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

Question

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;

Answer 1

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

Answer 2

try this:
CREATE OR REPLACE PROCEDURE ROW_CHANGER(
tRow IN MyTable%ROWTYPE,
col_name IN VARCHAR,
nValue IN NUMBER)
AS
dynamic_request VARCHAR(300);
BEGIN
dynamic_request := 'BEGIN :t_row.'||COL_NAME ||':= :n_value; END;';
EXECUTE IMMEDIATE dynamic_request
USING IN OUT TROW, IN nValue;
END;
this is because in your EXECUTE IMMEDIATE the tRow MyTable%ROWTYPE is not defined,
so we defined it with using statement.