What Is Union? | MySQL

Join is data retrieval operation that combines multiple query outputs of the same structure into a single output. By default the MySQL UNION removes all duplicate rows from the result set even if you don’t explicit using DISTINCT after the keyword UNION.
SELECT customerNumber id, contactLastname name
FROM customers
UNION
SELECT employeeNurrber id, firstname name
FROM employees
 id       name
103     Schmitt
112     King
114     Ferguson
119     Labrune
121     Bergulfsen