Understanding the Primary Purpose of SQL 'INNER JOIN' Clause

The SQL 'INNER JOIN' clause primarily serves to merge rows from two or more tables based on a related column between them. This functionality forms the backbone of relational databases, where multiple tables are interrelated through common identifiers.

Let's take an instance of two tables: Employees and Departments. Suppose we want to fetch a list of employees along with their respective department names. Here, both the tables have a common column, say, DepartmentID. This is where 'INNER JOIN' comes to the rescue.

SELECT Employees.Name, Departments.DepartmentName
FROM Employees
INNER JOIN Departments
ON Employees.DepartmentID = Departments.DepartmentID;

In this query, the SQL 'INNER JOIN' clause combines rows from both tables where the DepartmentID matches in both. The result is a new temporary table that includes rows from both tables where the join condition is true.

It's important to keep in mind that 'INNER JOIN' returns only the matching rows. If there are employees with a DepartmentID not existing in the Departments table, or departments with no employees assigned, those records will not be included in the results.

As a best practice, always make sure your 'INNER JOIN' operation has a join-predicate (the ON Employees.DepartmentID = Departments.DepartmentID part in the example). Without this, you'd end up with a Cartesian product, i.e., a combination of every row from the first table with every row from the second table, which most likely isn't what you want!

In summary, the SQL 'INNER JOIN' clause is a powerful tool in database management, especially when dealing with relational databases that involve multiple interlinked tables. As the correct answer to our quiz question indicates, its primary purpose is to combine rows from two or more tables based on a related column between them.