php&mysql 查找父类别名称

Question

我对内连接、左连接命令有疑问。

我的类别表是:

ID  | parent | title
1   | 0      | First Category
2   | 1      | Other Category

I have list categorys and I want get parents category title at sql command.

I have tried:

SELECT cat.ID, cat.title, cat2.title as parentcatname, cat.parent
FROM   categories cat INNER JOIN categories cat2 ON cat2.ID=cat.parent

但是它不起作用。

Answer 1

您必须使用LEFT JOIN才能提取所有类别，无论它们是否有父类别。 INNER JOIN 过滤掉所有不匹配的内容。

SELECT c.id, c.title, c.parent, p.title parent_title 
  FROM categories c LEFT JOIN categories p 
    ON c.parent = p.id

输出:

| ID |          TITLE | PARENT |   PARENT_TITLE |
-------------------------------------------------
|  1 | First Category |      0 |         (null) |
|  2 | Other Category |      1 | First Category |

这里是SQLFiddle 演示