SELECT * FROM employees 
WHERE email LIKE '%a%' OR department_id>90;

#等价

SELECT * FROM employees 
WHERE email LIKE '%a%' 
UNION 
SELECT * FROM employees 
WHERE department_id>90;

应用场景：要查询的结果来自于多个表，且多个表没有直接的连接关系，单查询的信息是 一致时

特点：列数一致，字段和顺序一致，