When my database teacher started talking about JOIN statments, my mind started thinking about how blind partisan politics is dangerous. I started reading an article right there in class. I stopped listening to the teacher and I started thinking in my own world. (The article was This one by the way. It’s pretty good no matter your political orientation, even iwth his bias acknowledged.)
Back to my main point: There are entire websites, and many YouTube channels covering SQL. In this article though, I simply want to explain what a JOIN statment is in theory, why it can be useful, and how to use it in SQLite3—this is not the language used in my database class but it is very esay to get running, unlike Oracle SQL.
The simplest join, in my opinion, is the JOIN ... ON pattern. This joins two tables by a common column. To show an example, I must first setup a very basic database. It may look intimidating at first, but just take your time in soaking in the information.
This database will hold 4 tables.
'books' which will have a 13-digit ISBN, and a book title associated with that ISBN.'customers' which will have a first names, last name, address, and ID for all customers.'orders' which will contain an order ID, and a customer ID associated with it.'order_items' which will contain an order ID, and an ISBN associated with that order. There may be multiple rows with the same order ID, as there may be multiple books per order.Here is the books table:
| ISBN | Title |
|---|---|
| xxxxxxxxxx | s1 |
| xxxxxxxxxx | s1 |
| xxxxxxxxxx | s1 |
| xxxxxxxxxx | s1 |
This is the customers table:
| ID | firstname | lastname | address |
|---|---|---|---|
| 1 | Joe | Smith | 123 Main St. NW |
| 1 | Joe | Smith | 123 Main St. NW |
| 1 | Joe | Smith | 123 Main St. NW |
| 1 | Joe | Smith | 123 Main St. NW |
This is the orders table:
| CustomerID | OrderID |
|---|---|
| 1 | 1 |
| 1 | 1 |
| 1 | 1 |
| 1 | 1 |
| 1 | 1 |
Finally, this is the orderItems table:
| orderID | ISBN |
|---|---|
| 1 | xxxxxxxxxx |
| 1 | xxxxxxxxxx |
| 1 | xxxxxxxxxx |
| 1 | xxxxxxxxxx |
| 1 | xxxxxxxxxx |
You don’t have to memorize any of this information. The point is that ou can understand the relationship between the parts of this database. These tables are relate to eachother.
JOIN...ONHere is an example of using the JOIN...ON pattern. Here, we must specify which two columns will join the table.
SELECT customers.firstname, customers.lastname, customers.address, orders.orderID
FROM customers
JOIN orders ON customers.ID=orders.customerID