ORA-22858: invalid alteration of datatype, ORA-22858: invalid alteration of datatype

When we try to modify a datatype from varchar2(400) to Clob in a Oracle table, we’ll get the following error:

 

The following steps is the work-around to solve this issue:

 

ALTER TABLE SCHEMANAME.TABLENAME ADD NEWCOLUMN CLOB;

 

UPDATE SCHEMANAME.TABLENAME SET NEWCOLUMN = OLDCOLUMN;

 

ALTER TABLE SCHEMANAME.TABLENAME drop COLUMN OLDCOLUMN;

 

ALTER TABLE SCHEMANAME.TABLENAME rename COLUMN NEWCOLUMN TO OLDCOLUMN;