Why should you care?

Orders without customers, or employees without departments, are useless reports — joins are how OracleCore builds real screens.

See it live — copy this example

Run these statements in SQL Developer or SQL*Plus against your lab PDB. Prefer a practice user — not SYS — for application SQL.

CREATE TABLE departments (
  dept_id NUMBER PRIMARY KEY,
  dept_name VARCHAR2(50)
);
CREATE TABLE employees (
  emp_id NUMBER PRIMARY KEY,
  emp_name VARCHAR2(100),
  dept_id NUMBER REFERENCES departments(dept_id),
  salary NUMBER(10,2)
);

INSERT INTO departments VALUES (10, 'Retail Banking');
INSERT INTO departments VALUES (20, 'IT');
INSERT INTO employees VALUES (101, 'Neha', 10, 55000);
INSERT INTO employees VALUES (102, 'Arjun', 20, 72000);
COMMIT;

SELECT e.emp_name, d.dept_name, e.salary
FROM employees e
INNER JOIN departments d ON e.dept_id = d.dept_id
ORDER BY e.salary DESC;

What happened?

• employees.dept_id points to departments.dept_id.
• INNER JOIN returns only employees who have a matching department.

Practice next

1. Create both tables and insert sample rows.
2. Run the INNER JOIN.
3. Change to LEFT JOIN and insert an employee with NULL dept_id to see the difference.
4. Add a third department with no employees.
5. Select dept_name and COUNT(*) with GROUP BY.

Remember

JOIN links tables on keys. INNER = matches only; LEFT = keep left rows. Always alias tables (e, d) for clarity.

HR roster

HR needs employee name with department for payroll export.

Outcome: One join query replaces two manual spreadsheets.