SQL & PL/SQL :: How To Select A Column Dynamically Inside A Query

I have a problem executing a function.

There are two tables,

1. Table with column names of a second table mapped to certain variables.

ex: Table1
col1, var1
col2, var2
col3, var3
--------
--------

2. Table with the values for columns as given in table 1 ex:

col1, col2, col3, col4, col5,
a , aa , 1 , x1 , p
b , ab , 2 , x2 , q
c , ac , 3 , x3 , r

I have to select values from table2.col1 do some processing and calculate values and store it in a table then do the same thing with col2 and so. This needs to be done for all the columns that appear in table1.For example in table 1 i have only three columns mentioned thus i have to process col1, col2 and col3 from table2. col4 and col5 will not be processed since they do not appear in the first table.

The problem is i have hundred columns in table 2 and the user can add up to hundred columns in table 1 as and when it is required.

I have created a cursor to first select column name from table 1 where variable is not null.For each value in cursor i put it in a local variable.

Second step is to select values from table2 where instead of column name i am using the local variable.But the problem is instead of choosing values from col1 the query returns the value as col1 (the value of local variable)