Question

出于分页原因，我正在向我的数据库运行两个查询。因此，每个查询几乎相同。我的COUNT（*）查询未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的专家。除了计数未提供正确数量的结果外，非计数查询也能正常工作。

对这个问题的任何启示都会很棒。

Answer 1

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

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

我原来的回答：

要获取总行数，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

或者您可以使用SQL_CALC_FOUND_ROWS + FOUND_ROWS()

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

SELECT语句可能包含LIMIT子句以限制该数字服务器返回到客户端的行数。在某些情况下，它是希望知道该语句将返回多少行没有LIMIT，但没有再次运行语句。获得此行计数，在SELECT中包含SQL_CALC_FOUND_ROWS选项声明，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，如果第一个查询中没有LIMIT语句，则获取将返回的行数：

SELECT FOUND_ROWS();

更新：SQL_CALC_FOUND_ROWS + FOUND_ROWS()似乎不太可靠，因为未知原因（不只是我：FOUND_ROWS() keeps returning 0）总是回零：

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

Answer 2

第二次查询的结果将返回相同的行数，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.

要将结果放到一行，请在完整结果集中使用PHP中的COUNT(DISTINCT p.host_id)或array_sum()。

两个看似相同的查询在使用count（分页）时返回不同的结果

2 个答案: