php＆mysql查找父类别名称

Question

I have problem about inner join, left join commands.

My category table is:

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

But ıt's not working.

Answer 1

You have to use LEFT JOIN to be able to pull all categories no matter have they parent category or not. INNER JOIN filters out all mismatches.

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

Output:

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

Here is SQLFiddle demo

Answer 2

If you want to get all parent categories then try query

SELECT cat_id FROM categories WHERE parent=0;

If you want get parent category of a category

SELECT C.cat_id, P.title FROM categories C LEFT JOIN categories P ON P.parent=C.cat_id;

I haven't tested above code but it should work fine.

Answer 3

You can always debug your SQL by entering it into a validator (loads online), phpMyAdmin's SQL tab or a editor with SQL validation. It looks to me like you have a small typo near your categories table selection.

Always dumb it down if your SQL isn't working. Note that JOINs (inner, left, right, ect) are meant to join TWO or MORE tables.

SELECT 
    one.ID, one.title, one.parent, one.title, one.parent, one.title
FROM 
    categories one 
LEFT JOIN 
    categories two
ON 
    one.parent = two.ID