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All the Perl that's Practical to Extract and Report

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  • Shouldn't it be (as it appears was suggested in your earlier post):

    SUM( COALESCE( price, 0 ) ) AS total

    If the coalesce is outside the sume, it doesn't fix the underlying issue of apply an aggregate function on a set of data that may contain a NULL value. Or, perhaps I'm missing something...
    • SUM ignores NULLs. So the sum of 1 and NULL is 1.

      SUM's behavior, of course, makes perfect sense: as 1+NULL yields NULL, so clearly the sum is 1.</sarcasm>

  • Now consider a simpler, yet silly, example: SELECT first_name FROM employee WHERE salary > 50000; What happens if the salary field is NULL? You'll get a list of employees whose known salary is NULL.

    I can't figure out what you meant to say there, but the query doesn't seem to go with the text.

    In this scenario, it is the case that everyone has a salary; you just don't know what some of them are. So here's the kicker: SELECT first_name FROM employee WHERE salary = salary; Th

    • Ack, fixed the typo regard "known salary is null".

      You don't know that though. Not within the terms of the database. If you did, they'd have a numeric salary and not NULL.

      That's entirely the point. Database "logic", when combined with NULLs, returns answers that we really know are not true. I don't mean "within the terms of the database". p = p is true, regardless of whether or not a poorly designed query language says otherwise.