Php mysql查询计算评级和

我有下表

id item_id rating 
 1    10     1
 2    10     2
 3    10     2
 4    10     3
 5    10     3
 6    10     3
 7    10     4

如何编写查询以获得如下结果：

$ratings = array(
    1 => 1,
    2 => 2,
    3 => 3,
    4 => 1,
    5 => 0
);

我需要使用此查询编写一个php函数来计算此函数的平均评分：

$total = 0;
$count = 0;
foreach($ratings as $number=>$frequency) {
  $total += $number * $frequency;
  $count += $frequency;
}
return $total / $count;

---

表上记录的基本计数不会产生

5

，因为评级上没有值

5

。但是，生成包含

1-5

中的值的子查询并使用

LEFT JOIN

将它们连接起来将得到您想要的结果

SELECT a.rating, COUNT(b.rating) totalCount
FROM
    (
      SELECT 1 rating UNION SELECT 2 UNION
      SELECT 3 UNION SELECT 4 UNION
      SELECT 5
    ) a
    LEFT JOIN tableName b
      ON a.rating = b.rating
GROUP BY a.rating

---

您好，这是迄今为止解决问题最简单的查询，但它不会显示等级5，因为它不在列表中<代码>：D

SELECT a.rating, COUNT(b.rating) totalCount
FROM
    (
      SELECT 1 rating UNION SELECT 2 UNION
      SELECT 3 UNION SELECT 4 UNION
      SELECT 5
    ) a
    LEFT JOIN tableName b
      ON a.rating = b.rating
GROUP BY a.rating