SQL & PL/SQL

dbms_lob.trim not working ?
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I am using Oracle 11g/Sqldeveloper. I learned that dbms_lob.trim could be used to trim the CLOB items. I have the query

SELECT dbms_lob.trim (WHATMEMO,3990) WHATMEMO_V
FROM TEXT_TAB A;

ORA-00904: "DBMS_LOB"."TRIM": invalid identifier
00904. 00000 - "%s: invalid identifier"
*Cause:
*Action:
Error at Line: 17 Column: 4

What we could do to recognize DBMS_LOB.TRIM function in SQL?

What is happening here?

Thanks in helping.