MySQL 8.0

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select version();


version()


8.0.30


CREATE TABLE mytable ( 
KEY1 INTEGER ,
KEY2 INTEGER ,
KEY3 INTEGER ,
KEY6 CHAR(1) ,
PLT_ID INTEGER );

INSERT INTO  mytable  VALUES
('1','1','1','A','1'),
('1','1','1','B','1'),
('1','1','0','C','1'),
('1','1','0','D','0'),
('1','1','1','E','0'),
('1','1','1','F','0');

Records: 6  Duplicates: 0  Warnings: 0


SELECT *
FROM mytable
WHERE (KeY1,Key2,Key3, PLT_ID) IN (SELECT KeY1,Key2,Key3, PLT_ID 
                                   FROM mytable
                                   GROUP BY KeY1,Key2,Key3, PLT_ID
                                   HAVING count(*)=1)


KEY1
KEY2
KEY3
KEY6
PLT_ID


1
1
0
C
1

1
1
0
D
0