Calling Function In a Select

Question

I have a function which returns the name of a client when given
the id of the customer as an input parameter. I want to call the
function inside a select with the customer id as the input
parameter not the actual id and get the name something like
select id_cust as "id", test_function(id_cust) as "surname"
from customers
where -----
Please help because I have tried and I get an error message
ora-14551 Cannot Perform DML Inside Query

Answer 1

It would help if you would post your table structure and the code for creating your function.  It sounds like your function is doing something other than what you have described that you want it to do, like some unnecessary DML.  See if you can use the following example:
SQL> CREATE TABLE customers
  2    (id_cust   NUMBER,
  3     last_name VARCHAR2 (30))
  4  /
Table created.
SQL> INSERT INTO customers
  2  VALUES (1, 'TEST1')
  3  /
1 row created.
SQL> INSERT INTO customers
  2  VALUES (2, 'TEST2')
  3  /
1 row created.
SQL> CREATE OR REPLACE FUNCTION test_function
  2    (p_id_cust IN NUMBER)
  3    RETURN        VARCHAR2
  4  AS
  5    v_surname        customers.last_name%TYPE := NULL;
  6  BEGIN
  7    SELECT    last_name
  8    INTO      v_surname
  9    FROM      customers
10    WHERE     id_cust = p_id_cust;
11    RETURN    v_surname;
12  END test_function;
13  /
Function created.
SQL> COLUMN "surname" FORMAT A30
SQL> SELECT id_cust AS "id",
  2         test_function (id_cust) AS "surname"
  3  FROM   customers
  4  WHERE  1 = 1
  5  /
        id surname
         1 TEST1
         2 TEST2