In episode 6 it is implied in the paragraph about count() that you would get the same result whatever field you counted, i.e. count(*) is the same as count(quant) is the same as count(person) etc. However, as the students learned in the previous episode aggregation functions do not include NULL values. I think it could be made a bit clearer in episode 6 that count(person) would not return 9 as there are NULL values in the person field. It would reinforce the learning of the previous episode and perhaps stop any confusion about what count does.
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