impala、HIVE、Mysql排序字段中含有null

如果升序排序的字段中含有null，null被排在什么位置，要不然最前，要不然最后。

我测试了几个数据库

mysql和hive中null都是在最前

impala中null是在最后

测试代码

SELECT *

FROM

(

    SELECT 1 as field1,'zhang' as field2 

    union

    SELECT 2 as field1,'wang' as field2

    union

    SELECT 3 as field1,'li' as field2

    union

    SELECT 4 as field1,'zhao' as field2

    union

    SELECT null as field1,'wu' as field2

) a

ORDER BY field1