MySQL - 在WHERE中查找平均数

Question

我试图根据WHERE子句找到某些行的总和的平均数。这是我到目前为止所做的：

SELECT SUM(rating), id
FROM rating
WHERE id=$id;");

所以类似“行数之和”/“返回的行数”

Answer 1

AVG(expr)：返回expr的平均值，例如

SELECT
    AVG(rating),
    id
FROM
    rating
WHERE
    id=$id
GROUP BY
    id

（编辑：忘记GROUP BY）

Answer 2

不是那么简单：

SELECT SUM(rating) AS sum, AVG(rating) AS avg, id
  FROM rating
 WHERE id = $id
 GROUP BY id;

Answer 3

MySQL实际上有一个内置的平均功能

SELECT AVG(rating) as average_rating,
id
FROM rating
WHERE id=$id
GROUP BY id

如果由于某种原因你真的想使用SUM，你可以使用

SELECT SUM(rating)/COUNT(rating) as average_rating,
id
FROM rating
WHERE id=$id
GROUP BY id

希望不是！