In the last video, we created a table with a single primary key attribute called the partition key. DynamoDB also lets you create tables that use two attributes as the unique identifier. One field is the partition key, also known as the hash key, and the other is the sort key, sometimes called the range key. This structure can be useful if your data has a hierarchy.

Here's one example of when you might want to use a compound primary key. Say you've created a table that's logging some actions performed by users on your site, like every time they logged in to the site. Every action has a username and a numeric timestamp, representing when the action happened. It might make sense to use a composite key to uniquely identify each action. The partition key would be the username and the sort key would be the timestamp. You can have many items with the same username and many items with the same timestamp, as long as you don't have any records with the same username and the same timestamp. By using a composite primary key here, it would be easy to find a single action and to query for all the actions taken by a single user.

Here's another example, keeping track of the high scores that users have gotten on some mobile games. Here's a situation where there are many users and there are a handful of games, but each user should only have one high score for each game. In this example, you could use the user ID as the partition key and the game title as the sort key. The unique identifier for each item would be the combination of the user ID and the game title.

Here's one last example, which is the one that we're going to use for this demonstration. Each time a customer places an order from our online store, they might purchase many different items. Each of those line items will become an entry in the Order Line Items table. The partition key will be the order ID and the sort key will be the line number for that order.

Here's another way to visualize that table. You can see that order 630178 has two line items for two different products and both of them are unshipped. The next order ID, 607161, only has one line item. Next, order 672102 has three line items. Some of them are shipped and some of them are unshipped. This is the structure that we'll design in the AWS console.

Let's go back to the DynamoDB console and create this table. Now that we already have one table in our account, the DynamoDB console loads with a dashboard of summary information. Let's click Create Table again to build the table for our order line items. The table name, Order Line Items, makes sense here. As we just discussed, we're going to use a composite primary key, so we'll need to check the checkbox for Add Sort Key. Now, you can see that the form asks for two primary key fields, the partition key and the sort key. Just like in the Orders table, our partition key will be order ID, which is a number, and the sort key will be the line number within each order. That's also a number. This time we can go ahead and use the default settings. Since we are not going to make any other changes to the configuration here, let's click Create. We're taken back to the list of tables, and you can see that the table is being created. After a minute or so, the table status will change to active and we'll be able to start using it.