Question

I knew that normal columns followed by aggregate functions are allowed only if a Group By including them follows.
But then why is the following working?

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.

However, 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.

It is best to respect the group by and add all non-aggregated columns, especially if there’s the possibility that you might someday migrate to a server that has ONLY_FULL_GROUP_BY turned on.