MySQL 8.0

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CREATE TABLE employee(salary INT, name VARCHAR(100), age INT, designation VARCHAR(100)
     , designation_virtual VARCHAR(100) AS (COALESCE(designation, '^'))
);
ALTER TABLE employee ADD CONSTRAINT employee_constraint UNIQUE key (salary,name, age, designation_virtual) ;
INSERT INTO employee(salary, name, age, designation) VALUES(1000, 'xyz', 10, NULL);
INSERT INTO employee(salary, name, age, designation) VALUES(1000, 'xyz', 10, NULL);
Duplicate entry '1000-xyz-10-^' for key 'employee.employee_constraint'

SELECT *
FROM employee;


salary
name
age
designation
designation_virtual


1000
xyz
10
null
^