MySql查询以获取每个国家/地区为每个目的花费的天数？ （获取第一张表中第二个表中所有记录的计数）

Question

我有三个表tl_log，tl_geo_countries，tl_purpose。我想在表'tl_purpose'中为每个目的获取表'tl_log'中每个国家/地区所花费的天数。 我试过下面的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

但与之相伴。

我无法获得'tl_log'中每个国家/地区的计数，而这些国家/地区'tl_log'中没有该数据。任何帮助是极大的赞赏。另外，如果这个问题很难理解，请告诉我。

预期输出：

以下是这三个表格的结构

tl_log

tl_geo_countries

tl_purpose

Answer 1

如果你想要所有可能的国家和目的组合，即使那些没有出现在日志表中的（这些将显示为0），你可以先做两个表的笛卡尔积（a { {1}} join）然后CROSS加入日志表：

LEFT

如果您只想要记录表中存在的国家/地区（但仍有所有可能的原因/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 
    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 ;

Answer 2

LEFT JOIN应替换为RIGHT JOIN