MySQL 8.0

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CREATE TABLE mytable(
   A VARCHAR(12) NOT NULL PRIMARY KEY
  ,B VARCHAR(13) NOT NULL
);
INSERT INTO mytable(A,B) VALUES ('EMPLOYEE','34');
INSERT INTO mytable(A,B) VALUES ('OTHER','mystring');
SELECT A, 
JSON_OBJECT(
   'key', IF(A = 'EMPLOYEE', CAST(B AS UNSIGNED), B)
) as B
FROM mytable


A
B


EMPLOYEE
{"key": "34"}

OTHER
{"key": "mystring"}


select 
	a, 
	case a
		when 'EMPLOYEE' then json_object('key', cast(b as unsigned))
		else json_object('key', b)
	end b
from mytable


a
b


EMPLOYEE
{"key": 34}

OTHER
{"key": "mystring"}