Question

How do I create a query that display every parent with its children rather than to display all parents first then children. For example, right now, the mySQL table shows all COMPUTERS and then DRIVES. I want it to display for every computer, display the drives associated with that computer. I.E.:

 COMPUTER   DRIVE        PID         ID 
 dell                    1 
 apple                   2
 apple                   3
 hp                      4
 hp                      5
            WD300FV                  1
            WD440GB                  1
            WD55                     2
            WD44X                    2...

I want it displayed like this where every computer is shown a list of its drives:

 COMPUTER   DRIVE        PID     
 dell                    1 
            WD300FV      1
            WD440GB      1
 apple                   2
            WD55         2
            WD44X        2
 apple                   3.....

Where computer has a parent ID and drive has an ID.

When I do this query SELECT * FROM table it doesn’t create rows as I would like it to be displayed. I want every row that shows a computer to display a list of drives below the computer row.

Answer 1

I would question your table structure. I would think computers and drives should be in two separate but related tables, since they are two different things, potentially with their own properties.

computer table:

computer_id |  manufacturer
---------------------------
1           |  dell
2           |  apple
3           |  hp

computer_id would be auto-incrementing primary key

drive table:

drive_id   |  computer_id  |  name
-----------------------------------
1          |  1            | WD300FV
2          |  1            | WD440GB
3          |  2            | WD55
4          |  2            | WD44X
5          |  3            | XYZ

drive_id would be auto-incrementing primary key

computer_id would have index (and possibly foreign key constraint if you are using InnoDB and wanted to enforce a relationship to computer table)

Then you would query like this:

SELECT c.computer_id, c.manufacturer, d.drive_id, d.name
FROM computer AS c
INNER JOIN drive AS d ON c.computer_id = d.computer_id
ORDER BY c.computer_id ASC, d.drive_id ASC

If, for whatever reason, you didn’t want to go with a normalized structure like I have proposed and instead use a de-normalized structure like similar to what you already have, you should at least consider doing your de-normalization in a more sane way. There is no need for your pid logic at all if you just structured your table like this:

computer  |  drive
------------------
dell      | WD300FV
dell      | WD4440GB
apple     | WD55
apple     | WD44X
hp        | XYZ

You are just adding a level of complexity that doesn’t need to be there. You could easily sort your result by “computer”:

SELECT computer, drive FROM table
ORDER BY computer ASC, drive ASC

With no schema changes at all you could query like this:

SELECT computer, drive, CASE WHEN pid = '' THEN id ELSE pid END as `sort_id`
FROM table
ORDER BY `sort_id` ASC, drive ASC