空字符串与NULL(Empty string vs NULL)

空字符串与NULL(Empty string vs NULL)

我有一张表，其中一些行有一些空白cells 。 我试图使用IS NULL函数选择这样的行。 但查询选择0行。

select * from zzz_fkp_registration_female where fname is null; (0 row(s) affected)

现在我将查询更改为：

select * from zzz_fkp_registration_female where fname is null or fname ='';

我得到了理想的结果。

为什么IS NULL没有给出结果？ IS NULL和''之间有什么区别，因为单元格是空的。 请解释。

I've a table where some rows have some blank cells. I tried to select such rows using IS NULL function. But the query select 0 rows.

select * from zzz_fkp_registration_female where fname is null; (0 row(s) affected)

Now I changed my query to:

select * from zzz_fkp_registration_female where fname is null or fname ='';

I got the desired result.

Why is IS NULL not giving the result? What is the difference between IS NULL and '' since the cells are empty. Please explain.

最满意答案

他们之间的一些差异：

NULL可以分配给任何类型，而不是与日期/数字字段不兼容的空字符串。 NULL是一个UNKNOWN值，它没有值而不是空字符串，这是一个值，但是为空值。 据我所知， NULL不应该捕获内存而不是空字符串。 null = null将导致null而不是''=''将导致TRUE 。

Some differences between them:

NULL can be assigned to any type, as opposed to empty string which won't be compatible with date/numerical fields. NULL is an UNKNOWN value, it doesn't have a value as opposed to an empty string, which is a value, but empty one. As far as I'm aware of, NULL shouldn't capture memory as opposed to an empty string which does. null = null will result in null as opposed to ''='' which will result in TRUE.