mysql 中NULL被如何处理.docx

binlog，redo log，undo log区别??mysql 中NULL被如何处理?1.enum字段与int字段有null情况2.null字段在timestamp，datetime中的情况3.select count在字段有null的情况4.not in 和not exists在有null值存在的情况举例：1.enum字段与int字段有null情况因为在MySQL中有三种状态：True、False、Unknown，任何NULL的比较操作都是Unknown状态，NULL跟任何比较操作返回结果都是unknown状态，而且所有查询条件都是unknown状态当做false处理。所以不会返回null这个组。mysql select default_group_type,count(*) from emailTemplateGroup where default_group_type takedownTool group by 1;+--------------------+----------+| default_group_type | count(*) |+--------------------+----------+| p2p??????????????? |????? 147 || Ad network???????? |????? 154 || searchEngine?????? |????? 154 || linkingsite??????? |????? 154 |+--------------------+----------+4 rows in set (0.00 sec)mysql select default_group_type,count(*) from emailTemplateGroup where default_group_type is null;+--------------------+----------+| default_group_type | count(*) |+--------------------+----------+| NULL?????????????? |????? 386 |+--------------------+----------+1 row in set (0.02 sec)mysql select p2ptakedownTool,Ad networktakedownTool,searchEngine takedownTool,linkingsitetakedownTool,takedownTooltakedownTool,null takedownTool;+-----------------------+------------------------------+---------------------------------+-------------------------------+--------------------------------+-----------------------+| p2ptakedownTool | Ad networktakedownTool | searchEngine takedownTool | linkingsitetakedownTool | takedownTooltakedownTool | null takedownTool |+-----------------------+------------------------------+---------------------------------+-------------------------------+--------------------------------+-----------------------+|???????????????????? 1 |??????????????????????????? 1 |?????????????????????????????? 1 |???????????????????????????? 1 |????????????????????????????? 0 |????????????????? NULL |+-----------------------+------------------------------+---------------------------------+-------------------------------+--------------------------------+-----------------------+m