Question

I have a table with 2 varchar2 columns. I have added new number column to existing table to make this column primary key. This table now includes 3 columns. I gave a try to use anonymous block as following:


declare
cnt number;
begin
select nvl(count(*),0) into cnt from sometable;
for i in 1..cnt
loop
update sometable set id=i where i=rownum;
end loop;
end;

Using this anonymous block updates this table unexpectedly.
My solution was to use the following statement:

create table sometablecopy as select row_number() over(order by sometable.col1) as id, sometable.* from sometable;
Nevertheless I am curios why doesn’t anonymous block produce expected primary key values with the help of rownum pseudocolumn? It must be rownum related issue.

Answer 1

Rownum is a pseudocolumn. Its assigned to rows as they are returned from the select. So you can’t say “select * from my_table where rownum = 42” since the row with rownum=42 hasn’t been defined yet, it will vary depending on your select and predicate (and “select * from my_table where rownum = 1” will return a single row, not the “first” row, whatever that would be). You could do something like (untested):

declare
  cursor sel_cur is
  select rowid as row_id
  from my_table
  order by orderby_col;

  v_ctr pls_integer := 0;
begin
  for rec in sel_cur
  loop
    v_ctr := v_ctr + 1;
    update my_table set pk_col = v_ctr where rowid = rec.row_id;
  end loop;
  commit;
exception
  when others then 
  rollback;
  raise;
end;

This assumes you have sufficient rollback to update the entire table.

Hope that helps.