NULL

NULL值是一种对列的特殊约束,我们创建一个新列时,如果没有明确的使用关键字not

null声明该数据列,Mysql会默认的为我们添加上NULL约束.

Mysql

的默认推荐设置.(即允许字段使用NULL值).而这一陋习很容易在使用NULL的场景中得出不确定的查询结果以及引起数据库性能的下降.

Null

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NULL

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row

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NULL

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NULL

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row

任何有返回值的表达式中有NULL参与时,都会得到另外一个NULL值.

(rootlocalhost

null),ifnull(concat(abc,null),First

null);

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ifnull(null,First

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First

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row

NULLNULL的结果为false,但是在我们使用distinct,group

by,order

中支持在含有NULL值的列上使用索引,但是Oracle不支持.这就是我们平时所说的如果列上含有NULL那么将会使索引失效.

严格来说,这句话对与

***************************Table:

sbtest1

mysql3306.sock)[sysbench]insert

into

mysql3306.sock)[sysbench]explain

select

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select_type

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SIMPLE

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row

mysql3306.sock)[sysbench]explain

select

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select_type

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SIMPLE

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row

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select_type

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SIMPLE

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row

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select_type

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SIMPLE

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row

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select_type

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SIMPLE

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row

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select_type

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SIMPLE

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row

列中使用NULL值容易引发不受控制的事情发生,有时候还会严重托慢系统的性能.

例如:

count(),max(),min(),结果并不符合我们的期望值.

null

NULL值并是占用原有的字段空间存储,而是额外申请一个字节去标注,这个字段添加了NULL约束.(就像额外的标志位一样)

these