Update random rows in sql table

Question

Do not ask me why but I have to do it and I would greatly appreciate any help.
Let us say that I want to update column foo_column of a table foo. The selection criteria is foo_selection_criteria. So my sql to update would be something like:
update foo set foo_column='some value' where <foo_selection_criteria>
Foo_selection_criteria can be any logical expression based on the value of columns in table foo. For example foo_selection_criteria could be foo.columna != '1';
There are m rows in the table that meet the foo_selection_criteria. I want to update only randomly selected 1/n (eg., 0.5 , 083 etc) of these m rows.
I tried using dbms_random.value procedure to update my sql above so that it reads something like
update foo set foo_column='some value' where <foo_selection_criteria> and dbms_random.value <= 1/n;
However it seems that dbms_random is evaluated only once in the update sql above. Is there any way I can avoid that dbms_value is calculated once and stick in the sql above ? Or is someone aware of another way to randomly update a random fraction of m rows that meed an update selection criteria ?
I would be grateful for any help/

Answer 1

Tubby wrote:
Why not just
ROWNUM = 1
that's random....I disagree. Rownum = 1 will in general return the first inserted row into a table.
There is no guarantee that it is the first inserted row, but in 99% of all cases it will be so ( i just made this statistic up, but prove me to be wrong ;)) .
And an 99% chance is not very random for me.