Check out example codes for "order child below the parent in mysqli". It will help you in understanding the concepts better.

Code Example 1

# If no-parent = 0 instead of null 
SELECT CONCAT(IF(parent = 0,'',CONCAT('/',parent)),'/',id) AS gen_order
FROM table 
ORDER BY gen_order

#and if no-parent = null
ORDER BY COALESCE(parent, id), parent IS NOT NULL, id

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