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db<>fiddle
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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