Why is the query returning an unexpected result

An introduction to SQL queries but better you look for an introductory SQL text

I assume that every employee is in the job_history table and every employee and therefore job is assigned to a department. Otherwise one will loose some employees.

Let's display the columns of the tables involved:

select employee_id,first_name,last_name,department_id
from employees;

select department_id, department_name
from departments;

select job_id,employeed_id,department_id
from job_history;

select job_id,job_title
from jobs;

First a list of all employees and their departement information

select *
from employee e, departments d
where e.department_id=d.department_id;

now select only the rows from the employees that are in the department 'Sales'

select *
from employee e, departments d
where e.department_id=d.department_id
and d.department_name='Sales';

for these employees now find the jobs have already head fron the job_history

select *
from employee e, departments d, jobhistory h
where e.department_id=d.department_id
and d.department_name='Sales'
and e.employee_id=h.employee_id;

Now for each job find the job title

select *
from employee e, departments d, jobhistory h, jobs j
where e.department_id=d.department_id
and d.department_name='Sales'
and e.employee_id=h.employee_id
and h.job_id=jobs.job_id;

Now we display only the columns we need

select e.first_name,e.last_name,j.job_title
from employee e, departments d, jobhistory h, jobs j
where e.department_id=d.department_id
and d.department_name='Sales'
and e.employee_id=h.employee_id
and h.job_id=jobs.job_id;

If we want to know in which department they worked in their job wee join another department table to get these information

select e.first_name,e.last_name,j.job_title,d2.department_name
from employee e, departments d, jobhistory h, jobs j,departments d2
where e.department_id=d.department_id
and d.department_name='Sales'
and e.employee_id=h.employee_id
and h.job_id=jobs.job_id
and h.depatment_id=d2.department_id;

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