Understanding LEFT JOIN in SQL

In SQL, a 'LEFT JOIN' is a type of command used to combine rows from two or more tables, based on a related column between them. The LEFT JOIN keyword returns all records from the left table (Employees in this case), and the matched records from the right table (Departments). The result is NULL on the right side, if there's no match.

The correct answer to perform a LEFT JOIN between 'Employees' and 'Departments' tables on the 'DepartmentID' column is:

SELECT * FROM Employees LEFT JOIN Departments ON Employees.DepartmentID = Departments.DepartmentID

This line of SQL code does the following:

• SELECT * chooses all columns from the joined table.
• FROM Employees LEFT JOIN Departments specifies which tables will be joined. 'Employees' is the left table and 'Departments' is the right table.
• ON Employees.DepartmentID = Departments.DepartmentID is the join condition that determines how the tables relate to each other.

Practical Example

Suppose you have an 'Employees' table that includes information about your employees, and a 'Departments' table that includes information about your company's departments. You want to create a list of all employees, including their department information. However, not all employees are assigned to a department. Using a LEFT JOIN, you can retrieve a complete list of employees, and where an employee does not have a department the department information will be returned as NULL.

Additional Insights

Remember that the LEFT JOIN keyword returns all the records from the left table and the matched records from the right table, filling with NULL where there's no match. It differs from standard JOIN (or INNER JOIN), where only matched records from both tables will be returned. Also, in a practical scenario, instead of selecting all columns using *, you might want to select only certain columns to reduce data and increase performance.

Therefore, Depending on the requirement and data distribution, you should choose the right type of join – LEFT JOIN, RIGHT JOIN, INNER JOIN, FULL JOIN, etc. Understanding the differences between these types is essential for efficient database queries and data manipulation.