Summary of the "SQL INNER JOIN" page on W3Schools:

• The INNER JOIN keyword in SQL is used to select records that have matching values in both tables.
• It demonstrates joining two tables—typically, for example, Products and Categories—using a shared key like CategoryID.
• Only rows with matches in both tables are returned (if a Product has a CategoryID not found in the Categories table, it will not appear in the result).
• The typical syntax is:

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

• Table names should be specified in the select statement when column names are present in both tables, to avoid ambiguity.
• JOIN and INNER JOIN are functionally identical—INNER is the default join type for JOIN.
• The page also shows how to perform INNER JOINs across more than two tables by chaining JOINs.
• Practical examples with small data sets are given, and interactive "Try it Yourself" SQL editors accompany examples for hands-on learning.
• An exercise section quizzes users on the purpose of the INNER JOIN (to select only the records that have matching values in both tables).[1]

1

SQL INNER JOIN

INNER JOIN

The INNER JOIN keyword selects records that have matching values in both tables.

Let's look at a selection of the Products table:

And a selection of the Categories table:

We will join the Products table with the Categories table, by using the CategoryID field from both tables:

Join Products and Categories with the INNER JOIN keyword:

SELECT ProductID, ProductName, CategoryName  
FROM Products  
INNER JOIN Categories ON Products.CategoryID = Categories.CategoryID;

Note: The INNER JOIN keyword returns only rows with a match in both tables. Which means that if you have a product with no CategoryID, or with a CategoryID that is not present in the Categories table, that record would not be returned in the result.

Syntax

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

Naming the Columns

It is a good practice to include the table name when specifying columns in the SQL statement.

Specify the table names:

SELECT Products.ProductID, Products.ProductName, Categories.CategoryName
FROM Products
INNER JOIN Categories ON Products.CategoryID = Categories.CategoryID;

The example above works without specifying table names, because none of the specified column names are present in both tables. If you try to include CategoryID in the SELECT statement, you will get an error if you do not specify the table name (because CategoryID is present in both tables).

JOIN or INNER JOIN

JOIN and INNER JOIN will return the same result.

INNER is the default join type for JOIN, so when you write JOIN the parser actually writes INNER JOIN.

JOIN is the same as INNER JOIN:

SELECT Products.ProductID, Products.ProductName, Categories.CategoryName  
FROM Products  
JOIN Categories ON Products.CategoryID = Categories.CategoryID;

JOIN Three Tables

The following SQL statement selects all orders with customer and shipper information:

Here is the Shippers table:

SELECT Orders.OrderID, Customers.CustomerName, Shippers.ShipperName  
FROM ((Orders  
INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID)  
INNER JOIN Shippers ON Orders.ShipperID = Shippers.ShipperID);