sql_join_inner

SQL INNER JOIN Keyword

Understanding SQL INNER JOIN

The INNER JOIN keyword selects records that have matching values in both tables. It is the most common type of join.

If a row in table1 finds a corresponding row in table2 based on the join condition, the INNER JOIN combines columns from both rows into a single result row. If there’s no match, the rows are excluded from the result.

SQL INNER JOIN Syntax

SELECT column_name(s)
FROM table1
INNER JOIN table2
ON table1.join_column = table2.join_column;

Alternatively, you can often just use JOIN (which defaults to INNER JOIN in most SQL databases):

SELECT column_name(s)
FROM table1
JOIN table2
ON table1.join_column = table2.join_column;

Visually, an INNER JOIN can be thought of as the intersection of two sets – only the elements (rows) that exist in both sets (based on the join condition) are returned.

Demo Database: Northwind

We’ll use the Northwind sample database with its Customers and Orders tables.

A simplified view of the “Customers” table:

CustomerID | CustomerName                      | Country
-----------|-----------------------------------|---------
1          | Alfreds Futterkiste             | Germany
2          | Ana Trujillo Emparedados y helados| Mexico
3          | Antonio Moreno Taquería         | Mexico
...

A simplified view of the “Orders” table:

OrderID | CustomerID | OrderDate
--------|------------|-----------
10308   | 2          | 1996-09-18
10309   | 37         | 1996-09-19
10310   | 77         | 1996-09-20
...

SQL INNER JOIN Example

The following SQL statement will return only customers who have placed orders, along with their order IDs:

SELECT c.CustomerName, o.OrderID
FROM Customers c
INNER JOIN Orders o ON c.CustomerID = o.CustomerID
ORDER BY c.CustomerName;

A selection from the result set might look like this:

CustomerName                      | OrderID
-----------------------------------|---------
Ana Trujillo Emparedados y helados| 10308
Antonio Moreno Taquería         | 10365
... (other customers with orders)

Important Note: The INNER JOIN keyword selects all rows from both tables as long as there is a match on the join condition (Customers.CustomerID = Orders.CustomerID). If there are customers in the “Customers” table that do not have any matching orders in the “Orders” table, those customers will NOT be listed in the result. Similarly, any orders without a matching customer (orphaned data, if possible) would also be excluded.

Common Use Cases:

• Retrieving related data from multiple tables (e.g., user profiles and their posts).
• Filtering records based on the existence of related data (e.g., products that have been ordered).
• Combining information for comprehensive reports where a direct relationship exists.