ORM - 'null' vs 'bla

Django 层次的null与blank的区别

blank、null默认都是False

null is for the DataBase column value

null=True sets NULL (versus NOT NULL) on the column in your DB. Blank values for Django field types such as DateTimeField or ForeignKey will be stored as NULL in the DB.

blank is for the field requirement in forms

blank=True determines whether the field will be required in forms. This includes the admin and your own custom forms. If blank=True then the field will not be required, whereas if it's False the field cannot be blank.

The combo of the two is so frequent because typically if you're going to allow a field to be blank in your form, you're going to also need your database to allow NULL values for that field. The exception is CharFields and TextFields, which in Django are never saved as NULL. Blank values are stored in the DB as an empty string ('').

A few examples:

models.DateTimeField(blank=True) # raises IntegrityError if blank
models.DateTimeField(null=True) # NULL allowed, but must be filled out in a form

Obviously those two options don't make logical sense to use (though, there might be a use case for null=True, blank=False if you want a field to always be required in forms, but optional when dealing with an object through something like the shell.)

models.CharField(blank=True) # No problem, blank is stored as ''
models.CharField(null=True) # NULL allowed, but will never be set as NULL

CHAR and TEXT types are never saved as NULL by Django, so null=True is unnecessary. However, you can manually set one of these fields to None to force set it as NULL. If you have a scenario where that might be necessary, you should still include null=True.

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db层次的null与空字符串的区别

如果打算搜索列值为NULL的列，不能使用expr = NULL测试。下述语句不返回任何行，这是因为，对于任何表达式，expr = NULL永远不为“真”： mysql> SELECT * FROM my_table WHERE phone = NULL; 。

要想查找NULL值，必须使用IS NULL测试。在下面的语句中，介绍了查找NULL电话号码和空电话号码的方式：

    mysql> SELECT * FROM my_table WHERE phone IS NULL; 
    mysql> SELECT * FROM my_table WHERE phone = '';