SQL Joins

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SQL joins are used to combine rows from two or more tables.

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SQL JOIN

An SQL JOIN clause is used to combine rows from two or more tables, based on a common field between them.

The most common type of join is: SQL INNER JOIN (simple join). An SQL INNER JOIN return all rows from multiple tables where the join condition is met.

Let's look at a selection from the "Orders" table:

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

Then, have a look at a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Country
1	Alfreds Futterkiste	Maria Anders	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mexico

Notice that the "CustomerID" column in the "Orders" table refers to the customer in the "Customers" table. The relationship between the two tables above is the "CustomerID" column.

Then, if we run the following SQL statement (that contains an INNER JOIN):

Example

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

Try it yourself »

it will produce something like this:

OrderID	CustomerName	OrderDate
10308	Ana Trujillo Emparedados y helados	9/18/1996
10365	Antonio Moreno Taquería	11/27/1996
10383	Around the Horn	12/16/1996
10355	Around the Horn	11/15/1996
10278	Berglunds snabbköp	8/12/1996

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Different SQL JOINs

Before we continue with examples, we will list the types the different SQL JOINs you can use:

• INNER JOIN: Returns all rows when there is at least one match in BOTH tables
• LEFT JOIN: Return all rows from the left table, and the matched rows from the right table
• RIGHT JOIN: Return all rows from the right table, and the matched rows from the left table
• FULL JOIN: Return all rows when there is a match in ONE of the tables

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