如何通过子查询获取MIN / MAX

Question

I have three tables like these

Employees

employee_ID(Pk) | department_ID(Fk)

Departments

department_ID(Pk) | location_ID(Fk)

Locations

location_ID(Pk) | city

What I want is the name of the city that has least employees. I've try something like the following sql below :

 SELECT l.city FROM employees e, departments d, locations l WHERE e.department_ID = d.department_ID AND d.location_ID = l.location_ID GROUP BY l.city ORDER BY 2 LIMIT 1 

But that's not a good one. I want it in the subquery and MIN function maybe COUNT function.I tried it but couldn't figure it out. Any ideas? Thanks a lot!

Answer 1

You were pretty close. Try this:

select l.city, count(*) as no_of_employees
from locations l
inner join departments d
  on d.location_id = l.location_id
inner join employees e
  on e.department_id = d.department_id
group by l.city
order by no_of_employees asc
limit 1

Example:

create table locations (location_id int, city varchar(20));
insert into locations values (1, 'LA'), (2, 'NY');

create table departments (department_id int, location_id int);
insert into departments values (1, 1), (2, 1), (3, 2), (4, 2);

create table employees (employee_id int, department_id int);
insert into employees values (1, 1), (2, 1), (3, 1), (4, 3), (5, 4);

Result of the query:

| city | no_of_employees |
|------|-----------------|
|   NY |               2 |

SQLFiddle example: http://sqlfiddle.com/#!9/75aa1/1

Using subquery as per requested in the comment, here's how you could do it - but don't do it! Use subquery only if needed.

select * from (
    -- get list of all city and employee count here
    select l.city, count(*) as no_of_employees
    from locations l
    inner join departments d
      on d.location_id = l.location_id
    inner join employees e
      on e.department_id = d.department_id
    group by l.city
) subquery1

-- compare the no of employees with min. employees from the same query
where no_of_employees = (

    -- find minimum number of employees here
    select min(no_of_employees) from (
        -- same query as subquery1
        select l.city, count(*) as no_of_employees
        from locations l
        inner join departments d
          on d.location_id = l.location_id
        inner join employees e
          on e.department_id = d.department_id
        group by l.city
    ) subquery2
)

Result:
| city | no_of_employees |
|------|-----------------|
|   NY |               2 |

SQLFiddle example: http://sqlfiddle.com/#!9/75aa1/4