UNPIVOT on an indeterminate number of columns

Question

How can I write a query that will unpivot a table that always has 1 row and many columns to a result set that has 2 columns: column_name and value. I understand the underly

Answer 1

It sounds like you want to unpivot the table (pivoting would involve going from many rows and 2 columns to 1 row with many columns). You would most likely need to use dynamic SQL to generate the query and then use the DBMS_SQL package (or potentially EXECUTE IMMEDIATE) to execute it. You should also be able to construct a pipelined table function that did the unpivoting. You'd need to use dynamic SQL within the pipelined table function as well but it would potentially be less code. I'd expect a pure dynamic SQL statement using UNPIVOT to be more efficient, though.

An inefficient approach, but one that is relatively easy to follow, would be something like

SQL> ed
Wrote file afiedt.buf

  1  create or replace type emp_unpivot_type
  2  as object (
  3    empno number,
  4    col   varchar2(4000)
  5* );
SQL> /

Type created.

SQL> create or replace type emp_unpivot_tbl
  2  as table of emp_unpivot_type;
  3  /

Type created.

SQL> ed
Wrote file afiedt.buf

  1  create or replace function unpivot_emp
  2  ( p_empno in number )
  3    return emp_unpivot_tbl
  4    pipelined
  5  is
  6    l_val varchar2(4000);
  7  begin
  8    for cols in (select column_name from user_tab_columns where table_name = 'EMP')
  9    loop
 10      execute immediate 'select ' || cols.column_name || ' from emp where empno = :empno'
 11         into l_val
 12       using p_empno;
 13      pipe row( emp_unpivot_type( p_empno, l_val ));
 14    end loop;
 15    return;
 16* end;
SQL> /

Function created.

You can then call that in a SQL statement (I would think that you'd want at least a third column with the column name)

SQL> ed
Wrote file afiedt.buf

  1  select *
  2*   from table( unpivot_emp( 7934 ))
SQL> /

     EMPNO COL
---------- ----------------------------------------
      7934 7934
      7934 MILLER
      7934 CLERK
      7934 7782
      7934 23-JAN-82
      7934 1301
      7934
      7934 10

8 rows selected.

A more efficient approach would be to adapt Tom Kyte's show_table pipelined table function.