SQL joins are used to combine rows from two or more tables. It 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 (also know as simple join). An SQL INNER JOIN returns all rows from multiple tables where the join condition is met.
Let’s look at a selection from the “Orders” table:
Then, have a look at a selection from the “Customers” table:
Notice 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, if we run the following SQL statement (that contains an INNER JOIN):
SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDate FROM Orders INNER JOIN Customers ON Orders.CustomerID=Customers.CustomerID;
it will produce something like this:
Different SQL JOINs
Before we continue with examples, we will list the types of 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