How does one evaluate an expression stored as a VARCHAR 2 in a table?

Question

I am attempting to write a single SQL statement that will select three columns of data from a table and based on the value of a string in one column perform a calculation using the other two columns. Here is an example. TableX contains three columns. For simplicity sake let's say Col1 and Col2 are Number(5) and Col3 is a VARCHAR2(25).
TableX
Col1 Col2 Col3
5 7 Col1+Col2
10 3 Col1*Col2
20 5 Col1/Col2
I would like to write an SQL statement like 'select Col1, Col2, evaluate(Col3) as "Eval" from TableX' and get the following result set back.
Col1 Col2 Eval
5 7 12
10 3 30
20 5 4
Is there an Oracle provided function like the hypothetical evaluate() provided above that will do this?
One potential solution would be to write a stored procedure that accepts the three columns as parameters and then performs an execute immediate and returns the result. However, I would like a more dynamic utility so I don't have to be resticted to a preset number of columns (3 in this example).
Any ideas would be greatly appreciated.
Thanks,
Matt

Answer 1

Matthew:
There is no evaluate function in Oracle, and as far as I know, there is no way to do what you want solely in sql. However, this function should do the trick.
SQL> CREATE OR REPLACE FUNCTION my_eval (
  2     p_tab_name IN VARCHAR2,
  3     p_rowid IN ROWID,
  4     p_calc_col IN VARCHAR2) RETURN NUMBER IS
  5
  6  sqlstr VARCHAR2(4000);
  7  result NUMBER;
  8  BEGIN
  9     sqlstr:= 'SELECT '||p_calc_col||' FROM '||p_tab_name||' WHERE rowid = :b';
10     EXECUTE IMMEDIATE sqlstr INTO result USING p_rowid;
11     RETURN result;
12  END;
13  /
Function created.
SQL> CREATE TABLE jtest (col1 number,col2 number,col3 varchar2(50));
Table created.
SQL> INSERT INTO jtest values (5, 7, 'Col1+Col2');
1 row created.
SQL> INSERT INTO jtest values (10, 3, 'Col1*Col2');
1 row created.
SQL> INSERT INTO jtest values (20, 5, 'Col1/Col2');
1 row created.
SQL> commit;
Commit complete.
SQL> SELECT col1,col2,col3,MY_EVAL('jtest',rowid,col3) evald
  2  FROM jtest
  3  /
      COL1       COL2 COL3                           EVALD
         5          7 Col1+Col2                         12
        10          3 Col1*Col2                         30
        20          5 Col1/Col2                          4
SQL> DROP TABLE jtest;
Table dropped.
SQL> CREATE TABLE jtest (col1 number,col2 number,col3 number,col4 varchar2(50));
Table created.
SQL> INSERT INTO jtest values (5, 7, 2, '(Col1+Col2)/Col3');
1 row created.
SQL> INSERT INTO jtest values (10, 3, 5, 'Col1*(Col2+Col3)');
1 row created.
SQL> INSERT INTO jtest values (20, 5, 2, 'Col1/Col2/Col3');
1 row created.
SQL> commit;
Commit complete.
SQL> SELECT col1,col2,col3,col4,MY_EVAL('jtest',rowid,col4) evald
  2  FROM jtest;
      COL1       COL2       COL3 COL4                           EVALD
         5          7          2 (Col1+Col2)/Col3                   6
        10          3          5 Col1*(Col2+Col3)                  80
        20          5          2 Col1/Col2/Col3                     2This could easily be extended to handle character functions as well.
John