SQL INNER JOIN

An SQL INNER JOIN is used to combine rows from two or more tables based on a related column between them. It returns only the rows that have matching values in both tables.

Let's illustrate this with an example. Consider two tables: Customers and Orders.

Table: Customers

Table: Orders

SQL INNER JOIN Query

To find all orders with the corresponding customer names, you can use the following INNER JOIN query:

SELECT Customers.CustomerID, Customers.CustomerName, Orders.OrderID, Orders.OrderDate
FROM Customers
INNER JOIN Orders ON Customers.CustomerID = Orders.CustomerID;

Output of the Query

Explanation

1. FROM Customers: The query starts with the Customers table.
2. INNER JOIN Orders ON Customers.CustomerID = Orders.CustomerID: The query combines the Customers table with the Orders table using the CustomerID column. The INNER JOIN ensures that only the rows with matching CustomerID values in both tables are included in the result.
3. SELECT Customers.CustomerID, Customers.CustomerName, Orders.OrderID, Orders.OrderDate: This specifies the columns to be included in the output.

Why Some Rows Are Excluded

• The Orders table has an OrderID of 104 with a CustomerID of 4, but there is no CustomerID of 4 in the Customers table. Thus, this row is excluded from the result.
• Similarly, if there were any customers without orders, they would also be excluded because the INNER JOIN requires a match in both tables.

This is a basic example to demonstrate the concept of an INNER JOIN. You can extend this to more complex scenarios involving multiple tables and conditions.