MySQL:使用GROUP BY时count(id)返回1而不是实际的总行数

Question

我的查询在下面(它搜索父母和孩子4个级别上下).这样,我得到一个"num"值中找到的行数:大约~22k行.

SELECT COUNT(fts_trip_id) AS num 
FROM feed_trips_se
JOIN departures ON d_departure_id=fts_trip_departure_id
JOIN destinations ON destinations.d_destination_id=d_departure_id
LEFT JOIN destinations AS destChild1 ON destChild1.d_destination_parent_id=destinations.d_destination_id
LEFT JOIN destinations AS destChild2 ON destChild2.d_destination_parent_id=destChild1.d_destination_id
LEFT JOIN destinations AS destChild3 ON destChild3.d_destination_parent_id=destChild2.d_destination_id
LEFT JOIN destinations AS destChild4 ON destChild4.d_destination_parent_id=destChild3.d_destination_id
LEFT JOIN destinations AS destParent1 ON destParent1.d_destination_id=destinations.d_destination_parent_id
LEFT JOIN destinations AS destParent2 ON destParent2.d_destination_id=destParent1.d_destination_parent_id
LEFT JOIN destinations AS destParent3 ON destParent3.d_destination_id=destParent2.d_destination_parent_id
LEFT JOIN destinations AS destParent4 ON destParent4.d_destination_id=destParent3.d_destination_parent_id
WHERE fts_trip_date>=NOW()
ORDER BY fts_trip_date ASC, fts_trip_price ASC

但是,由于我使用了大量LEFT JOIN,因此〜22k的数量不是正确的行程数量.我需要按实际的fts_trip_id对这些行进行分组.

不幸的是,当我进行GROUP BY时,我获得的行数与正确的行程数(~8k)相同,而不是获得正确行程数的单行.然后所有行都显示一个数字,最常见的是1,但也有10或13.

老实说,当我使用GROUP BY函数时,我不知道为什么我没有得到~8k的单行值.谁能帮我？

Answer 1

您可以使用COUNT(DISTINCT):COUNT(DISTINCT fts_trip_id)来计算ID.GROUP BY只会计算每个id的结果数.