24.1 Why Joins?

Data in a relational database is normalized and split into multiple tables. A JOIN clause is used to combine rows from two or more tables, based on a related column between them.

24.2 INNER JOIN

The INNER JOIN keyword selects records that have matching values in both tables.

• If a row exists in Table A but doesn't have a matching row in Table B, that row is discarded from the result.

24.3 LEFT (OUTER) JOIN

The LEFT JOIN keyword returns all records from the left table, and the matched records from the right table.

• The result is NULL from the right side if there is no match.
• Use Case: "List all students and the courses they are enrolled in, including students who haven't enrolled in any course yet."

Join Visualization Table:

24.4 Syntax Example:

SELECT Employees.Name, Departments.DeptName
FROM Employees
INNER JOIN Departments ON Employees.DeptID = Departments.DeptID;