Provide a one-off default now (will be set on all existing rows with a null value for this column)

To set a one-off default value for all existing rows with a null value for a specific column, you can use an SQL UPDATE statement. Here's an example:

UPDATE your_table
SET your_column = 'default_value'
WHERE your_column IS NULL;

In the above statement, replace `your_table` with the name of your table and `your_column` with the name of the column you want to update. Also, replace `'default_value'` with the desired default value you want to set.

Executing this SQL statement will update all rows where the specified column is null and set it to the provided default value.

相关问题

You are trying to add a non-nullable field 'Number' to author without a default; we can't do that (the database needs something to populate existing rows). Please select a fix: 1) Provide a one-off default now (will be set on all existing rows with a null value for this column) 2) Quit, and let me add a default in models.py

It seems like you are facing a database migration issue in Django. In this case, you have added a non-nullable field 'Number' to the 'author' model without a default value, and there are existing rows with null values for this column.

To solve this issue, you have two options:

1. Provide a one-off default now: You can provide a default value for the 'Number' field that will be set on all existing rows with a null value for this column. You can do this by running the following command in your Django project directory:

   python manage.py makemigrations --empty yourappname

This will create an empty migration file for your app. Then add the following code to the `operations` list in the `migrations.RunPython` method:

   from django.db import migrations

   def set_default_value(apps, schema_editor):
       Author = apps.get_model('yourappname', 'Author')
       Author.objects.filter(Number=None).update(Number=0)

   class Migration(migrations.Migration):

       dependencies = [
           ('yourappname', 'previous_migration'),
       ]

       operations = [
           migrations.RunPython(set_default_value),
       ]

Replace 'yourappname' and 'previous_migration' with your actual app name and the previous migration name respectively. Then run the following command:

   python manage.py migrate

This will apply the migration and set the default value for the 'Number' field on all existing rows with null values.

2. Quit, and let me add a default in models.py: If you don't want to provide a default value for the 'Number' field, you can quit the migration process and add a default value in the 'author' model in your models.py file. You can do this by adding the following code to your model:

   class Author(models.Model):
       name = models.CharField(max_length=100)
       # Add default value for Number field
       Number = models.IntegerField(default=0)

Then run the following command to create a new migration:

   python manage.py makemigrations

This will create a new migration file with the default value for the 'Number' field. Then run the following command to apply the migration:

   python manage.py migrate

This will apply the migration and add the default value for the 'Number' field in the 'author' model.

FutureWarning: Value based partial slicing on non-monotonic DatetimeIndexes with non-existing keys is deprecated and will raise a KeyError in a future Version.如何解决

这个警告是因为在非单调的DatetimeIndex上使用基于值的部分切片，而且切片中包含索引中不存在的键。这会在将来版本中引发 KeyError 异常。

要解决这个问题，可以使用基于位置的切片（使用 iloc）或者确保 DatetimeIndex 是单调的。如果你想使用基于值的切片，可以使用 reindex() 方法来确保索引中包含切片中的所有键：

df = df.reindex(index=slice(start_date, end_date))

其中 `start_date` 和 `end_date` 是你想要的时间范围。这将重新索引数据帧以包含所有日期，并在不存在的日期上填充 NaN 值。

另外，如果你确定索引已经单调，可以使用 sort_index() 方法来排序索引：

df = df.sort_index()

这将确保索引单调，并且可以消除警告。