In this section, we will delve into the concept of INNER JOIN, one of the most commonly used types of joins in SQL. INNER JOIN allows you to combine rows from two or more tables based on a related column between them.

1. Definition: An INNER JOIN returns only the rows that have matching values in both tables.
2. Syntax: The basic syntax for an INNER JOIN is:

   SELECT columns
   FROM table1
   INNER JOIN table2
   ON table1.column = table2.column;
   

3. Use Case: INNER JOIN is used when you need to retrieve data that exists in both tables.

Let's consider two tables: employees and departments.

To retrieve a list of employees along with their department names, you can use the following INNER JOIN query:

1. SELECT Clause: Specifies the columns to be retrieved.
2. FROM Clause: Specifies the primary table (employees).
3. INNER JOIN Clause: Specifies the secondary table (departments) and the condition for the join (employees.department_id = departments.department_id).

Problem: Write an INNER JOIN query to retrieve the first_name, last_name, and department_name of employees who work in the "Sales" department.

Solution:

Problem: Write an INNER JOIN query to find the employee_id and department_name for all employees, but only include departments that have employees.

Solution:

1. Missing ON Clause: Always ensure you include the ON clause to specify the condition for the join.
2. Ambiguous Column Names: When columns have the same name in both tables, use table aliases or fully qualify the column names to avoid ambiguity.
3. Performance Considerations: INNER JOINs can be resource-intensive on large datasets. Ensure your tables are indexed on the join columns to improve performance.

In this section, you learned about the INNER JOIN operation, its syntax, and practical applications. You also practiced writing INNER JOIN queries and learned about common mistakes to avoid. Understanding INNER JOIN is crucial for effectively combining data from multiple tables in SQL. In the next section, we will explore other types of joins, starting with LEFT JOIN.