Assign value to a field of rowtype where `field name` is a string

I want to assign a value to a rowtype's field but I don't know how to do it.

Suppose that I have a table X inside my database.

Suppose also that I have the following variables

a ( X%ROWTYPE ), representing a row of the table X
b ( VARCHAR2 ), containing a column name of the table X
c ( VARCHAR2 ), containing what I want to store inside a.b

What I want to do : something like a.b := c.

I've come up with something like this :

EXECUTE IMMEDIATE 'SELECT '|| c || ' INTO a.' || b || ' FROM DUAL';

Apparently, this isn't the right way to go. I get a ORA-0095: missing keyword error.

Can anyone help me with this ?

Here is the complete code :

DECLARE
    tRow            MyTable%ROWTYPE;
    col_name        VARCHAR(10) := 'Length';
    nValue          NUMBER(12,4) := 0.001;
    dynamic_request VARCHAR(300);
BEGIN 
    dynamic_request := 'SELECT '|| nValue || ' INTO tRow.' || col_name || ' FROM DUAL';
    EXECUTE IMMEDIATE dynamic_request;
END;

Solution

Ok, I solved it !

Short answer : Using a global variable does the trick

Answer Development

Let us consider two facts about dynamic PL/SQL blocks (i.e., PL/SQL blocks written as strings, to be executed trough an EXECUTE IMMEDIATE statement)

[1] There is no such thing as variable scope when you create a dynamic PLSQL block. What I mean by that is, if you do something like this :

CREATE OR REPLACE PROCEDURE DynamicVariableAssignment(
   theString IN VARCHAR2
 ) 
IS
BEGIN 
   EXECUTE IMMEDIATE 'BEGIN theString := ''test''; END; ';
END;

it will simply not work because the scope of theString is not transfered to the dynamic PL/SQL block. In other words, the dynamic PL/SQL block doesn't "inherit" of any variable, wherever it is executed.

[2] You might say "OK, no panic, I can give input/output arguments to my dynamic PL/SQL block, right ?". Sure you can, but guess what : you can only give SQL types as in/out ! True PL/SQL types on the other hand, such as a myTable%rowtype, are not accepted as an input for a dynamic PL/SQL block. So the answer of hmmftg won't work either :

-- I've reduced the code to the interesting part
dynamic_request := 'BEGIN :t_row.' || col_name || ':= 0.001; END;';
EXECUTE IMMEDIATE dynamic_request USING IN OUT tRow;
-- (where tRow is of type myTable%ROWTYPE)

since tRow is of MyTable%ROWTYPE, it is not a valid SQL type and is therefore not valid as an input to the dynamic PL/SQL block.

The Solution Who would have thought that global variables would come and save the day ? As we said in [1], we have no reference to any variable outside the dynamic PL/SQL block. BUT we can still access global variables defined in package headers !

Let us assume that I have a package kingPackage in which I define the following :

tempVariable  myTable%ROWTYPE;

Then I can do this :

FINAL CODE (body only)

-- Copy tRow into temp variable
kingPackage.tempVariable := tRow;

-- We modify the column of the temp variable
vString := 'BEGIN kingPackage.tempVariable.' || col_val || ' := ' || TO_CHAR(vNumber) ||'; END;'; 
EXECUTE IMMEDIATE vString;    

-- The column value has been updated \o/ 
tRow := kingPackage.tempVariable;

There you go, fellas ! Have a nice day