mysql - 普通列后跟聚合函数但不在分组依据中

Question

我知道只有在包含它们的 Group By 之后才允许后跟聚合函数的普通列。
但是为什么下面的工作？

mysql> select payee,sum(amount) from checks;  
+---------+-------------+  
| payee   | sum(amount) |  
+---------+-------------+  
| Ma Bell |      893.76 |  
+---------+-------------+  
1 row in set (0.00 sec)  

Answer 1

This behavior is an "extension" to MySql :

MySQL extends the use of GROUP BY so that the select list can refer to nonaggregated columns not named in the GROUP BY clause.

然而，This behavior is actually a configurable setting in MySql :

ONLY_FULL_GROUP_BY

Do not permit queries for which the select list or (as of MySQL 5.1.10) HAVING list refers to nonaggregated columns that are not named in the GROUP BY clause.

最好尊重 group by 并添加所有非聚合列，特别是如果有一天您可能会迁移到具有 的服务器>ONLY_FULL_GROUP_BY 开启。