SQL count 函数在 equals 运算符和 in 运算符之间的区别

Question

这个问题与以下hackerrank挑战有关： https://www.hackerrank.com/challenges/challenges/problem

有效的版本返回正确的结果集，而另一个返回不完整的结果集，因为 count(*) 在 in 子句中无法按预期工作

这两个代码片段之间的唯一区别是 or 的第一个子句中的 c_count 而不是 count(*)。

由于 c_count 等于 count(*)，所以应该没有语义差异。如果count(*) in 语法错误，为什么count(*) = 有效？

这行得通：

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

这行不通：

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

这是原始查询。 /**/评论指出两者的区别：

工作：

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;

不工作：

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

试试这个.....h.c_count in.....在c_count之前使用表格别名“h”。

祝你好运。