Question

我有一些带有一些产品的数据库，它们可以被许多类别“标记”。有些产品没有指定类别，我需要找到它们。无法找到没有类别的记录。我创建了这个表：

项

+----+------+
| ID | Item |
+----+------+
|  1 | qwe  |
|  2 | asw  |
|  3 | wgr  |
+----+------+

分类

+----+----------+
| ID | Category |
+----+----------+
|  1 | xxx      |
|  2 | yyy      |
+----+----------+

关系

+---------+--------+
| id_item | id_cat |
+---------+--------+
|       1 |      1 |
|       1 |      2 |
|       2 |      1 |
+---------+--------+

这是（不工作）查询：

SELECT item.id COUNT(relationship.id_item) AS n
FROM item
JOIN relationships GROUP
BY relationships .n
WHERE item.id = relationships.id_item
AND relationships.n =0;

Answer 1

您想要一个左连接，然后找到那些不匹配的连接：

SELECT i.id, COUNT(relationship.id_item) AS n
FROM item i LEFT OUTER JOIN
     relationships r
     on i.id = r.id_item
WHERE r.id_item is null;

Answer 2

您可以将子查询与NOT IN子句

一起使用

SELECT i.id
FROM item i
WHERE i.id NOT IN ( SELECT r.id_item
    FROM RELATIONSHIP r);

在多对多关系中选择没有类别的记录

2 个答案: