PostgreSQL join query issue

Question

I am learning PostgreSQL and was practicing questions.

I am stuck at this question here

My Solution does not seem to work and I am not sure why:

SELECT first_name, last_name, department_id, department_name 
FROM employees 
JOIN departments ON departments.department_id = employees.department_id;

Their solution:

SELECT first_name, last_name, department_id, department_name 
FROM employees 
JOIN departments USING (department_id);

I'd really appreciate the help.

Answer 1

Your solution does not work because the field names are ambiguous. Try prefixing like this

SELECT 
  employees.first_name, 
  employees.last_name, 
  departments.department_id, 
  departments.department_name 
FROM employees 
JOIN departments ON departments.department_id = employees.department_id;

The problem does not appear when you use USING , because then there will be only a single department_id column in the join result.