MySQL JOIN 同一张表上的多个连接？答案

【问题标题】：MySQL JOIN Multiple Joins on the same table?MySQL JOIN 同一张表上的多个连接？
【发布时间】：2011-10-18 05:55:21
【问题描述】：

SELECT people.first_name AS "First Name", people.last_name AS "Last Name", countries.name AS "Country1", territories.name AS "Territory1", cities.name AS "City1", countries.name AS "Country2", territories.name AS "Territory2", cities.name AS "City2"
FROM adb_people AS people
JOIN root_cities AS cities ON people.city1 = cities.id
AND people.city2 = cities.id
JOIN root_territories AS territories ON people.prov_state1 = territories.id
AND people.prov_state2 = territories.id
JOIN root_countries AS countries ON people.country1 = countries.id

我在这里要做的是将 Country1 (id) 链接到 Country1 (name) 并仅显示名称。此代码示例仅在 Country1,Territory1,City1 与 Country2,Territory2,City2 相同时才有效

我想我的问题是我如何进行 JOIN。我是 SQL 方面的新手。我已经阅读了互联网上的 JOINS（谷歌搜索并阅读了前几个教程），但是在这种情况下，我所阅读的任何内容都没有任何帮助。

对于我在这里做错的事情，我真的很感激。也许是朝着正确的方向轻推？

【问题讨论】：

标签： mysql join

【解决方案1】：

每个国家/城市/地区需要 2 个单独的连接。以下是基本语法，您可能需要稍微更改它，因为我还没有通过解析器：

SELECT people.first_name AS "First Name", people.last_name AS "Last Name", 
countries1.name AS "Country1", territories1.name AS "Territory1", cities1.name AS "City1", 
countries2.name AS "Country2", territories2.name AS "Territory2", cities2.name AS "City2"
FROM adb_people AS people
JOIN root_cities AS cities1 ON people.city1 = cities1.id
  AND people.city2 = cities1.id
JOIN root_territories AS territories1 ON people.prov_state1 = territories1.id
  AND people.prov_state2 = territories1.id
JOIN root_countries AS countries1 ON people.country1 = countries1.id
JOIN root_cities AS cities2 ON people.city2 = cities2.id
  AND people.city2 = cities2.id
JOIN root_territories AS territories2 ON people.prov_state2 = territories2.id
  AND people.prov_state2 = territories2.id
JOIN root_countries AS countries2 ON people.country2 = countries2.id

【讨论】：

呃，我为什么不开始尝试呢。我现在将实施它，并让你知道它是如何进行的。谢谢！
这个答案是不是还有只有city1=city2才有效的问题？
不应该，不。我只是编辑了查询，因为我忘记从复制/粘贴中更改选择部分中的表别名，如果这就是您所指的？
呵呵是的，上面的sql语句是错误的，但是给出的答案是对的。不使用该示例，而是将我的 SQL 语句更改为使用两个标识符进行两次 JOIN 就是答案！

【解决方案2】：

这样就可以了。

SELECT people.first_name AS "First Name", people.last_name AS "Last Name", countries.name AS "Country1", territories.name AS "Territory1", cities.name AS "City1", countries2.name AS "Country2", territories2.name AS "Territory2", cities2.name AS "City2"
FROM adb_people AS people
JOIN root_cities AS cities ON people.city1 = cities.id
JOIN root_cities AS cities2 ON people.city2 = cities.id
JOIN root_territories AS territories ON people.prov_state1 = territories.id
JOIN root_territories AS territories2 ON people.prov_state2 = territories.id
JOIN root_countries AS countries ON people.country1 = countries.id
JOIN root_countries AS countries2 ON people.country2 = countries.id

【讨论】：

【解决方案3】：

SELECT 
  people.first_name AS "First Name", 
  people.last_name AS "Last Name",
  countries.name AS "Country1",
  territories.name AS "Territory1",
  cities.name AS "City1",
  countries2.name AS "Country2",
  territories2.name AS "Territory2", 
  cities2.name AS "City2"
FROM 
   adb_people AS people
   JOIN root_cities AS cities ON people.city1 = cities.id
   jOIN root_cities AS cities2 people.city2 = cities2.id
   JOIN root_territories AS territories ON people.prov_state1 = territories.id
   JOIN root_territories AS territories2 ON people.prov_state2 = territories2.id
   JOIN root_countries AS countries ON people.country1 = countries.id
   JOIN root_countries AS countries2 ON people.country2 = countries2.id

【讨论】：