为什么选择'abc'+ null = null [英] Why select 'abc' + null =null

问题描述

你好朋友，

我想知道为什么选择``abc''+ null会导致null.

谢谢，
Jagjot

Hello Friends,

I would like to know that why select ''abc'' + null results in null.

Thanks,
Jagjot

推荐答案

这是因为 CONCAT_NULL_YIELDS_NULL [ ^ ]一个儿子.您可以在查询中将其关闭，结果将受到影响.
虽然不建议您将其关闭，但应谨慎使用.

This is because CONCAT_NULL_YIELDS_NULL [^] is by default set as ON. YOu can turn it off in your query and the result will be affected.
Although, turning it off is not recommended and should be used carefully.

您可以将其关闭.

SET CONCAT_NULL_YIELDS_NULL {开|关闭}

http://msdn.microsoft.com/en-us/library/ms176056.aspx [ ^ ]

You can switch it off.

SET CONCAT_NULL_YIELDS_NULL { ON | OFF }

http://msdn.microsoft.com/en-us/library/ms176056.aspx[^]

当连接null值时，concat null产生sp_dboption的null设置，或者SET CONCAT_NULL_YIELDS_NULL确定一个表达式为NULL时的行为.concat null产生null或SET CONCAT_NULL_YIELDS_NULL启用时，字符串" NULL返回NULL.如果concat null产生null或禁用SET CONCAT_NULL_YIELDS_NULL，则结果为字符串"."

这是从 http://msdn.microsoft.com/en-us/library/aa276862(v = sql.80).aspx [ ^ ].

"When you concatenate null values, either the concat null yields null setting of sp_dboption or SET CONCAT_NULL_YIELDS_NULL determines the behavior when one expression is NULL. With either concat null yields null or SET CONCAT_NULL_YIELDS_NULL enabled ON, ''string'' + NULL returns NULL. If either concat null yields null or SET CONCAT_NULL_YIELDS_NULL is disabled, the result is ''string''."

This was from http://msdn.microsoft.com/en-us/library/aa276862(v=sql.80).aspx[^].

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