合并结果集
union----合并结果集
对合并后的结果集中的重复数据也会自动去重
select sName from students
union
select tName from Teachers;
如果不想去重 则使用 union all
select sName from students
union all
select tName from Teachers;
union----合并结果集
select sName from students
union
select tName from Teachers;
select sName from students
union all
select tName from Teachers;