Question

I have a table with three NCLOB columns. For each NCLOB I want a count of how many are not ‘TC’ or ‘NC’. The case when … end approach works for NVARCHAR2 columns but not for NCLOB. How can I test the values of an NCLOB in the projection list?

Oracle Database 11g Release 11.1.0.6.0

This minimal example demonstrates the root issue.

create table t (
  alien_body_part nclob
);

insert into t(alien_body_part) values(null);
insert into t(alien_body_part) values('TC');
insert into t(alien_body_part) values('NC');
insert into t(alien_body_part) values('Extended Mandible');

select case when alien_body_part in ('TC', 'NC') then 0 else 1 end from t
                 *
ERROR at line 1:
ORA-00932: inconsistent datatypes: expected - got NCLOB

Answer 1

Only compare the first characters:

SQL> SELECT dbms_lob.substr(alien_body_part, 4000, 1) body_part,
  2         CASE
  3            WHEN dbms_lob.substr(alien_body_part, 4000, 1)
  4                 IN ('TC', 'NC') THEN
  5             0
  6            ELSE
  7             1
  8         END is_nc_or_tc
  9    FROM t;

BODY_PART              IS_NC_OR_TC
---------------------- -----------
                                 1
TC                               0
NC                               0
Extended Mandible                1

In this case since one side of the comparison is only 2 characters long, comparing the first 3 characters would be sufficient (as a NCLOB will be equal to ‘TC’ only if it is 2 characters long AND those characters equal ‘TC’ obviously).

Also neither the CASE nor the IN is the cause of the error here (you can’t compare directly a CLOB/NCLOB in SQL), consider:

SQL> select * from t where alien_body_part = 'TC';

select * from t where alien_body_part = 'TC'

ORA-00932: inconsistent datatypes: expected - got NCLOB