SQL & PL/SQL

hi, experts,

I have a clob column in the database.
I use this sql to know maximum length on the clob field.


select max(dbms_lob.getlength(clob_filed)) from table;

then use this sql to read the clob field


select dbms_lob.substr(clob_filed, 1000000, 1) from exportlog;

I run this sql in sql developer and sqlplus in command prompt.

both always return NULL.
how to read? I need to export them.