如果类别为空，则SQL删除类别

Question

我遇到了麻烦，我有一个产品分类。我的问题是，如果类别为空且没有产品，我如何从数据库中删除类别。

我的代码：

$sql = "DELETE 
FROM category AS a 
INNER JOIN products AS b 
ON a.cat_id = b.cat_id 
WHERE a.user_id = ? 
AND a.cat_id = ? LIMIT 1";

Answer 1

我会使用not exists运算符来查找products表中[不]的类别：

DELETE 
FROM   category c
WHERE  NOT EXISTS (SELECT *
                   FROM   products p
                   WHERE  c.cat_id = p.cat_id);

Answer 2

DELETE FROM category AS a 
Where a.cat_id = ?
And not exists(
 Select 1
 From products p
 Where p.cat_id = a.cat_id
);

此查询匹配绑定变量上的cat I＆f，并过滤掉那些具有相应产品的内容。