PostgreSQL JOINS: Make a join with three tables to find the name, jobs, department name and ID of all the employees working in London

3. Write a SQL query to make a join with three tables employees, departments and locations to find the name, including first_name and last_name, jobs, department name and ID, of the employees working in London.

Sample Solution:

Code:

-- This SQL query retrieves employee details along with their department information for employees working in London.

SELECT e.first_name, -- Selects the first_name column from the employees table
       e.last_name, -- Selects the last_name column from the employees table
       e.job_id, -- Selects the job_id column from the employees table
       e.department_id, -- Selects the department_id column from the employees table
       d.department_name -- Selects the department_name column from the departments table
FROM employees e -- Specifies the first table from which to retrieve data, in this case, the employees table
JOIN departments d -- Joins the employees table with the departments table
ON (e.department_id = d.department_id) -- Specifies the join condition based on the department_id column
JOIN locations l ON -- Joins the departments table with the locations table
(d.location_id = l.location_id) -- Specifies the join condition based on the location_id column
WHERE l.city = 'London'; -- Filters the rows to include only those where the city is 'London'

Explanation:

• This SQL query retrieves employee details along with their department information for employees working in London.
• The SELECT statement selects the first name, last name, job ID, department ID, and department name columns.
• The FROM clause specifies the first table from which to retrieve data, which is the employees table, aliased as e.
• The JOIN keyword is used to join the employees table (e) with the departments table (d) based on the department_id column.
• The ON clause specifies the join condition based on the matching department_id.
• Another JOIN operation is performed to join the departments table (d) with the locations table (l) based on the location_id column.
• The WHERE clause filters the rows to include only those where the city is 'London'.
• The result set will contain employee details along with their department information for employees working in London.

Sample table: employees

Sample table: locations

Sample table: departments

Output:

pg_exercises-# FROM employees e
pg_exercises-# JOIN departments d
pg_exercises-# ON (e.department_id = d.department_id)
pg_exercises-# JOIN locations l ON
pg_exercises-# (d.location_id = l.location_id)
pg_exercises-# WHERE l.city = 'London';
 first_name | last_name | job_id | department_id | department_name
------------+-----------+--------+---------------+-----------------
 Susan      | Mavris    | HR_REP |            40 | Human Resources
(1 row)

Relational Algebra Expression:

Relational Algebra Tree:

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