كيف يمكنك إسقاط عمود مع قيود القيمة الافتراضية غير المطلوبة ومؤشرات غير معروفة

Question

ما هي طريقة تحديد ما إذا كان هناك عائق افتراضي على عمود واسمه، وأسماء أي فهارس من أجل إسقاطها قبل إسقاط العمود؟

Answer 1

المنفعة التالية Procs ستحقق المهمة.

if (exists (select * from [dbo].sysobjects where (id = object_id(N'[dbo]._spDropDefaultValueConstraint')) and (xtype = 'P')))
 drop procedure [dbo]._spDropDefaultValueConstraint
GO

create procedure [dbo]._spDropDefaultValueConstraint
 @TableName varchar(256),
 @ColumnName varchar(256)
as
/* This proc will drop the default value constraint on
 a column even when you don't know what its name is.
*/
declare @ConstraintName varchar(256)
set @ConstraintName = (
 select 
  dobj.name
 from sys.columns col 
  left outer join sys.objects dobj 
   on dobj.object_id = col.default_object_id and dobj.type = 'D' 
 where col.object_id = object_id('[dbo].'+@TableName) 
 and dobj.name is not null
 and col.name = @ColumnName)

if(isnull(@ConstraintName, '') <> '')
 exec('alter table [dbo].['+@TableName+'] drop constraint ['+@ConstraintName+']')

GO

-------------------------------------------------------------------------------------------

if (exists (select * from [dbo].sysobjects where (id = object_id(N'[dbo]._spDropIndexesForColumn')) and (xtype = 'P')))
 drop procedure [dbo]._spDropIndexesForColumn
GO

create procedure [dbo]._spDropIndexesForColumn
 @TableName varchar(256),
 @ColumnName varchar(256)
as
/* This proc will drop all indexes on a column, both indexes
 and unique constraints as well as multi-part indexes that reference it.
*/
declare @IndexName varchar(256)
declare @IsPrimaryKey bit
declare @IsUniqueConstraint bit

declare crIndexes cursor for
 select
  ind.name, ind.is_primary_key, ind.is_unique_constraint
 from 
  sys.indexes ind
  inner join sys.index_columns ic on ind.object_id = ic.object_id and ind.index_id = ic.index_id
  inner join sys.columns col on ic.object_id = col.object_id and ic.column_id = col.column_id 
  inner join sys.tables t on ind.object_id = t.object_id
 where
  t.name = @TableName and
  col.name = @ColumnName      
open crIndexes
fetch next from crIndexes into @IndexName, @IsPrimaryKey, @IsUniqueConstraint
while(@@fetch_status = 0) begin
 if(@IsPrimaryKey = 1) or (@IsUniqueConstraint = 1)
  exec('alter table [dbo].['+@TableName+'] drop constraint ['+@IndexName+']')
 else
  exec('drop index [dbo].['+@TableName+'].['+@IndexName+']')
 fetch next from crIndexes into @IndexName, @IsPrimaryKey, @IsUniqueConstraint
end
close crIndexes
deallocate crIndexes

GO

-------------------------------------------------------------------------------------------

if (exists (select * from [dbo].sysobjects where (id = object_id(N'[dbo]._spDropColumn')) and (xtype = 'P')))
 drop procedure [dbo]._spDropColumn
GO

create procedure [dbo]._spDropColumn
 @TableName varchar(256),
 @ColumnName varchar(256)
as
/* This proc will drop a column (first dropping the default value
 constraint and any indexes if they exist) if it exists.
*/
if (exists (select * from [dbo].sysobjects where (id = object_id('[dbo].'+@TableName)) and (xtype = 'U'))) and
   (exists (select * from [dbo].syscolumns where (id = object_id('[dbo].'+@TableName)) and (name = @ColumnName))) begin
 exec [dbo]._spDropIndexesForColumn @TableName, @ColumnName
 exec [dbo]._spDropDefaultValueConstraint @TableName, @ColumnName
 exec('alter table [dbo].['+@TableName+'] drop column ['+@ColumnName+']')
end
GO

من السهل إذن استدعاء كما يلي:

exec [dbo]._spDropColumn 'TableName', 'ColumnName'

لم أتطلع إلى قيود المفاتيح الأجنبية لأننا لا نستخدمها، ولكن ربما يمكن إدراجها أيضا.