14.组合查询

关于规则中列顺序，前后必须一致。否则会出现如下情况：
use test;
select cust_name ,cust_email,cust_contact from customers where cust_state in (‘IL’,’IN’,’MI’) union all
select cust_name,cust_contact,cust_email from customers where cust_name = ‘Fun4All’

返回结果为：

列的顺序一致，才会返回正确结果
use test;
select cust_name ,cust_email,cust_contact from customers where cust_state in (‘IL’,’IN’,’MI’) union all
select cust_name,cust_email,cust_contact from customers where cust_name = ‘Fun4All’

返回结果：