问题
There appears to be at least two ways to add a default constraint using straight T-SQL. Am I correct that the only difference between the two below is that the second method specifically creates a name for the constraint, and the first method has one generated by SQL Server?
ALTER TABLE [Common].[PropertySetting] ADD DEFAULT ((1)) FOR [Active];
ALTER TABLE [Common].[PropertySetting] ADD CONSTRAINT [DF_PropertySetting_Active) DEFAULT ((1)) FOR [Active];
回答1:
Pretty much, yes for an ALTER TABLE
You can add a columnn with default in one step for CREATE or ALTER too.
ALTER TABLE foo ADD bar varchar(100) CONSTRAINT DF_Foo_Bar DEFAULT ('bicycle')
ALTER TABLE foo ADD bar varchar(100) DEFAULT ('bicycle')
As you noted, the system generates a name if one is not supplied. CONSTRAINT constraint_name is optional says MSDN. The same applies to any column or table CONSTRAINT
Edit If the column was already created, and you only want to add the constraint use:
ALTER TABLE TableName ADD CONSTRAINT DF_Foo_Bar DEFAULT 'bicycle' FOR FieldName;
来源：https://stackoverflow.com/questions/4307075/command-for-adding-a-default-constraint