SQL - 避免重复的列值

Question

SELECT t.id, apa.district AS district
FROM tbl_activity AS t
JOIN tbl_activity_package AS ap ON ap.activity_id = t.id
JOIN tbl_activity_package_address AS apa ON apa.activity_package_id = ap.id
GROUP BY t.id
ORDER BY apa.district

查询后，输出为：

我想按地区返回一行。我怎么能在查询中做到这一点？

Answer 1

您希望按district分组，而不是id分组。

但是，这会让您陷入困境：您希望为Evora区显示哪个id值？你想要117 id，还是118 id？你将如何确定它？您需要一个特定的规则来满足您的需求。作为一个例子，这将给你更大的：

SELECT MAX(t.id) id, apa.district AS district
FROM tbl_activity AS t
JOIN tbl_activity_package AS ap ON ap.activity_id = t.id
JOIN tbl_activity_package_address AS apa ON apa.activity_package_id = ap.id
GROUP BY apa.district
ORDER BY apa.district

将MAX（）更改为MIN（）将为您提供较低的ID。您也可以拥有更复杂的条件，但这些条件需要首先将其构建为可以加入的派生表（子查询），以将您的条件映射到特定的ID号。