In SQL, an INNER JOIN is used to retrieve rows from multiple tables where there is a match between the columns being joined. The INNER JOIN keyword selects records that have matching values in both tables.

Here’s a basic structure of an INNER JOIN:

Example Scenario

Suppose you have two tables:

1. Employees

   • EmployeeID
   • FirstName
   • LastName
   • DepartmentID

2. Departments

   • DepartmentID
   • DepartmentName

To get a list of employees along with their department names, you would use an INNER JOIN like this:

Explanation

• Employees.FirstName, Employees.LastName, Departments.DepartmentName: These are the columns you want to retrieve.
• FROM Employees: This specifies the first table to retrieve data from.
• INNER JOIN Departments: This specifies that you are joining the Departments table with the Employees table.
• ON Employees.DepartmentID = Departments.DepartmentID: This specifies the condition for the join, which is that the DepartmentID column in both tables must match.

Key Points

1. Matching Rows: INNER JOIN returns only those rows where there is a match in both tables.
2. Multiple Joins: You can join more than two tables in a single query by chaining INNER JOIN clauses.
3. Aliases: For readability, you can use table aliases. For example:
   SELECT e.FirstName, e.LastName, d.DepartmentName FROM Employees e INNER JOIN Departments d ON e.DepartmentID = d.DepartmentID;