SQL Query to Join Employees, Departments, and

Joining Three Different Tables Using SQL

Objective

To demonstrate how to write an SQL query that links three different tables using JOIN and WHERE clauses.

Assumptions

1. Table1 (Employees): Contains employee details.
2. Table2 (Departments): Contains department details.
3. Table3 (Salaries): Contains salary details of employees.

Schema

• Table1 (Employees):

  • employee_id
  • first_name
  • last_name
  • department_id

• Table2 (Departments):

  • department_id
  • department_name

• Table3 (Salaries):

  • employee_id
  • salary

SQL Query

-- Linking three tables using JOIN and WHERE clause
SELECT 
    e.employee_id,
    e.first_name,
    e.last_name,
    d.department_name,
    s.salary
FROM 
    Employees e
JOIN 
    Departments d ON e.department_id = d.department_id
JOIN 
    Salaries s ON e.employee_id = s.employee_id
WHERE
    s.salary > 50000;  -- Filter to only include employees with a salary greater than 50,000

Explanation

1. SELECT Statement:

   • Retrieves specific columns from the combined tables: employee_id, first_name, last_name, department_name, and salary.

2. FROM Clause:

   • Specifies the primary table to query from, which is Employees (aliased as e).

3. JOIN Clauses:

   • JOIN Departments d ON e.department_id = d.department_id: Links the Employees table to the Departments table based on the department_id column.
   • JOIN Salaries s ON e.employee_id = s.employee_id: Links the Employees table to the Salaries table based on the employee_id column.

4. WHERE Clause:

   • Adds a condition to filter the results where the salary is greater than 50,000.

Practical Example

Imagine you want to view a list of employees, their corresponding department names, and salaries for all employees earning more than $50,000. You can run the above SQL query in your database management system (DBMS) to get the desired results.

Conclusion

This query efficiently joins three tables and filters results based on the specified condition. Following best practices, the code is modular and can be easily extended or modified for additional requirements.

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