ORA-00904 and ORA-06512 when I try to MAX all columns - oracle

Question

create or replace PROCEDURE COMPARAISON_TEST as
begin
declare
cursor c_tab is
select table_name, column_name , data_type
from all_tab_columns;
v_sql VARCHAR2 (32000);
begin
FOR r_tab in c_tab LOOP
v_sql := 'SELECT ' ||
r_tab.table_name || ' TABLE_NAME, ' ||
r_tab.column_name || ' COLUMN_NAME, ' ||
'(SELECT MAX(' || r_tab.column_name || ') FROM ' || r_tab.table_name || ') VALUE ' ||
'FROM DUAL ';
execute immediate v_sql;
end LOOP;
end;
end COMPARAISON_TEST;
ERRORS
ORA-00904: "DUAL" : invalid identifier
ORA-06512: à "ODS.COMPARAISON_TEST", line 20
ORA-06512: à "ODS.COMPARAISON_TEST", line 20
ORA-06512: à line 2

Answer 1

Problem is that you're doing it in the dark.
Always, before actually executing dynamic SQL, display it on the screen so that you'd see the string to be executed.
This is what you did (I added WHERE clause to a cursor, to shorten the output):
SQL> declare
2
3 cursor c_tab is
4 select table_name, column_name , data_type
5 from all_tab_columns
6 where owner = 'SCOTT' and table_name = 'DEPT';
7
8 v_sql VARCHAR2 (32000);
9
10 begin
11
12 FOR r_tab in c_tab LOOP
13 v_sql := 'SELECT ' ||
14 r_tab.table_name || ' TABLE_NAME, ' ||
15 r_tab.column_name || ' COLUMN_NAME, ' ||
16 '(SELECT MAX(' || r_tab.column_name || ') FROM ' || r_tab.table_name || ') VALUE ' ||
17 'FROM DUAL ';
18 dbms_output.put_line(v_sql);
19 --execute immediate v_sql;
20 end LOOP;
21 end;
22 /
SELECT DEPT TABLE_NAME, DEPTNO COLUMN_NAME, (SELECT MAX(DEPTNO) FROM DEPT) VALUE
FROM DUAL
SELECT DEPT TABLE_NAME, DNAME COLUMN_NAME, (SELECT MAX(DNAME) FROM DEPT) VALUE
FROM DUAL
SELECT DEPT TABLE_NAME, LOC COLUMN_NAME, (SELECT MAX(LOC) FROM DEPT) VALUE FROM
DUAL
PL/SQL procedure successfully completed.
SQL>
See all those invalid strings? They can't be executed. In order to make them valid, include table and column names into single quotes, e.g.
SELECT 'DEPT' TABLE_NAME, 'DEPTNO' COLUMN_NAME, (SELECT MAX(DEPTNO) FROM DEPT) VALUE
FROM DUAL;
This is what you might have wanted:
SQL> set serveroutput on
SQL>
SQL> declare
2 cursor c_tab is
3 select table_name, column_name , data_type
4 from user_tab_columns
5 where table_name = 'DEPT';
6 v_sql VARCHAR2 (32000);
7 v_val varchar2(200);
8 begin
9 FOR r_tab in c_tab LOOP
10 v_sql := 'SELECT MAX(' || r_tab.column_name || ') FROM ' ||
11 r_tab.table_name;
12 execute immediate v_sql into v_val;
13 dbms_output.put_line(r_tab.table_name||'.'||r_tab.column_name||': '|| v_val);
14 end LOOP;
15 end;
16 /
DEPT.DEPTNO: 40
DEPT.DNAME: SALES
DEPT.LOC: NEW YORK
PL/SQL procedure successfully completed.
SQL>
If you want to store result into a table, then you'd do something like this:
SQL> create table maxes
2 (table_name varchar2(30),
3 column_name varchar2(30),
4 max_value varchar2(30)
5 );
Table created.
SQL> declare
2 cursor c_tab is
3 select table_name, column_name , data_type
4 from user_tab_columns
5 where table_name = 'DEPT';
6 v_sql VARCHAR2 (32000);
7 v_val varchar2(200);
8 begin
9 FOR r_tab in c_tab LOOP
10 v_sql := 'SELECT MAX(' || r_tab.column_name || ') FROM ' ||
11 r_tab.table_name;
12 execute immediate v_sql into v_val;
13 insert into maxes (table_name, column_name, max_value)
14 values
15 (r_tab.table_name, r_tab.column_name, v_val);
16 end LOOP;
17 end;
18 /
PL/SQL procedure successfully completed.
SQL> select * From maxes;
TABLE_NAME COLUMN_NAME MAX_VALUE
--------------- --------------- ---------------
DEPT DEPTNO 40
DEPT DNAME SALES
DEPT LOC NEW YORK
SQL>

Answer 2

You should double single quotes that you want to appear within the string.
v_sql := 'SELECT ''' ||
r_tab.table_name || ''' TABLE_NAME, ''' ||
r_tab.column_name || ''' COLUMN_NAME, ' ||
'(SELECT MAX(' || r_tab.column_name || ') FROM ' || r_tab.table_name || ') VALUE ' ||
'FROM DUAL ';