COUNT operator

COUNT from a column

Find the total number of names of all the students.

Instead of COUNT(*) we can use COUNT(name) to solve this problem.

The total number of students is 18.

It’s not obvious but if we are not using * then sql by default adds ALL. Let’s see the same code with ALL.

Besides ALL the other option is to use DISNTICT.

The result of COUNT query is also 18.

Let’s count total number of courses.

This time we got 17 in result.

So why the difference.

That’s because one student(Hugh) does not have any course. For that student the value of course is null.

COUNT does not count null values.