使用计数（分页）时，两个看似相同的查询返回不同的结果

Question

出于分页原因，我正在对我的数据库运行两个查询。因此，每个查询几乎是相同的。我的 COUNT(*) 查询没有返回非计数查询的结果数。我很困惑为什么会这样。查询如下。

SELECT p.host_id, p.rating_support, p.rating_tech, MAX(p.rating_overall) AS rating_overall, p.publish_rating, h.name, prices.price, prices.term_duration
FROM plans p
INNER JOIN hosts AS h ON h.id = p.host_id
INNER JOIN (SELECT plan_id, price, term_duration FROM prices WHERE price > 0 AND price < 50 AND term_duration = 1) prices ON prices.plan_id = p.id
WHERE p.published = 1 AND h.published = 1
GROUP BY p.host_id
ORDER BY rating_overall desc LIMIT 0, 12

SELECT COUNT(*) AS count
FROM plans p
INNER JOIN hosts AS h ON h.id = p.host_id
INNER JOIN (SELECT plan_id, price, term_duration FROM prices WHERE price > 0 AND price < 50 AND term_duration = 1) prices ON prices.plan_id = p.id
WHERE p.published = 1 AND h.published = 1
GROUP BY p.host_id

我不是 MySQL 专家。除了计数没有提供正确数量的结果之外，非计数查询也能完美运行。

任何关于这个问题的观点都会很棒。

Answer 1

在 Dems 的评论 (hunt down and upvote him 某处 :) 的帮助下，我创建了这个查询。请注意，我删除了子查询，因为它似乎没有必要：

SELECT
  COUNT( DISTINCT p.host_id )
FROM       plans p
INNER JOIN hosts h ON h.id = p.host_id
INNER JOIN prices  ON prices.plan_id = p.id
                  AND prices.price > 0
                  AND prices.price < 50
                  AND prices.term_duration = 1
WHERE p.published = 1
  AND h.published = 1

---

我原来的答案：

要获得总行数，您必须将GROUP BY 查询包装到外部SELECT：

SELECT COUNT(*)
FROM (
  SELECT NULL -- we are just counting, so we need no actual data -> a bit faster
  FROM       plans p
  INNER JOIN hosts h ON h.id = p.host_id
  INNER JOIN prices  ON prices.plan_id = p.id
                    AND prices.price > 0
                    AND prices.price < 50
                    AND prices.term_duration = 1
  WHERE p.published = 1
    AND h.published = 1
  GROUP BY p.host_id
) AS all_rows_without_data

或者你可以使用SQL_CALC_FOUND_ROWS + FOUND_ROWS()

http://dev.mysql.com/doc/refman/5.0/en/information-functions.html#function_found-rows

一个 SELECT 语句可能包含一个 LIMIT 子句来限制数量 服务器返回给客户端的行数。在某些情况下，它是 想知道语句将返回多少行 没有 LIMIT，但没有再次运行该语句。获得 此行数，在 SELECT 中包含 SQL_CALC_FOUND_ROWS 选项 语句，然后调用 FOUND_ROWS() ：

首先，只需选择所需的行，但添加SQL_CALC_FOUND_ROWS：

SELECT SQL_CALC_FOUND_ROWS
  p.host_id, p.rating_support, p.rating_tech,
  MAX(p.rating_overall) AS rating_overall,
  p.publish_rating, h.name, prices.price, prices.term_duration
FROM       plans p
INNER JOIN hosts AS h ON h.id = p.host_id
INNER JOIN prices  ON prices.plan_id = p.id
                  AND prices.price > 0
                  AND prices.price < 50
                  AND prices.term_duration = 1
WHERE p.published = 1 AND h.published = 1
GROUP BY p.host_id
ORDER BY rating_overall desc
LIMIT 0, 12;

其次，获取第一个查询中没有LIMIT语句时会返回的行数：

SELECT FOUND_ROWS();

更新：SQL_CALC_FOUND_ROWS + FOUND_ROWS() 似乎不太可靠，总是因为未知原因返回零（不仅仅是我：FOUND_ROWS() keeps returning 0）：

http://sqlfiddle.com/#!2/7304d/8

Answer 2

第二个查询的结果将返回相同数量的行，但第一行不会返回总数。

结果将给出每行每组的计数：

3
5
1
6
etc.
etc.

要将结果放入一行，请在 PHP 中对完整结果集使用 COUNT(DISTINCT p.host_id) 或 array_sum()。