How do you drop a column with an unamed default value constraint and unknown indexes

Question

What is a method of determining if there is a default constraint on a column and its name, and the names of any indexes in order to drop them before dropping the column?

Answer 1

The following utility procs will accomplish the task.

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

It's then easy to invoke as follows:

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

I haven't looked at foreign key constraints as we don't use them, but perhaps they could be included too.