Default Constraint
declare @key nvarchar(200);
select @key = c.name from sys.all_columns a
inner join sys.tables b on a.object_id = b.object_id
inner join sys.default_constraints c on a.default_object_id = c.object_id
where b.name='tablename' and a.name = 'columnname'
exec('alter table tablename drop constraint ' + @key)
// in case you wanted to remove the column
alter table tablename drop column columnname
Foreign Key Constraint
SELECT KCU.CONSTRAINT_NAME
FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE AS KCU
WHERE KCU.TABLE_NAME = 'Message' AND KCU.COLUMN_NAME = 'MaintenanceID'
