MySQL JOIN or SUBQUERY on SET from current result row

Question

I have a table named DEPARTMENTS that has a column named EMPLOYEES which is a SET:

department | employees
----------------------
sales      | 100,107
support    | 120,121

And another table named EMPLOYEES:

employee | active
-----------------
100      | true
107      | false
120      | true
121      | true

I'm trying to void looping through the results and making another connection just to determine if the EMPLOYEE is ACTIVE. Basically, I need another column with the ACTIVE employees, the result something like:

department | employees | active
-------------------------------
sales      | 100,107   | 100

I tried what I thought was an obvious:

SELECT departments.*, (SELECT GROUP_CONCAT(employee) FROM employees WHERE employee IN(departments.employees) AND active = true) AS active ...

Which failed and I don't understand why, because if I do this manually:

SELECT departments.*, (SELECT GROUP_CONCAT(employee) FROM employees WHERE employee IN(100,107) AND active = true) AS active ...

Which works.

Any advice appreciated:) Seems like a simple problem but for some reason escaping me..

Answer 1

Join the tables and aggregate with group_concat() .
The function find_in_set() can be used to link each emploee to each departmment.

Assuming that the data type of the column active in employees is Boolean :

select d.department, d.employees,
       group_concat(e.employee) active
from departments d left join employees e
on find_in_set(e.employee, d.employees) and e.active
group by d.department, d.employees

See the demo .

If active 's data type is varchar change the ON clause:

and e.active = 'true'

See the demo .

Results:

| department | employees | active  |
| ---------- | --------- | ------- |
| sales      | 100,107   | 100     |
| support    | 120,121   | 120,121 |