drop default constraint in sql

DECLARE @ObjectName NVARCHAR(100)
DECLARE @table_name NVARCHAR(100)
SELECT @ObjectName = default_constraints.name, @table_name = tables.name
FROM
sys.all_columns
    INNER JOIN
sys.tables
    ON all_columns.object_id = tables.object_id
    INNER JOIN
sys.schemas
    ON tables.schema_id = schemas.schema_id
 INNER JOIN
sys.default_constraints
    ON all_columns.default_object_id = default_constraints.object_id
WHERE
schemas.name = 'dbo'
AND tables.name = 'Appeal'
AND all_columns.name = 'AppealGuid'
EXEC ('ALTER TABLE '+@table_name+' DROP CONSTRAINT ' + @ObjectName)