MySQL 8.0

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-- INIT database
CREATE TABLE TableA (
  ID  INT,
  ChildID INT,
  ParentID INT
);



INSERT INTO TableA (ID,ChildID,ParentID) VALUES (1,89,891);
INSERT INTO TableA (ID,ChildID,ParentID) VALUES (2,99,991);
INSERT INTO TableA (ID,ChildID,ParentID) VALUES (3,991,99);
-- QUERY database

Delete S1 from TableA S1
    Inner Join TableA S2 
    On S1.ChildID=S2.ParentID
    and S1.ParentID=S2.ChildID
    AND S1.ID < S2.ID;
Select * from TableA;


ID
ChildID
ParentID


1
89
891

3
991
99


INSERT INTO TableA (ID,ChildID,ParentID) VALUES (2,99,991);
Delete S1 from TableA S1
    Inner Join TableA S2 
    On S1.ChildID=S2.ParentID
    and S1.ParentID=S2.ChildID
    AND S1.ID > S2.ID;
Select * from TableA;


ID
ChildID
ParentID


1
89
891

2
99
991