Question

Assume the following table records:

TABLE: foo
==========================
| foo_id | foo_parent_id |
==========================
| 1      | NULL          |
| 2      | NULL          |
| 3      | 1             |
| 4      | 2             |
| 5      | 1             |
| 6      | 1             |
| 7      | 2             |
| 8      | 1             |
| 9      | NULL          |
--------------------------

I want to get, say, the first 10 parent records (those records with foo_parent_id = NULL) immediately followed by, say, the first 2 child records of that parent record. So, I’m looking for a result like this:

1, NULL
3, 1
5, 1
2, NULL
4, 2
7, 2
9, NULL

How do I query something like this?

Answer 1

Here’s one idea. But it’s based on lots of assumptions about the way your data is setup. Ever increasing IDs down the tree, only two levels, etc.

SELECT f.foo_id,f.foo_parent_id FROM foo f
foo f

–give me the top X number of parent_ids
(This is good, you just adjust the LIMIT 10 to vary the number of parent levels to show)

INNER JOIN 
(select foo_id from foo where foo_parent_id is null order by foo_parent_id 
LIMIT 10
) top_foo_parent
      on isnull(f.foo_parent_id,f.foo_id) = top_foo_parent.foo_id
WHERE

(This part is kind of hacky, as you have to put an ever longer string of these to get past two children)

–it’s the first child, or…

(f.foo_id in (select MIN(foo_id) from foo fc1 where fc1.foo_parent_id =f.foo_parent_id)
 )
 or

–it’s the second child, or…

(f.foo_id in (select MIN(foo_id) from foo fc1 where fc1.foo_parent_id =f.foo_parent_id  and fc1.foo_id not in (select MIN(foo_id) from foo fc2 where fc2.foo_parent_id=f.foo_parent_id))
 )
 or 

–it’s the parent

 f.foo_parent_id is null
order by isnull(f.foo_parent_id,f.foo_id)*100 + f.foo_id

So what we’re doing here is basically ordering by the parent_id column and then the child columns underneath it with a slight twist. If the parentid column is NULL then we use the actual ID. This means that for ordering purposes our table looks like this:

==============================================================================
| foo_id | foo_parent_id |   isnull(f.foo_parent_id,f.foo_id)
==============================================================================
| 1      | NULL           |         (1)
| 2      | NULL           |         (2)
| 3      |  1             |         1
| 4      |  2             |         2
| 5      |  1             |         1
| 7      |  2             |         2
----------------------------------------------------------------------

Then we multiply that ordering column *100

==============================================================================
| foo_id | foo_parent_id |   isnull(f.foo_parent_id,f.foo_id)*100
==============================================================================
| 1      | NULL           |         100
| 2      | NULL           |         200
| 3      |  1             |         100
| 4      |  2             |         200
| 5      |  1             |         100
| 7      |  2             |         200
----------------------------------------------------------------------

and lastly we add our foo_id column to it

==============================================================================
| foo_id | foo_parent_id |   isnull(f.foo_parent_id,f.foo_id)*100 + foo_id
==============================================================================
| 1      | NULL           |         101
| 2      | NULL           |         202
| 3      |  1             |         103
| 4      |  2             |         204
| 5      |  1             |         105
| 7      |  2             |         207
----------------------------------------------------------------------

Now we order the table by that virtual column and…

==============================================================================
| foo_id | foo_parent_id |   ORDER BY isnull(f.foo_parent_id,f.foo_id)*100 + foo_id
==============================================================================
| 1      | NULL           |         101
| 3      |  1             |         103
| 5      |  1             |         105
| 2      | NULL           |         202    
| 4      |  2             |         204
| 7      |  2             |         207
----------------------------------------------------------------------

There we go!