oracle query select a column (where column name is unknown) - oracle

Question

My column name is retrieved from another query, then using that result as the column name I want to query to get the value. I have found a duplicate question here but I could not find the answer. So my code looks like this:
DECLARE
COL_NO NUMBER(3,0);
COL_NAME VARCHAR2(30);
COL_VALUE VARCHAR2(100);
BEGIN
COL_NO:=0;
COL_NAME:=NULL;
COL_VALUE:=NULL;
SELECT COUNT(*) INTO COL_NO FROM user_tab_columns WHERE table_name='SALARYSLIP';
FOR A IN 4..COL_NO LOOP
SELECT column_name INTO COL_NAME FROM user_tab_columns WHERE column_id=A AND table_name='SALARYSLIP';
--SELECT COL_NAME INTO COL_VALUE FROM SALARYSLIP WHERE ID=SALARYSLIP.ID;
DBMS_OUTPUT.PUT_LINE(COL_VALUE);
END LOOP;
END;
The commented query is supposed to fetch the column value but is not working obviously. What should I write there? TIA.

Answer 1

I'm not exactly sure what you are trying to accomplish. Counting the umber of columns in the table and iterating from 4 to that number doesn't seem particularly logical. In the desired SQL statement, it's also not clear where you are getting the value for the id that you want to use.
If you want to determine the column name at runtime, then you would need to use dynamic SQL. Something like
FOR A IN 4..COL_NO LOOP
SELECT column_name INTO COL_NAME FROM user_tab_columns WHERE column_id=A AND table_name='SALARYSLIP';
--SELECT COL_NAME INTO COL_VALUE FROM SALARYSLIP WHERE ID=SALARYSLIP.ID;
EXECUTE IMMEDIATE 'SELECT ' || col_name ||
' FROM salaryslip ' ||
' WHERE id = :1'
INTO col_value
USING <<whatever ID value you want>>;
DBMS_OUTPUT.PUT_LINE(COL_VALUE);
END LOOP;
For simplicity, I'm not using a local variable for the SQL statement that is assembled before being dynamically executed. Normally, you would want to have a local variable with the SQL statement so that you can do things like log the statement that was assembled in case it doesn't do what you expected.

Answer 2

I think the logic needs to be addressed first - i.e. whether the required results is in terms of the rows from the SALARYSLIP table, OR the columns of the SALARYSLIP table.
SELECT COUNT(*) INTO COL_NO FROM user_tab_columns WHERE table_name='SALARYSLIP'; ( this would get the number of columns from the SALARYSLIP table )
After this, what is the desired result to iterate through the for loop, based on the number of columns on the SALARYSLIP table? i.e. since, generally, the output is based on the rows being retrieved.
Where is the "ID" value coming from? It's not being initialized / declared in the code.
From what I gather, the column_name of the table is required to be retrieved at runtime. Then, the data for this column should be displayed for the SALARYSLIP table.
Try this :
DECLARE
COL_NO NUMBER(3,0);
COL_NAME VARCHAR2(30);
COL_VALUE VARCHAR2(100);
ROWNO NUMBER(3,0);
BEGIN
COL_NO:=0;
COL_NAME:=NULL;
COL_VALUE:=NULL;
SELECT COUNT(*) INTO COL_NO FROM user_tab_columns WHERE table_name='SALARYSLIP'; --number of columns in SALARYSLIP
SELECT Count(*) INTO rownno FROM SALARYSLIP; --number of rows in SALARYSLIP
--go through each column
FOR A IN 1..COL_NO LOOP
SELECT column_name INTO COL_NAME FROM user_tab_columns WHERE column_id=A AND table_name='SALARYSLIP'; --column_name
--go through each row in the SALARYSLIP table
--for each row, get the specific column of the row, and print this column's value
FOR B IN 1..rowno LOOP
SELECT COL_NAME INTO COL_VALUE FROM SALARYSLIP WHERE =SALARYSLIP.COL_NAME = COL_NAME;
DBMS_OUTPUT.PUT_LINE(COL_VALUE);
END LOOP;
END LOOP;
END;