【IT168 评论】SQL Server删除操作是最常用的操作之一，下面就为您介绍如何在有默认约束情况下实现SQL Server删除数据，希望对您能有所启迪。
在SQL Server 中，如果给表的一个字段设置了默认值，就会在系统表sysobjects中生成一个默认约束。
如果想删除这个设置了默认值的字段(假设此字段名column1)，执行“ALTER TABLE table1 DROP COLUMN column1”时就会报错：
The object .DF__xxxxxxxxxxx. is dependent on column .column1..
ALTER TABLE DROP COLUMN column1failed because one or more objects access this column.
所以SQL Server删除此字段时需要先将系统表中的对应默认约束删除， 可以使用下面的脚本进行删除：
-- this script drops the default constraint which is generated by the setting of default value.
-- SQL Server删除约束
以下是代码片段：
DECLARE @tablename VARCHAR(100), @columnname VARCHAR(100), @tab VARCHAR(100)
SET @tablename='表名'
SET @columnname='字段名'
declare @defname varchar(100)
declare @cmd varchar(100)
select @defname = name
FROM sysobjects so
JOIN sysconstraints sc
ON so.id = sc.constid
WHERE object_name(so.parent_obj) = @tablename
AND so.xtype = 'D'
AND sc.colid =
(SELECT colid FROM syscolumns
WHERE id = object_id(@tablename) AND
name = @columnname)
select @cmd='alter table '+ @tablename+ ' drop constraint '+ @defname
if @cmd is null print 'No default constraint to drop'
exec (@cmd)
SET @tablename='表名'
SET @columnname='字段名'
declare @defname varchar(100)
declare @cmd varchar(100)
select @defname = name
FROM sysobjects so
JOIN sysconstraints sc
ON so.id = sc.constid
WHERE object_name(so.parent_obj) = @tablename
AND so.xtype = 'D'
AND sc.colid =
(SELECT colid FROM syscolumns
WHERE id = object_id(@tablename) AND
name = @columnname)
select @cmd='alter table '+ @tablename+ ' drop constraint '+ @defname
if @cmd is null print 'No default constraint to drop'
exec (@cmd)
-- 删除约束结束
在删除对应的默认约束后，执行：
ALTER TABLE table1 DROP COLUMN column1
即可删除字段。
