Re: I don't understand something... - Mailing list pgsql-general

Alexander, that's a classic one,
rewrite your last query as :

SELECT count(employee_id) from employees where employee_id not in
    (select manager_id from employees WHERE manager_id  IS NOT NULL);

NULLS semantics are sometimes not so obvious.

Στις Monday 03 October 2011 09:33:12 ο/η Alexander Pyhalov έγραψε:
> Hello.
> I was asked a simple question. We have table employees:
>   \d employees
>                                              Table "public.employees"
>       Column     |            Type             |
>      Modifiers
> ----------------+-----------------------------+-----------------------------------------------------------------
>   employee_id    | integer                     | not null default
> nextval('employees_employee_id_seq'::regclass)
>   first_name     | character varying(20)       |
>   last_name      | character varying(25)       | not null
>   email          | character varying(25)       | not null
>   phone_number   | character varying(20)       |
>   hire_date      | timestamp without time zone | not null
>   job_id         | character varying(10)       | not null
>   salary         | numeric(8,2)                |
>   commission_pct | numeric(2,2)                |
>   manager_id     | integer                     |
>   department_id  | integer                     |
> Indexes:
>      "employees_pkey" PRIMARY KEY, btree (employee_id)
>      "emp_email_uk" UNIQUE, btree (email)
>      "emp_department_ix" btree (department_id)
>      "emp_job_ix" btree (job_id)
>      "emp_manager_ix" btree (manager_id)
>      "emp_name_ix" btree (last_name, first_name)
> Check constraints:
>      "emp_salary_min" CHECK (salary > 0::numeric)
> Foreign-key constraints:
>      "employees_department_id_fkey" FOREIGN KEY (department_id)
> REFERENCES departments(department_id)
>      "employees_job_id_fkey" FOREIGN KEY (job_id) REFERENCES jobs(job_id)
>      "employees_manager_id_fkey" FOREIGN KEY (manager_id) REFERENCES
> employees(employee_id)
> Referenced by:
>      TABLE "departments" CONSTRAINT "dept_mgr_fk" FOREIGN KEY
> (manager_id) REFERENCES employees(employee_id)
>      TABLE "employees" CONSTRAINT "employees_manager_id_fkey" FOREIGN
> KEY (manager_id) REFERENCES employees(employee_id)
>      TABLE "job_history" CONSTRAINT "job_history_employee_id_fkey"
> FOREIGN KEY (employee_id) REFERENCES employees(employee_id)
>
> Now we want to select count of all employees who doesn't have any
> subordinates (query 1):
>   SELECT count(employee_id) from employees o where not exists  (select 1
> from employees  where manager_id=o.employee_id);
>   count
> -------
>      89
> (1 row)
>
> We can select count of all managers (query 2):
> SELECT count(employee_id) from employees where employee_id  in (select
> manager_id from employees);
>   count
> -------
>      18
> (1 row)
>
> But if we reformulate the first query in the same way, answer is
> different (query 3):
> SELECT count(employee_id) from employees where employee_id not in
> (select manager_id from employees) (query 3);
>   count
> -------
>       0
> (1 row)
>
> I don't understand why queries 1 and 3 give different results. They
> seems to be the same... Could someone explain the difference?
>
> --
> Best regards,
> Alexander Pyhalov,
> system administrator of Computer Center of Southern Federal University
>



--
Achilleas Mantzios