An SQL join is used to combine rows from multiple tables. An SQL join is performed whenever two or more tables is listed in the SQL FROM clause of an SQL statement.
There are different kinds of SQL joins. Let's take a look at a few examples.
SQL Inner Join (simple join)
Chances are, you've already written an SQL statement that uses an SQL inner join. It is the most common type of SQL join. SQL inner joins return all rows from multiple tables where the join condition is met.
For example,
SELECT suppliers.supplier_id, suppliers.supplier_name, orders.order_date FROM suppliers, orders WHERE suppliers.supplier_id = orders.supplier_id;
This SQL inner join example would return all rows from the suppliers and orders tables where there is a matching supplier_id value in both the suppliers and orders tables.
Let's look at some data to explain how SQL inner joins work:
We have a table called suppliers with two fields (supplier_id and supplier_ name). It contains the following data:
| supplier_id | supplier_name |
|---|---|
| 10000 | IBM |
| 10001 | Hewlett Packard |
| 10002 | Microsoft |
| 10003 | NVIDIA |
We have another table called orders with three fields (order_id, supplier_id, and order_date). It contains the following data:
| order_id | supplier_id | order_date |
|---|---|---|
| 500125 | 10000 | 2003/05/12 |
| 500126 | 10001 | 2003/05/13 |
If we run the SQL statement (that contains an inner join) below:
SELECT suppliers.supplier_id, suppliers.supplier_name, orders.order_date FROM suppliers, orders WHERE suppliers.supplier_id = orders.supplier_id;
Our result set would look like this:
| supplier_id | name | order_date |
|---|---|---|
| 10000 | IBM | 2003/05/12 |
| 10001 | Hewlett Packard | 2003/05/13 |
The rows for Microsoft and NVIDIA from the supplier table would be omitted, since the supplier_id's 10002 and 10003 do not exist in both tables.
Outer Join
Another type of join is called an SQL outer join. This type of join returns all rows from one table and only those rows from a secondary table where the joined fields are equal (join condition is met).
For example,
select suppliers.supplier_id, suppliers.supplier_name, orders.order_date from suppliers, orders where suppliers.supplier_id = orders.supplier_id(+);
This SQL outer join example would return all rows from the suppliers table and only those rows from the orders table where the joined fields are equal.
The (+) after the orders.supplier_id field indicates that, if a supplier_id value in the suppliers table does not exist in the orders table, all fields in the orders table will display as <null> in the result set.
The above SQL outer join statement could also be written as follows:
select suppliers.supplier_id, suppliers.supplier_name, orders.order_date from suppliers, orders where orders.supplier_id(+) = suppliers.supplier_id;
Let's look at some data to explain how SQL outer joins work:
We have a table called suppliers with two fields (supplier_id and name). It contains the following data:
| supplier_id | supplier_name |
|---|---|
| 10000 | IBM |
| 10001 | Hewlett Packard |
| 10002 | Microsoft |
| 10003 | NVIDIA |
We have a second table called orders with three fields (order_id, supplier_id, and order_date). It contains the following data:
| order_id | supplier_id | order_date |
|---|---|---|
| 500125 | 10000 | 2003/05/12 |
| 500126 | 10001 | 2003/05/13 |
If we run the SQL statement (that contains an outer join) below:
select suppliers.supplier_id, suppliers.supplier_name, orders.order_date from suppliers, orders where suppliers.supplier_id = orders.supplier_id(+);
Our result set would look like this:
| supplier_id | supplier_name | order_date |
|---|---|---|
| 10000 | IBM | 2003/05/12 |
| 10001 | Hewlett Packard | 2003/05/13 |
| 10002 | Microsoft | <null> |
| 10003 | NVIDIA | <null> |
The rows for Microsoft and NVIDIA would be included because an SQL outer join was used. However, you will notice that the order_date field for those records contains a <null> value.
