为啥修改不了数据类型

为什么修改不了数据类型啊
我创建了一个表，但是ponum 定义的长度太短，想给他改长点，用以以下语句，可一直提示错误
ALTER TABLE purchase_order4 ALTER COLUMN ponum CHARACTER(10);

CREATE TABLE purchase_order4
(
  ponum character(5) NOT NULL,
  qty character(10),
  orderdate date,
  duedate date,
  receiveddate date,
  CONSTRAINT purchase_order4_pkey PRIMARY KEY (ponum)
)
------解决方案--------------------

--查询约束
sp_helpconstraint purchase_order4
--删除约束
ALTER TABLE purchase_order4 
drop constraint purchase_order4_pkey

然后你再修改下字段试试..
------解决方案--------------------

ALTER TABLE purchase_order4  add constraint purchase_order4_pkey PRIMARY KEY(ponum)

还有别忘了再加回约束..
------解决方案--------------------

CREATE TABLE  purchase_order4

(
  ponum character(5) NOT NULL,
  qty character(10),
  orderdate datetime,
  duedate datetime,
  receiveddate datetime,
  CONSTRAINT purchase_order4_pkey PRIMARY KEY (ponum)
) 
--改变前字符长度为5
/*
Column_name                                                                                                                      Type                                                                                                                             Computed                            Length      Prec  Scale Nullable                            TrimTrailingBlanks                  FixedLenNullInSource                Collation
-------------------------------------------------------- -------------------------------------------------------- ----------------------------------- ----------- ----- ----- ----------------------------------- ----------------------------------- ----------------------------------- --------------------------------------------------------