In this chapter, we expanded on our knowledge of the SQL language, using it to perform increasingly complex queries and operations on our data. With the addition of transactions, we can execute multiple SQL queries in an atomic action with varying degrees of isolation and locking. We should also have an idea of how to qualify and sort our data, use functions, and modify the schema of our tables (including the costs associated with this).
With the knowledge presented in the previous four chapters, we now know enough about databases to begin using them from within PHP code. In the next chapter, we will look at how to connect to servers, execute queries, and see the results from within our web applications.