Question

I have a table with employees and a table with errors. In my application you can assign some errors to an employee. So,
Employee A will have Error1(type 1), Error2(type 1), and Error3(type2) assigned
Employee B will have Error3(type2) assigned
and
Employee C will have Error4(type2), Error5(type2) assigned.
Hope you understand the logic.

Now:
I need to make a select which will get count of employees that have assigned strictly errors of type 2(for example).
My case will return: 2(is employee B, and employee C)

How do you suggest to make this?
I was thinking to do a select where to have employee name, total of errors assigned, and total errors assigned which are type2, but i don’t know how to make that 😐

any help will be much appreciated.

Answer 1

Use subqueries.

SELECT COUNT (id) 
FROM employees
WHERE EXISTS (SELECT * FROM errors WHERE type = 2 AND employee_id = employees.id)
AND NOT EXISTS (SELECT * FROM errors WHERE type != 2 AND employee_id = employees.id)

As for your query with employee name, total number of errors, and number of errors with type 2, you can do it like this:

SELECT employees.name, count(error.id) all_errors, sum(case when error.type = 2 then 1 else 0 end) type_2_errors
FROM employees, errors
WHERE errors.employee_id = employees.id
GROUP BY employees.name