolled should be a boolean column. Make it so if it isn't. Then you can use it directly. Smaller, faster, cleaner, shorter code.
I replaced the [INNER] JOIN with a LEFT [OUTER] JOIN on a suspicion. Typically, you want to keep all results, even if the same department is not found in the other table. Maybe even a FULL [OUTER] JOIN?
Also, USING (department) as join condition conveniently outputs that column only once, so we can make do with SELECT * in the outer SELECT.
Finally, subqueries are shorter and faster than CTEs. Not much since Postgres 12, but still. See:
Does WITH query store the results of referred tables?

Answer 2

Join the results of the two queries, using sub-queries, don't join the tables.
That way you're joining 1 row of enrollment data per department to 1 row of employee data per department.
SELECT
*
FROM
(
SELECT tableA.department,
sum(case when enrolled = 'Yes' and tableA.project = 'ARQ' then 1 else 0 end) as ARQ_E,
sum(case when enrolled = 'Yes' and tableA.project = 'TC' then 1 else 0 end) as TC_E,
sum(case when enrolled = 'Yes' then 1 else 0 end) as TOTAL_ENROLLED,
sum(case when enrolled != 'Yes' and tableA.project = 'ARQ' then 1 else 0 end) as ARQ_N,
sum(case when enrolled != 'Yes' and tableA.project = 'TC' then 1 else 0 end) as TC_N,
sum(case when enrolled != 'Yes' then 1 else 0 end) as TOTAL_NOT_ENROLLED,
count (*) AS Total
FROM tableA
GROUP BY tableA.department
)
AS enroll
INNER JOIN
(
SELECT tableB.department,
sum(tableB.total_employees) AS Total_EMPLOYEES
FROM tableB
GROUP BY tableB.department
)
AS employee
ON employee.department = enroll.department

Answer 3

As the join will multiply the summ, you can first sum the values and then join them
WITH CTE1 as (SELECT tableA.department,
sum(case when enrolled = 'Yes' and tableA.project = 'ARQ' then 1 else 0 end) as ARQ_E,
sum(case when enrolled = 'Yes' and tableA.project = 'TC' then 1 else 0 end) as TC_E,
sum(case when enrolled = 'Yes' then 1 else 0 end) as TOTAL_ENROLLED,
sum(case when enrolled != 'Yes' and tableA.project = 'ARQ' then 1 else 0 end) as ARQ_N,
sum(case when enrolled != 'Yes' and tableA.project = 'TC' then 1 else 0 end) as TC_N,
sum(case when enrolled != 'Yes' then 1 else 0 end) as TOTAL_NOT_ENROLLED,
count (*) AS Total
FROM tableA
GROUP BY tableA.department),
CTE2 as (SELECT tableB.department,
sum(tableB.total_employees) AS TOTAL_EMPLOYEES
FROM tableB
GROUP BY tableB.department)
SELECT
CTE1.department, ARQ_E, TC_E, TOTAL_ENROLLED, ARQ_N, TC_N, TOTAL_NOT_ENROLLED, TOTAL, T_EMPL,CTE2.TOTAL_EMPLOYEES
FROM CTE1 JOIN CTE2 ON CTE1.department = CTE2.department