Question

我在这里有这个计算/查询：

SELECT  u.username,
        (a.totalCount * 7) +
        (b.totalCount * 3) +
        (c.totalCount * 1) AS totalScore
        FROM users u 
        LEFT JOIN
            (
                SELECT user_id, COUNT(user_id) totalCount
                FROM items 
                GROUP BY user_id
            ) a ON a.user_id= u.user_id
        LEFT JOIN
            (
                SELECT user_id, COUNT(user_id) totalCount
                FROM comments
                GROUP BY user_id
            ) b ON b.user_id= u.user_id
        LEFT JOIN
            (
                SELECT user_id, COUNT(user_id) totalCount
                FROM ratings
                GROUP BY user_id
            ) c ON c.user_id = u.user_id

        ORDER BY totalScore DESC LIMIT 10;

问题是，如果a，b或c返回0，则整个totalScore为0.我无法弄清楚发生了什么？我没有将最终的结果乘以0，我不认为？

Answer 1

我认为这是一个空问题（使用LEFT JOIN，这可能很容易发生）。

NULL + 1 + 2 = NULL

所以使用COALESCE（如果为null然后...）运算符

所以

SELECT  u.username,
        (COALESCE(a.totalCount, 0) * 7) +
        (COALESCE(b.totalCount, 0) * 3) +
        (COALESCE(c.totalCount, 0) * 1) AS totalScore

和一点SqlFiddle

SQL空数据取消所有计算

1 个答案: