php – 计算MySQL中的投票频率

2023年5月16日 135次阅读

我正在创建一个网站,用户可以在该网站上对页面的类别进行投票.他们可以投票表明该页面属于a,b,c或d类.

我需要在所有投票中找到MySQL行中最常出现的类别.

每次用户提交投票时,它都会提交他们投票的“类别”和“page_id”.

到目前为止我有这个：

SELECT    page_id, category
FROM      categories
GROUP BY  page_id

我不能使用COUNT(*)WHERE category =’a’然后对每个类别重复它,因为实际项目中还有更多类别.

最佳答案 如果你的表看起来像这样：

SELECT * from categories;
+---------+----------+
| page_id | category |
+---------+----------+
|       1 | a        |
|       1 | b        |
|       1 | a        |
|       1 | c        |
|       1 | a        |
|       1 | b        |
|       1 | a        |
|       2 | d        |
|       2 | d        |
|       2 | c        |
|       2 | d        |
|       3 | a        |
|       3 | b        |
|       3 | c        |
|       4 | c        |
|       4 | d        |
|       4 | c        |
+---------+----------+
17 rows in set (0.00 sec)

然后您可能想尝试此查询：

SELECT   c1.page_id, MAX(freq.total),
         (
            SELECT   c2.category
            FROM     categories c2
            WHERE    c2.page_id = c1.page_id
            GROUP BY c2.category
            HAVING   COUNT(*) = MAX(freq.total)
            LIMIT    1
         ) AS category
FROM     categories c1 
JOIN     (
            SELECT   page_id, category, count(*) total 
            FROM     categories 
            GROUP BY page_id, category
         ) freq ON (freq.page_id = c1.page_id) 
GROUP BY c1.page_id;

哪个返回：

+---------+-----------------+----------+
| page_id | MAX(freq.total) | category |
+---------+-----------------+----------+
|       1 |               4 | a        |
|       2 |               3 | d        |
|       3 |               1 | a        |
|       4 |               2 | c        |
+---------+-----------------+----------+
4 rows in set (0.00 sec)

将结果与实际频率分布进行比较：

SELECT page_id, category, COUNT(*) FROM categories GROUP BY page_id, category;
+---------+----------+----------+
| page_id | category | COUNT(*) |
+---------+----------+----------+
|       1 | a        |        4 |
|       1 | b        |        2 |
|       1 | c        |        1 |
|       2 | c        |        1 |
|       2 | d        |        3 |
|       3 | a        |        1 |
|       3 | b        |        1 |
|       3 | c        |        1 |
|       4 | c        |        2 |
|       4 | d        |        1 |
+---------+----------+----------+
10 rows in set (0.00 sec)

请注意,对于page_id = 3,没有前导频率,在这种情况下,此查询不保证在这种情况下将选择哪个类别.

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