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Databases Reporting with SQL Aggregate and Numeric Functions Counting Groups

Why is the a NULL genre value complicating this challenge and howto solve it elegantly?

So this was my query to find the count of all unique genres in books:

SELECT COUNT(*) AS 'total_genres' FROM (SELECT DISTINCT genre FROM books) G;

Now this returned 7 which i figured eventually was one more than the actual number because of NULL values in some rows for genre... So this worked:

SELECT COUNT(*)-1 AS 'total_genres' FROM (SELECT DISTINCT genre FROM books) G;

But this seems like a recreational solution... Is there a nicer more robust way to solve this?

I am not sure about this one, but have you already tried SELECT COUNT(*) AS 'total_genres' FROM (SELECT DISTINCT genre FROM books WHERE genre IS NOT NULL)?

Thanks Egarat! This seems to be one better way to solve this! I am not sure how I can comment under your answer or mark it as best answer but maybe theydo not have this option anymore...

1 Answer

Hey Gleb,

I was too lazy to scroll down and put the answer in the corresponding field and just used the comment field instead (that's why you could not mark it as the correct solution)... :-) I am glad to be of help, if you don't know the reason why this works - just let me know.

But here is the solution again:


Happy coding!