Mysql与Oracle group by的不同之处

在Oracle里，分组与聚合必须是成对出现的，”非分组的字段“必须做聚合操作，否则执行就会报错。而在Mysql里则完全不同。

mysql> select actor.actor_id,actor.first_name from actor join actor_info on (actor.actor_id=actor_info.actor_id) group by actor.first_name;+----------+-------------+| actor_id | first_name  |+----------+-------------+|       71 | ADAM        ||      165 | AL          ||      173 | ALAN        ||      125 | ALBERT      ||       29 | ALEC        ||       65 | ANGELA      ||       76 | ANGELINA    ||       49 | ANNE        ||       34 | AUDREY      ||      196 | BELA        ||       83 | BEN         ||        6 | BETTE       |

select actor.first_name,count(actor.actor_id) from actor join actor_info on (actor.actor_id=actor_info.actor_id) group by actor.first_name;+-------------+----------+| first_name  | count(*) |+-------------+----------+| ADAM        |        2 || AL          |        1 || ALAN        |        1 || ALBERT      |        2 || ALEC        |        1 || ANGELA      |        2 || ANGELINA    |        1 || ANNE        |        1 || AUDREY      |        2 || BELA        |        1 || BEN         |        2 |

其中，actor_id未作聚合，却能输出值，当某个first_name的分组记录数大于1的时候，mysql将会输出该分组第一条记录

。