Oracle: Changing VARCHAR2 column to CLOB

Question

I have an encountered an issue where the data I was trying to store in my varchar2(4000) column was too big, so I wish to change the column to one more suitable for storing large amounts of textual data. Specifically, a serialized array.

1. Firstly, is CLOB the best data type for me to use for this purpose? Is there a more appropriate data type?

2. Secondly, when I try to alter the column using the usual snyntax:

    ALTER TABLE table MODIFY column CLOB 

   I get the following error: ORA-22858: invalid alteration of datatype

   What's the most straightforward way to alter this table without losing any data?

Answer 1

The most straightforward way, given that the operation of moving from a varchar column to a CLOB is disallowed, would be to create a new column and move the data from the old column to the new column:

ALTER TABLE some_table ADD (foo CLOB);
UPDATE some_table SET foo = old_column;
ALTER TABLE some_table DROP COLUMN old_column;
ALTER TABLE some_table RENAME COLUMN foo TO old_column;

Answer 2

The VARCHAR2 column cannot be directly converted to CLOB but it can be done in 2 steps:

• Convert column datatype from VARCHAR2 to LONG.
• Convert column datatype from LONG to CLOB.

ALTER TABLE table MODIFY column long;
ALTER TABLE table MODIFY column clob;

Answer 3

For Oracle 11g:

ALTER TABLE table MODIFY column long;
ALTER TABLE table MODIFY column clob;