情景

原来表中的列定义成VARCHAR2类型，众所周知，VARCHAR2类型最大支持长度为4000。如果由于业务需要，想把此列转换为CLOB类型，在Oracle中直接通过ALTER语句转换是行不通的。下面根据具体事例讲解在Oracle数据库中如何把表列由VARCHAR2类型转换为CLOB类型。

示例准备

1. 新建两张张表TB_WITHOUT_DATA（此VARCHAR2列不包含数据）和TB_WITH_DATA（此Varchar2列包含数据)

create table TB_WITHOUT_DATA
(
  id NUMBER,
  name VARCHAR2(100),
  description VARCHAR2(2000)
);

create table TB_WITH_DATA
(
  id NUMBER,
  name VARCHAR2(100),
  description VARCHAR2(2000)
);

insert into TB_WITH_DATA VALUES (1,'David Louis','He is capable of resolving such kind of issue');
insert into TB_WITH_DATA VALUES (2,'German Noemi','She is very beatiful and charming');
insert into TB_WITH_DATA VALUES (3,'Oliver Queen','He is main actor in the Green Arrow');
insert into TB_WITH_DATA VALUES (4,'Mark Williams','He plays snooker very well');
insert into TB_WITH_DATA VALUES (5,'Sita Rama Raju Kata','I do not know this guy');
insert into TB_WITH_DATA VALUES (6,'Promethus','This is a very nice movie');
commit;

错误方法

ALTER TABLE TB_WITHOUT_DATA MODIFY description clob;

SQL Error: ORA-22858: invalid alteration of datatype
22858. 00000 - "invalid alteration of datatype"
*Cause: An attempt was made to modify the column type to object, REF,
nested table, VARRAY or LOB type.
*Action: Create a new column of the desired type and copy the current
column data to the new type using the appropriate type
constructor.

解决方法

方法一：对于此列没有数据的可通过以下方法修改-首先把该列改成Long类型，然后再改成clob类型

alter table TB_WITHOUT_DATA modify description long;--首先改成Long类型
alter table TB_WITHOUT_DATA modify description clob;--在Long类型的基础上改成clob类型

注：对于此列已经存在数据的，不能通过此方法，否则会报如下错误：

alter table TB_WITH_DATA modify description long;--更改包含数据的列

SQL Error: ORA-01439: column to be modified must be empty to change datatype
01439. 00000 -  "column to be modified must be empty to change datatype"

方法二：此方法适合此列包含数据和此列不包含数据两种情况

步骤一：把原来表中该列重命名

alter table TB_WITHOUT_DATA rename column description to description_bak;
alter table TB_WITH_DATA rename column description to description_bak;

步骤二：在表中增加该列，并指定改列类型为clob

alter table TB_WITHOUT_DATA add description clob;
alter table TB_WITH_DATA add description clob;

update TB_WITH_DATA set description=description_bak;
commit;

alter table TB_WITHOUT_DATA drop column description_bak;
alter table TB_WITH_DATA drop column description_bak;

1) 表结构验证

DESC TB_WITHOUT_DATA
Name        Null Type          
----------- ---- ------------- 
ID               NUMBER        
NAME             VARCHAR2(100) 
DESCRIPTION      CLOB 

DESC TB_WITH_DATA
Name        Null Type          
----------- ---- ------------- 
ID               NUMBER        
NAME             VARCHAR2(100) 
DESCRIPTION      CLOB  

select * from TB_WITH_DATA;

        ID NAME                       DESCRIPTION                                     
---------- -------------------------- ------------------------------------------------
         1 David Louis                He is capable of resolving such kind of issue   
         2 German Noemi               She is very beatiful and charming               
         3 Oliver Queen               He is main actor in the Green Arrow             
         4 Mark Williams              He plays snooker very well                      
         5 Sita Rama Raju Kata        I do not know this guy                          
         6 Promethus                  This is a very nice movie                       

 6 rows selected 

方法三：此方法适合此列包含数据和此列不包含数据两种情况

在讲解方法三之前，需要包表恢复到准备阶段，由于时间关系，直接通过drop然后re-create方法,脚本如下：