How many unique?
But what if we want to calculate the number of unique site visitors? Sounds like a combination of selecting unique and then counting...
Yes, that's the point! To calculate the number of unique observations over a certain column use COUNT(DISTINCT( ... )) statement. For example,
SELECT COUNT(DISTINCT(user_id)) FROM visitors
This query will return a single value - the number of unique user_id in the visitors table.
Oppgave
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Count the number of different cars models in the audi_cars table.
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