查询按单列分组

Question

I'd like to spit out database results which are grouped by a parent ID.

I have the following table:

`pages`
    `id` int
    `parent_id` int
    `title` varchar

Some of the pages have a parent_id of 0, meaning they do not have a parent.

Lets say I have the following rows in the pages table:

id | parent_id | title
1  | 0         | Colors
2  | 1         | Red
3  | 1         | Green
4  | 1         | Blue
5  | 0         | Devices
6  | 5         | Mobile
7  | 5         | Desktop
8  | 5         | Tablet

Given these rows, I want to output the following on my PHP page:

Colors
    Red
    Green
    Blue

Devices
    Mobile
    Desktop
    Tablet

I know that I can use a query to get the rows with a parent_id of 0, and then another query to get the rows with a parent_id of that id .

Is it possible to write this as a single query?

Answer 1

Yes, but it's not pretty: You have a hierarchical data structure (parent/child), and to arrange things in that fashion in a single query requires recursive queries, which mysql doesn't support.

You'll need to do a self-join for every level of hierarchy you have:

SELECT parent.*, child.*
FROM yourtable AS parent
LEFT JOIN yourtable AS child ON parent.id = child.parent_id

For every level of your tree, you'll need another join/alias as above.

Answer 2

I think you are looking the following solution:

SELECT 
  t1.title as parent, t2.title
FROM 
  pages AS t1
LEFT JOIN pages AS t2 
  ON t1.id = t2.parent_id
WHERE
  t2.id IS NOT NULL

You will get the result as like:

parent  title
Colors  Red
Colors  Green
Colors  Blue
Devices Mobile
Devices Desktop
Devices Tablet

Now you have to arrange it in you php side.

Here is the sqlfiddle link: http://sqlfiddle.com/#!9/11fa6/3