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Introduction
Function COUNT
Function AVG
Functions SUM, MAX, MIN
Revision

## Instruction

Good job! It is less commonly known that you can use COUNT functions with any expression as argument.

You can write:

SELECT COUNT(price*2)
FROM project


This query counts the doubled prices. (Yes, the result will be the same as counting single prices.) We will see the real-world usage of this construction later in this section and in the next part of the course.

## Exercise

The feedback of 0 doesn't mean anything to agency managers. They want to find out how many projects got meaningful (non-zero) feedback.

Count the number of project with non-zero feedback.

### Stuck? Here's a hint!

The function NULLIF(x,y) checks if x equals to y and if it does, returns a NULL. Otherwise returns x.