MySQL：平均子查询（不在 FROM/GROUP BY 中）

Question

我想显示我的数据库中产品组的平均价格。因此，我想返回该产品组内所有产品的平均价格。

首先，我想知道该组有多少价格变化。此查询成功返回所有日期，并带有此查询...

SELECT DISTINCT(DATE(gbp.prdate)) as dte
FROM sku AS s
  INNER JOIN mt_price gbp ON s.id = gbp.
WHERE (gbp.prdate < NOW() - INTERVAL ? DAY) AND (s.grp = ?)`

然后我在 php 中使用 foreach 生成以下查询...

$i = 1;
$qry = "SELECT s.id AS sid, s.sku, s.title AS s_title,";
foreach ($dates as $date) {
  $qry .= "(SELECT price FROM price WHERE price.sid = s.id AND prdate <= DATE(?) ORDER BY prdate LIMIT 1) AS ?, ";
  $datearr[] = $date;
}
$qry = substr($qry, 0, -2) . " ";
$qry .= "FROM sku AS s
  WHERE (s.grp = ?)
  GROUP BY s.id
  ORDER BY s.id";
$results = $db->prepare($qry);
foreach($datearr as $darr) {
  $results->bindParam($i++,$darr->dte,PDO::PARAM_STR);
  $results->bindParam($i++,$darr->dte,PDO::PARAM_STR);
}

上面的一个例子给了我这个查询......

SELECT s.id AS sid, s.rid, s.sku, s.title AS s_title,
  (SELECT price FROM price WHERE price.sid = s.id AND prdate <= DATE('2015-01-08') ORDER BY prdate LIMIT 1) AS '2015-01-08',
  (SELECT price FROM price WHERE price.sid = s.id AND prdate <= DATE('2015-02-12') ORDER BY prdate LIMIT 1) AS '2015-02-12',
  (SELECT price FROM price WHERE price.sid = s.id AND prdate <= DATE('2015-01-26') ORDER BY prdate LIMIT 1) AS '2015-01-26',
  (SELECT price FROM price WHERE price.sid = s.id AND prdate <= DATE('2015-02-08') ORDER BY prdate LIMIT 1) AS '2015-02-08',
  (SELECT price FROM price WHERE price.sid = s.id AND prdate <= DATE('2015-01-30') ORDER BY prdate LIMIT 1) AS '2015-01-30',
  (SELECT price FROM price WHERE price.sid = s.id AND prdate <= DATE('2015-02-02') ORDER BY prdate LIMIT 1) AS '2015-02-02'

来自 sku AS s 哪里（s.cat = 'TLPT01'） 按 s.id 分组 按 s.id 订购

当我按上述方式运行此查询时，我得到了我所期望的结果，每个 SKU 只有一个价格（低至LIMIT）和正确的价格。然后它将随着DATE 的变化正确显示 SKU 的价格。耶！

所以，我想要得到所有这些结果的平均值。如果我在s.id 前面使用AVG，我会得到SKU 的平均值，这显然不是一个有意义的结果。我想要返回的所有 SKU 的平均价格（因此，GROUP BY 应该只给我一行信息）。当我将GROUP BY 放在子查询之前时，我得到了一个错误。

Answer 1

尝试如下进行子查询 (SELECT AVG(price) FROM price WHERE price.sid = s.id AND prdate

Answer 2

我发现将子查询移动到LEFT JOIN 解决方案。然后我可以对从这些 JOIN 返回的价格执行AVG。我仍然不知道为什么我不能只 AVG 子查询，但是这种方法（虽然不是超级快）...有效。

SELECT s.cat, s.id AS sid, s.rid, s.sku, s.title AS s_title, AVG(pr1.price)
FROM sku AS s
  LEFT JOIN (SELECT sid, price, prdate FROM price WHERE prdate <= DATE('2014-12-21') ) AS pr1 ON s.id = pr1.sid
  ... <more from foreach, add more AVG on top line> ...
WHERE (s.cat = 'TLPT01')
GROUP BY cat

Answer 3

你不是在编码你所说的； 每个日期的平均值。在子查询上使用平均查询（简化）：

select date, avg(avg_price) from (
    select group_id, date, avg(price) avg_price
    from mytable
    group by group_id, date
) x
group by date

内部查询将为每个 group_id 和日期生成一个 avg 值，而外部查询仅按日期平均这些 group_id/date 值。