To differentiate between employees and customers in a union, you can add a column as shown in the following query:
a) SELECT name, email, 'employee' AS type FROM employees
UNION
SELECT name, email, 'customer' AS type FROM customers;
b) SELECT name, email, 'employee' AS type FROM employees
UNION ALL
SELECT name, email, 'customer' AS type FROM customers;
c) SELECT name, email, 'employee' AS type FROM employees
INTERSECT
SELECT name, email, 'customer' AS type FROM customers;
d) SELECT name, email FROM employees
UNION
SELECT name, email FROM customers
