Introduction
Function COUNT
Function AVG
Functions SUM, MAX, MIN
27. SUM with NULLs only
Revision

## Instruction

Good! Just like with AVG, SUM will return a NULL when all the values are NULL. Let's check it out.

## Exercise

Remember the translator with id = 5 Adriana Fuentes? Let's go back there again. Sum all the words from all her projects except for project_id = 13 so that you only sum NULLs.

### Stuck? Here's a hint!

Use a WHERE clause with two conditions joined by AND.

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