equals运算符与in运算符之间的SQL计数函数差异

Question

This question is related to following hackerrank challenge: https://www.hackerrank.com/challenges/challenges/problem

The version that works returns the correct result set while the other returns an incomplete result set since count(*) does not work as expected with in clause

Only difference between the two code pieces is c_count instead of count(*) in the first clause of or.

Since c_count is equal to count(*), there should be no semantic difference. If count(*) in is wrong syntax, why does count(*) = work?

This works:

select count(*) as c_count ... having c_count in (select ...) or count(*) = (select ...);

This won't work:

select count(*) as c_count ... having count(*) in  (select ...) or count(*) = (select ...);

Here's the original queries. /**/ comment calls out the difference between the two:

Working:

SELECT h.hacker_id
    ,h.name
    ,count(*) AS c_count
FROM Hackers h
JOIN Challenges c ON h.hacker_id = c.hacker_id
GROUP BY h.hacker_id, h.name
HAVING 
    /*USING c_count ALIAS*/ 
    c_count IN (
        SELECT hacker_c_counts2.c_count2
        FROM (
            SELECT count(*) AS c_count2
            FROM Challenges c2
            GROUP BY c2.hacker_id
            ) AS hacker_c_counts2
        GROUP BY hacker_c_counts2.c_count2
        HAVING count(*) = 1
        )
    OR count(*) = (
        SELECT max(hacker_c_counts.c_count1)
        FROM (
            SELECT count(*) AS c_count1
            FROM Challenges c1
            GROUP BY c1.hacker_id
            ) AS hacker_c_counts
        )
ORDER BY count(*) DESC,  h.hacker_id;

Not Working:

SELECT h.hacker_id
    ,h.name
    ,count(*) AS c_count
FROM Hackers h
JOIN Challenges c ON h.hacker_id = c.hacker_id
GROUP BY h.hacker_id, h.name
HAVING 
    /*USING count(*) directly*/ 
    count(*) IN (
        SELECT hacker_c_counts2.c_count2
        FROM (
            SELECT count(*) AS c_count2
            FROM Challenges c2
            GROUP BY c2.hacker_id
            ) AS hacker_c_counts2
        GROUP BY hacker_c_counts2.c_count2
        HAVING count(*) = 1
        )
    OR count(*) = (
        SELECT max(hacker_c_counts.c_count1)
        FROM (
            SELECT count(*) AS c_count1
            FROM Challenges c1
            GROUP BY c1.hacker_id
            ) AS hacker_c_counts
        )
ORDER BY count(*) DESC, h.hacker_id;

Answer 1

Try this .....having h.c_count in..... Use the table Alias "h" before the c_count.

Good luck.