MySql: ORDER BY parent and child

Question

I have a table like:

+------+---------+-
| id   | parent  |
+------+---------+
| 2043 |    NULL |
| 2044 |    2043 |
| 2045 |    2043 |
| 2049 |    2043 |
|          


        

Answer 1

The solution above didn't work for me, my table used 0 instead of NULL. I found this other solution: you create a column with the concatened parent id and child id in your query and you can sort the result by it .

SELECT CONCAT(IF(parent = 0,'',CONCAT('/',parent)),'/',id) AS gen_order
FROM table 
ORDER BY gen_order

Answer 2

Including sorting children by id:

ORDER BY COALESCE(parent, id), parent IS NOT NULL, id

SQL Fiddle example

Explanation:

• COALESCE(parent, id): First sort by (effectively grouping together) the parent's id.
• parent IS NOT NULL: Put the parent row on top of the group
• id: Finally sort all the children (same parent, and parent is not null)

Answer 3

If your table uses 0 instead of null to indicate an entry with no parent:

id   | parent
-------------
1233 | 0
1234 | 1233
1235 | 0
1236 | 1233
1237 | 1235

Use greatest instead of coalesce and check the value does not equal 0:

ORDER BY GREATEST(parent, id), parent != 0, id