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Oracle中表列由VARCHAR2类型改成CLOB
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;错误信息:
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;
步骤三:对此列包含数据的需要包数据从步骤一重命名列中拷出(对于此列没有数据的此步骤省略)
update TB_WITH_DATA set description=description_bak; commit;
步骤四:删除步骤一中的备份列
alter table TB_WITHOUT_DATA drop column description_bak; alter table TB_WITH_DATA drop column description_bak;
步骤五:验证
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 CLOB2) 数据验证
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:
drop table TB_WITHOUT_DATA; drop table TB_WITH_DATA; 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;
步骤一:重命名两张表
rename TB_WITHOUT_DATA to TB_WITHOUT_DATA_BAK; rename TB_WITH_DATA to TB_WITH_DATA_BAK;
步骤二:创建两张新表(通过以下语句创建两张表)
create table TB_WITHOUT_DATA as select id, name, to_clob(description) description from TB_WITHOUT_DATA_BAK; create table TB_WITH_DATA as select id, name, to_clob(description) description from TB_WITH_DATA_BAK;
表结构与数据验证:
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; 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
步骤三:删除备份表:
DROP TABLE TB_WITHOUT_DATA_BAK; DROP TABLE TB_WITH_DATA_BAK;
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如果您们在尝试的过程中遇到什么问题或者我的代码有错误的地方,请给予指正,非常感谢!
联系方式:david.louis.tian@outlook.com
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