Ordering
Eliminating duplicate results
Aggregation
Grouping
HAVING: filtering and ordering groups
Let's practice
Instruction
Naturally, the asterisk (*) isn't the only option available in the function COUNT(). For example, we may ask the database to count the values in a specific column:
SELECT COUNT(customer_id) FROM orders;
What's the difference between COUNT(*) and COUNT(customer_id)? Well, the first option counts all rows in the table and the second option counts all rows where the column customer_id has a specified value. In other words, if there is a NULL in the column customer_id, that row won't be counted.
Exercise
Check how many non-NULL values in the column position there are in the table employees. Name the column non_null_no.
Stuck? Here's a hint!
Type:
SELECT COUNT(position) AS non_null_no FROM employees;
