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All the Perl that's Practical to Extract and Report

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  • count(expression) counts the number of times expression is not NULL. Which explains your answer. I always use 'count(*)' if I want to count the number of rows - several databases have optimized 'count(*)'. Others use 'count(1)' to count rows.
    • Regrettably, this is something that much documentation [mysql.com] does not make clear. I'm expecting count() to return the number of instances of a particular value. Given that NULL can be thought of as "unknown", I suppose one could argue that it makes sense that it doesn't count the number of values. However, it also seems reasonable for one to assume that count(some_field) will return how many unknown values there are.

      Your explanation is perfectly correct. I'm just frustrated that an arguably "intuitive" answe

      • I guess some of the strange logic is this: if you want to get a count of the number of unique values, you use COUNT(DISTINCT field). If you want to get a count of records, you use COUNT(*). Now what do you want when you ask for COUNT(field)? Do you want a count of distinct values? Use COUNT(DISTINCT field). Do you want a count of records? Use COUNT(*). If you grabbed field and then counted the number of values (not distinct), you'd get absolutely the same results as COUNT(*), right?

        So I guess some of the tortured logic is, "COUNT(field) should do something completely different since the other two effects can already be achieved with different code. What else can we do? Oh, I know! Let's have it return a count of all the NON-NULL values! After all, it's got to have some reason for actually looking at field; otherwise the coder would've just said COUNT(*)."

        Weird, but sensible, in the way that any language decision that comes out of a committee is "sensible."

        --
        J. David works really hard, has a passion for writing good software, and knows many of the world's best Perl programmers