I am trying to write a search query on MYSQL to find the users
People who didn't attend any concert should not be reference in your Attended table by definition.

So you could maybe try to do your requests with a left or right join and put a criteria on records with null id, maybe something like this:

SELECT People.FirstName from People 
LEFT JOIN Attended on Attended.Customer = People.ID
WHERE Attended.Customer is null;


This is how I thought my query :

By using a left join, I'll get all records from left table (here People), with matching record from right table (here Attended).

The missing records (i.e. records on which there is no match with given on clause) are replaced by null value - thus, if I can't find an Attended record for my people ID, I'll get Null for Attended Customer column. 

My WHERE clause filter all Attended record which are not null, leaving me with only people who don't have matching Attended record - these are people who don't go to concert.