How to Refer to a Column by ID or Index Number - sql

Question

In Oracle PL/SQL, I have run a query and am trying to read through each column for each row one by one so I can concatenate them together with a delimiter (hard format requirement). The script is used on multiple tables of varying sizes, so the number of columns is not known in advance. I used
SELECT COUNT(column_name) INTO NumColumns FROM all_tabs_cols
WHERE table_name = Table_Array(i);
where Table_Array has already been defined. This is in the middle of a for loop and has successfully gotten me a total number of columns. Table_Cursor is a SELECT * statement. After this I am trying to do something like
FOR j IN 0..NumColumns-1 LOOP
FETCH TABLE_CURSOR.column(j) INTO DataValue;
DBMS_OUTPUT.PUT(DataValue || '/');
END LOOP
The above is pseudo code. It illustrates the concept I am after. I do not know PL/SQL well enough to know how to get a value like this out of a row. I am also worried about accidentally advancing the cursor while doing this. How can I accomplish this task?

Answer 1

You must use some form of dynamic SQL. Here is a quick example:
It builds the SQL statement that will select the '/' separated columns from the table you want. Then it uses dynamic SQL to run that SQL statement.
DECLARE
p_table_name VARCHAR2(30) := 'DBA_OBJECTS';
l_sql VARCHAR2(32000);
TYPE varchar2tab IS TABLE OF VARCHAR2(32000);
l_array varchar2tab;
BEGIN
SELECT 'SELECT ' || listagg(column_name,' ||''/''||') within group ( order by column_id ) || ' FROM ' || owner || '.' || table_name || ' WHERE ROWNUM <= 100'
INTO l_sql
FROM dba_tab_columns
where table_Name = 'DBA_OBJECTS'
group by owner, table_Name;
EXECUTE IMMEDIATE l_sql BULK COLLECT INTO l_array;
FOR i in l_array.first .. l_array.last LOOP
dbms_output.put_line(l_array(i));
END LOOP;
END;

Answer 2

This is how your code should look:
SELECT F1 || ', ' || F2 || ', ' || ... || ', ' || FN
FROM TABLE
NO LOOPS
Here is how you can generate code that does not use loops.
Note, if you want you can take out the where statement and generate the code for the whole database.
Test with just one table first.
SELECT 'SELECT '|| LISTAGG(COLUMN_NAME, ' || '', '' || ') || ' FROM '||TABLE_NAME as sql_stm
FROM ALL_TAB_COLUMNS
WHERE TABLE_NAME='tablename'
GROUP BY TABLE_NAME;