找到没有关系的多对多

Question

我有一个 items table、一个 tags table 和一个 reletations table。我需要找到未标记的项目。

通过这个（有效的）查询，我找到了每个项目的标签编号：

SELECT i.id, i.text, COUNT( ti.item ) AS count
FROM items AS i
JOIN topic_item AS ti ON ti.item = i.id
GROUP BY ti.item

如果尝试修改查询：

ti.item != i.id

或者我添加

WHERE count = 0

但这不是正确的方法。哪种方法正确？

Answer 1

尝试left join如下

SELECT i.id, i.text, COUNT( ti.item ) AS count
FROM items AS i
LEFT JOIN topic_item AS ti ON ti.item = i.id
WHERE ti.item is null
GROUP BY ti.item

Answer 2

您可以使用 left join，并获取没有匹配标签的项目：

SELECT i.id, i.text
FROM items AS i
LEFT JOIN topic_item AS ti ON ti.item = i.id
WHERE ti.item is null

您也可以使用 not exists:

SELECT i.id, i.text
FROM items AS i
WHERE not exists(
  select * from topic_item AS ti
  where ti.item = i.id
)

find many to many with no relation