MySQL 8.0

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CREATE TABLE TABLENAME
    (`ID` int, `SQ` int, `Code` int)
;
    
INSERT INTO TABLENAME
    (`ID`, `SQ`, `Code`)
VALUES
    (123456, 1, 123),
    (654321, 1, 369),
    (123456, 2, 234)
;

Records: 3  Duplicates: 0  Warnings: 0


INSERT INTO TABLENAME (ID,SQ,Code) 
SELECT 1223344,( SELECT COALESCE(MAX(SQ),0) +1 FROM TABLENAME  WHERE ID =  1223344) ,666 
WHERE  NOT EXISTS ( SELECT 1 FROM TABLENAME  WHERE ID =  1223344 AND Code = 666)

Records: 1  Duplicates: 0  Warnings: 0


INSERT INTO TABLENAME (ID,SQ,Code) 
SELECT 123456,( SELECT COALESCE(MAX(SQ),0) +1 FROM TABLENAME  WHERE ID =  123456) ,666 
WHERE  NOT EXISTS( SELECT 1 FROM TABLENAME  WHERE ID =  123456 AND Code = 666)

Records: 1  Duplicates: 0  Warnings: 0


INSERT INTO TABLENAME (ID,SQ,Code) 
SELECT 1223344,( SELECT COALESCE(MAX(SQ),0) +1 FROM TABLENAME  WHERE ID =  1223344) ,666 
WHERE  NOT EXISTS( SELECT 1 FROM TABLENAME  WHERE ID =  1223344 AND Code = 666)

Records: 0  Duplicates: 0  Warnings: 0


SELECT * FROM TABLENAME


ID
SQ
Code


123456
1
123

654321
1
369

123456
2
234

1223344
1
666

123456
3
666