SQL Joins

The SQL JOIN Clause

The JOIN clause is used to combine rows from two or more tables, based on a related column between them.

Here are the different types of JOINs in SQL:

• (INNER) JOIN: Returns only rows that have matching values in both tables
• LEFT (OUTER) JOIN: Returns all rows from the left table, and only the matched rows from the right table
• RIGHT (OUTER) JOIN: Returns all rows from the right table, and only the matched rows from the left table
• FULL (OUTER) JOIN: Returns all rows when there is a match in either the left or right table

Look at an order in "Orders" table:

Then, look at a customer in the "Customers" table:

Here we see that the "CustomerID" column in the "Orders" table refers to the "CustomerID" in the "Customers" table. The relationship between the two tables above is the "CustomerID" column.

Then, we can create the following SQL statement (that contains an INNER JOIN), that selects records that have matching values in both tables:

Example

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

and it will produce something like this: