1 union all 联合查询

将满足多种独立查询条件的结果集整合到一起，不去除重复数据

 select * from accounts_user where age > 20 union all select * from accounts_user where age > 25;

2 union 联合查询

将满足多种独立查询条件的结果集整合到一起，去除重复数据

 select * from accounts_user where age > 20 union select * from accounts_user where age > 25;