转自：https://blog.csdn.net/e_wsq/article/details/7561209
步骤：
1.建立一个临时varchar2字段用来保存数据
2.将clob的内容截取后更新到varchar2字段中
update table_wonder set tempcolumn = dbms_lob.substr(clobcolumn,4000);
3.drop掉clob字段
4.将临时varchar2字段改名
alter table table_wonder rename column tempcolumn to column;
另外rename语句在9.2以上版本支持，在pl/sql developer 7.0版本的command window中不支持，需要使用sqlplus。
列出lob类型不支持的sql语句
| 不支持的语句 | 不支持的用法例子 |
|---|---|
| SELECT DISTINCT |
SELECT DISTINCT clobCol from... |
| SELECT clause
ORDER BY |
SELECT... ORDER BY clobCol |
| SELECT clause
GROUP BY |
SELECT avg(num) FROM... GROUP BY clobCol |
| UNION, INTERSECT, MINUS
(Note that UNION ALL works for LOBs.) |
SELECT clobCol1 from tab1 UNION SELECT clobCol2 from tab2; |
| Join queries |
SELECT... FROM... WHERE tab1.clobCol = tab2.clobCol |
| Index columns |
CREATE INDEX clobIndx ON tab(clobCol)... |