SQL返回始终为false的条件的结果

Question

I'm confused on why the following SQL sentence returns one row:

#MySQL
select max(1) from dual where 0 = 1

#SQLite
select max(1) where 0 = 1 

I'm really confused about this result. Can anyone please shed some light over this behavior?

Thanks in advance!

Answer 1

An aggregate like max without a group by always returns one row. If the underlying rowset it empty, it will return a single row with null . Otherwise, it returns the maximum of the column you specified.

The where clause affects which rows the maximum is calculated for. It does not change the fact that the query has to return a maximum.

Answer 2

因为MAX是标量函数，所以它需要返回结果，而不是表结果查询。

Answer 3

It its returning the max value (or NULL in the case of no matching rows), so a single row returned seems logical

http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html#function_max