So far, you’ve been writing mostly queries that return an entire set of data, but as you write more purposeful SQL using larger sets of data, you’ll find that you need only a subset of the data instead of all the rows. You did work a bit in chapter 4 to reduce the number of rows returned using LIMIT and OFFSET, but those commands aren’t helpful for finding specific rows.
You may want only a report of sales for the past month, a list of orders with pending status, or a list of customers in New Hampshire, for example. All these scenarios have conditions for specific data being returned, and we apply those conditions using filtering. Filtering means taking the broader results of your data set and applying one or more conditions to restrict the data being returned. To do this, you primarily use a different clause: the WHERE clause.
It’s highly likely that most of the SQL you’ll write in your career will include a WHERE clause because there’s a nearly infinite number of ways you may need to find data that meets specific criteria. The WHERE clause is incredibly powerful, with so many ways to filter data that it will take a few chapters to review them. Let’s get started!