15. Count signups from country

So far we’ve learned how to filter all user records from Germany . Instead of counting all records by hand, we can ask SQL to count it for us. Here’s the query:

SELECT 
  COUNT(*)
FROM users
WHERE
  country = 'de'

As you can see it’s our normal query to select all records but instead of selecting all columns (*) we use SQL aggregate function COUNT.

Aggregate functions

It’s helpful to think about the query above as a usual SELECT * query which filters all records and then aggregates them. Instead of printing all records, it gives us a single number, aggregated value, calculated from all filtered records. SQL provides an aggregate function to count, sum, calculate min/max/average values, and more. Later in the course, we’ll use these functions to answer questions like “Min/max/average user age”, “Average revenue per user” and so on.

Let’s look closer at function syntax: COUNT(*). COUNT is a function name and * is the argument of a function. Usually, the argument is a column name or a wildcard symbol * (meaning all columns). Here’s another example of an aggregate function: MIN(age):

SELECT 
  MIN(age)
FROM users
WHERE
  country = 'de'

This query filters users from Germany and then it aggregates all of those users by finding the one with minimum age and printing it for us. Aggregate functions are vital for reporting in Marketing or Product Management – we’re measuring a number of signups, sums or averages of revenues. We’ll see many examples of aggregate functions in the SQL Habit course. It’s time for practice!