Perfect! To count registrations in 2016, we use the following WHERE clause:

WHERE registration_date >= '2016-01-01'
  AND registration_date <  '2017-01-01';

There is also an alternative way of writing this query. Take a look:

SELECT COUNT(customer_id) AS registration_count_2016
FROM customers
WHERE registration_date >= '2016-01-01'
  AND registration_date <  '2016-01-01'::date + INTERVAL '1' year;

This time, instead of providing the end date specifically, we used the INTERVAL 'number' datepart function. It takes two arguments:

1. number – the number of intervals to add.
2. datepart – the interval unit we want to add, such as: year or month.

In our example, '2016-01-01'::date + INTERVAL '1' year means "add one year to the date of Jan 1, 2016". Remember to cast the string with the date to date (by typing ::date), so the database knows that you want to operate on time values.