Congratulations on learning SQL! Now that you've learned all that, here are some ideas for how to keep going.
Try SQL in other environments
On Khan Academy, we created a playground for you to try out SQL with small amounts of fake data. We encourage you to keep experimenting with it here, but you'll likely want to use your skills in an environment with real data.
Try SQL for data analysis. These days, SQL is often used to analyze data that comes from all sorts of places. One way that you can start analyzing data immediately is with Google's BigQuery, which includes a SQL-like language, and lets you upload data or use public data sets. Read the BigQuery documentation or read this BigQuery tutorial.
Try SQL on the server-side. SQL can be used to manage the data in a app's server-side database. A user uses the app via the "frontend" (webpage or mobile app), the frontend sends HTTP requests to the "backend" (server), and the backend issues SQL commands to the database. You can set up a server-side app on your own computer using many different languages/frameworks, two popular stacks are LAMP and LAPP.
Try SQL in a webpage. If you don't have a server setup or data to analyze, you could use SQLite inside a webpage, to store data that the user creates while using the webpage. The data won't be stored when the user leaves the page, however. You can spin-off this example of using SQLite in a webpage.
Learn more SQL
We covered a lot of SQL, but as you use it more, particularly for creating read/write databases to power apps, you should familiarize yourself with all the features available. You can learn more about indexes and query planning, constraints, triggers, views, and foreign keys. It's best if you know which SQL environment you'll be using those features in first, so you can read the most accurate documentation for it.
You could also learn different versions of SQL that are used with popular database vendors, like MySQL, PostGreSQL, Oracle, MS SQL, and DB2, perhaps in combination with learning SQL on the server-side.
Go deeper on databases
SQL is a way to interact with databases, so by learning SQL, you've also learned a fair bit about how databases work. However, you can go much deeper in the area of databases - learning more about the theories, principles, and design - like relational design theory, relational algebra and unified modeling language. One way to learn those is with Stanford's self-paced Intro to Databases class.