我有三个表 tl_log、tl_geo_countries、tl_purpose。我正在尝试为表“tl_purpose”中的每个目的获取表“tl_log”中每个国家/地区花费的天数。 我尝试了下面的 mysql 查询
SELECT t.country_id AS countryID,t.reason_id AS reasonID,count(t.reason_id) AS
days,c.name AS country, p.purpose AS purpose
FROM `tl_log` AS t
LEFT JOIN tl_geo_countries AS c ON t.country_id=c.id
LEFT JOIN tl_purpose AS p ON t.reason_id=p.id
GROUP BY t.reason_id,t.country_id ORDER BY days DESC
但是着陆了。
我无法在表“tl_log”中不存在的“tl_log”中获取每个国家/地区的目的计数。任何帮助是极大的赞赏。另外,如果问题难以理解,请告诉我。
预期输出:
下面是这三个表的结构
tl_log
tl_geo_countries
tl_purpose
最佳答案
如果您想要所有可能的国家和目的组合,即使是那些没有出现在日志表中的(这些将显示为 0),您可以首先对两个表进行笛卡尔乘积(a CROSS
join) 然后LEFT
join 到日志表:
SELECT
c.id AS countryID,
p.id AS reasonID,
COUNT(t.reason_id) AS days,
c.name AS country,
p.purpose AS purpose
FROM
tl_geo_countries AS c
CROSS JOIN
tl_purpose AS p
LEFT JOIN
tl_log AS t
ON t.country_id = c.id
AND t.reason_id = p.id
GROUP BY
p.id,
c.id
ORDER BY
days DESC ;
如果您只想要日志表中存在的国家/地区的记录(但仍然是所有可能的原因/目的),则需要稍作修改:
SELECT
c.id AS countryID,
p.id AS reasonID,
COUNT(t.reason_id) AS days,
c.name AS country,
p.purpose AS purpose
FROM
( SELECT DISTINCT
country_id
FROM
tl_log
) AS dc
JOIN
tl_geo_countries AS c
ON c.id = dc.country_id
CROSS JOIN
tl_purpose AS p
LEFT JOIN
tl_log AS t
ON t.country_id = c.id
AND t.reason_id = p.id
GROUP BY
p.id,
c.id
ORDER BY
days DESC ;
关于MySql 查询以获取在每个国家/地区为每个目的花费的天数? (获取第一个表中存在的第二个表中的所有记录的计数),我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/12104672/