MySQL 8.0

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select version();

version()

8.0.30

create table SRC 
(
    Z varchar(20) null,
    A varchar(63) null,
    B varchar(30) null
);

insert into SRC
select 'XYZ', '1876301/347', null;

Records: 1  Duplicates: 0  Warnings: 0

SELECT * FROM SRC;

Z
A
B

XYZ
1876301/347
null

SELECT '1876301/347' + 1;

'1876301/347' + 1

1876302

SELECT '1876301/347' * 4;

'1876301/347' * 4

7505204

SELECT '1876301' * 4;

'1876301' * 4

7505204

SELECT Z IN ('XYZ') FROM SRC;

Z IN ('XYZ')

1

create table TGT
(
    C varchar(11) null
);
insert into TGT
(
    C
)
select 
    CASE
        WHEN if(Z in ('XYZ'), A, NULL) IS NULL AND B IS NULL THEN NULL
        WHEN if(Z in ('XYZ'), A, NULL) IS NULL AND B IS NOT NULL THEN B
        WHEN if(Z in ('XYZ'), A, NULL) IS NOT NULL AND B IS NULL THEN A
        WHEN if(Z in ('XYZ'), A, NULL) IS NOT NULL AND if(Z in ('XYZ'), A, NULL) = B THEN A 
        ELSE if(Z in ('XYZ'), A, NULL)
    END AS C
from 
    SRC 

Records: 1  Duplicates: 0  Warnings: 0

SELECT * FROM TGT;

C

1876301/347