为什么SQL Server会继续创建DF约束？

Question

我正在尝试在SQL中创建升级和回退脚本.升级脚本添加如下列:

IF NOT EXISTS (SELECT * FROM sys.columns WHERE Name = N'ColumnName' 
    AND object_id = OBJECT_ID(N'[dbo].[TableName]'))

ALTER TABLE TableName
    ADD ColumnName bit NOT NULL DEFAULT(0) 

回退脚本会删除列,如下所示:

IF EXISTS (SELECT * FROM sys.columns WHERE Name = N'ColumnName' 
    AND object_id = OBJECT_ID(N'[dbo].[TableName]'))

ALTER TABLE TableName
    DROP COLUMN ColumnName

但是,回退脚本会抛出此错误:

Msg 5074, Level 16, State 1, Line 5
    The object 'DF__TableName__ColumnName__1BF3D5BD' is dependent on column 'ColumnName'.
Msg 4922, Level 16, State 9, Line 5
    ALTER TABLE DROP COLUMN ColumnName failed because one or more objects access this column.

我知道如何删除约束,但约束的名称每次都会更改(后缀更改).我要么需要SQL Server来停止创建这个随机命名的约束,要么我需要能够使用通配符字符删除我的脚本中的约束,因为名称会更改.

Answer 1

这是default由于DEFAULT(0)新添加的列中添加的约束.

您可以自己命名,因此它具有已知的固定名称,而不是依赖于自动生成名称.

ALTER TABLE TableName
    ADD ColumnName bit NOT NULL CONSTRAINT DF_Some_Fixed_Name DEFAULT(0) 

然后一起删除列和约束

ALTER TABLE dbo.TableName
DROP CONSTRAINT DF_Some_Fixed_Name, COLUMN ColumnName

Answer 2

运行这个：

declare @name as nvarchar(255);
SELECT @name = name FROM dbo.sysobjects 
WHERE name LIKE 'DF__XXX__YYY__%' and type = 'D'

IF @name IS NOT NULL BEGIN
    EXEC('ALTER TABLE XXX DROP CONSTRAINT ' + @name);
END