ERROR at line 1: ORA-00911: invalid character ORA-06512: at line 17 - sql

Question

I am not a frequent user of database & once in a while i need to create/run/execute a few PL/SQL blocks. I have a similar situation right now, where I have the below block which while executing as SYS as SYSDBA in oracle database user throws the error :-
DECLARE
*
ERROR at line 1:
ORA-00911: invalid character
ORA-06512: at line 17
The PL/SQL Block is as below :-
DECLARE
TYPE RefCurTyp IS REF CURSOR;
alter_tbl VARCHAR2(200);
a_null CHAR(1);
tbl VARCHAR2(200);
clmn VARCHAR2(200);
dtyp VARCHAR2(200) ;
dlth VARCHAR2(200);
c RefCurTyp;
BEGIN
open c for 'select utc.table_name, utc.column_name, utc.data_type, utc.data_length FROM user_tab_columns utc, user_tables ut
WHERE utc.data_type = ''VARCHAR2'' AND utc.char_used =''B'' AND ut.table_name = utc.table_name';
LOOP
dbms_output.put_line(clmn);
FETCH c INTO tbl, clmn, dtyp, dlth;
EXIT WHEN c%NOTFOUND;
EXECUTE IMMEDIATE
'alter table '||tbl||' modify ('||clmn||' '||dtyp||'('||dlth||' CHAR))';
END LOOP;
CLOSE c;
END;
Even after pounding my head on it for 3 days i am unable to figure out the issue with this. Any input is appreciated.
While executing the same code via TOAD i get :-

Answer 1

You can use dbms_output to display the dynamic statement you are executing. To make sure you see and execute the same thing it's simpler to put the statement into a variable (you have one you aren't using). If you change the cursor type you don't need the local variables though, you can construct the statement as part of the cursor query, and then refer to it multiple times; you also won't have to escape your single quotes:
set serveroutput on
BEGIN
FOR r IN (
SELECT 'alter table ' || utc.table_name || ' modify (' || utc.column_name || ' '
|| utc.data_type || '(' || utc.data_length || ' CHAR))' as alter_stmt
FROM user_tab_columns utc
JOIN user_tables ut ON ut.table_name = utc.table_name
WHERE utc.data_type = 'VARCHAR2' AND utc.char_used ='B'
)
LOOP
dbms_output.put_line(r.alter_stmt);
execute immediate r.alter_stmt;
END LOOP;
END;
/
I suspect you have a table or column name that contains an invalid character and was created with a quoted identifier. That will probably be obvious from the output you see immediately before it fails. You can easily add double quotes to all of the identifiers by concatenating them as part of the statement generation:
BEGIN
FOR r IN (
SELECT 'alter table "' || utc.table_name || '" modify ("' || utc.column_name || '" '
|| utc.data_type || '(' || utc.data_length || ' CHAR))' as alter_stmt
FROM user_tab_columns utc
JOIN user_tables ut ON ut.table_name = utc.table_name
WHERE utc.data_type = 'VARCHAR2' AND utc.char_used ='B'
)
LOOP
dbms_output.put_line(r.alter_stmt);
execute immediate r.alter_stmt;
END LOOP;
END;
/
You said you're running this while connected as SYS, and you're looking at user_tables, so you are altering tables owned by SYS - which seems like a very bad idea. Even if you don't intend to modify built-in data dictionary tables, this will do so, and that would imply you've been creating your own objects in the SYS schema - which is generally considered a very bad idea. You should create a separate user account and only create and modify objects in that schema.
In my 11g instance SYS has a table that generates output from my first query as:
alter table _default_auditing_options_ modify (A VARCHAR2(1 CHAR));
... which would get ORA-00911 because of the underscores. If the identifiers are quoted then it would work:
alter table "_default_auditing_options_" modify ("A" VARCHAR2(1 CHAR));
... but, once again, you should not be altering built-in tables.