SQL for Data Scientists

CHAPTER 3The WHERE Clause

Now that you have the basic idea of how SQL queries look and have learned how to return the columns of data that you want from a single database table, we can talk about how to filter that result to include only the rows that you want returned.

The WHERE Clause

The WHERE clause is the part of the SELECT statement in which you list conditions that are used to determine which rows in the table should be included in the results set. In other words, the WHERE clause is used for filtering.

If you have programmed in other languages, you have likely encountered other conditional statements such as “IF” statements, which use boolean logic (think “AND” or “OR”) to determine what action to take, based on whether certain conditions are met. SQL uses boolean logic to check the available data against conditions in your WHERE clause to determine whether to include each row in the output.

I use the WHERE clause in almost every query I write as a data scientist to accomplish things like narrowing down categories of records to be displayed in a report, or filtering a dataset to a particular date range from the past that will be used to train a predictive model.

Filtering SELECT Statement Results

The WHERE clause goes after the FROM statement and before any GROUP BY, ORDER BY, or LIMIT statements in the SELECT query:

• SELECT [columns to return]
• FROM [table]
• WHERE [conditional filter statements]
• ORDER BY [columns to sort on]

For example, to get a list of product ...