Oracle 21c

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Create Table tbl AS 
Select 'TASK3' as DEPENDENT, 'TASK2' as TASK From Dual Union All
Select 'TASK1',	'TASK5' From Dual Union All
Select 'TASK2',	'TASK5' From Dual Union All
Select 'TASK4',	'TASK3' From Dual;
4 rows affected

Select * From tbl;


DEPENDENT
TASK


TASK3
TASK2

TASK1
TASK5

TASK2
TASK5

TASK4
TASK3


WITH 
  row_paths AS
    ( Select     t.DEPENDENT, t.TASK,  
                 t.DEPENDENT || '-' || t.TASK as TASK_ORDER,
                 o.DEPENDENT || '-' || o.TASK as TASK_SUBORDER
      From       tbl t
      Left Join  tbl o ON( o.TASK = t.DEPENDENT )
    )
  Select * From row_paths Order By TASK Desc, DEPENDENT



DEPENDENT
TASK
TASK_ORDER
TASK_SUBORDER


TASK1
TASK5
TASK1-TASK5
-

TASK2
TASK5
TASK2-TASK5
TASK3-TASK2

TASK4
TASK3
TASK4-TASK3
-

TASK3
TASK2
TASK3-TASK2
TASK4-TASK3


WITH 
  row_paths AS
    ( Select     t.DEPENDENT, t.TASK, 
                 t.DEPENDENT || '-' || t.TASK as TASK_ORDER,
                 o.DEPENDENT || '-' || o.TASK as TASK_SUBORDER
      From       tbl t
      Left Join  tbl o ON( o.TASK = t.DEPENDENT )
    ) 
Select     rp.*, 
           Case When rp.TASK_SUBORDER != '-' 
                Then REPLACE(rp.TASK_ORDER, rp.DEPENDENT, TASK_SUBORDER)
           Else rp.TASK_ORDER
           End as SELF_LEAF_TO_ROOT_PATH
From       row_paths rp
Order By   rp.TASK Desc, rp.DEPENDENT



DEPENDENT
TASK
TASK_ORDER
TASK_SUBORDER
SELF_LEAF_TO_ROOT_PATH


TASK1
TASK5
TASK1-TASK5
-
TASK1-TASK5

TASK2
TASK5
TASK2-TASK5
TASK3-TASK2
TASK3-TASK2-TASK5

TASK4
TASK3
TASK4-TASK3
-
TASK4-TASK3

TASK3
TASK2
TASK3-TASK2
TASK4-TASK3
TASK4-TASK3-TASK2

DEPENDENT	TASK	TASK_ORDER	TASK_SUBORDER
TASK1	TASK5	TASK1-TASK5	-
TASK2	TASK5	TASK2-TASK5	TASK3-TASK2
TASK4	TASK3	TASK4-TASK3	-
TASK3	TASK2	TASK3-TASK2	TASK4-TASK3