Update n random rows in SQL

Question

I have table which is having about 1000 rows.I have to update a column(\"X\") in the table to \'Y\' for n ramdom rows. For this i can have following query

update         


        

Answer 1

The following solution works just fine. It's performant and seems to be similar to sample():

create table t1 as 
    select level id, cast ('item'||level as varchar2(32)) item 
    from dual connect by level<=100000; 

Table T1 created.

update t1 set item='*'||item 
where exists (
    select rnd from (
        select dbms_random.value() rnd
        from t1
    ) t2 where t2.rowid = t1.rowid and rnd < 0.15
);

14,858 rows updated.

Elapsed: 00:00:00.717 

Consider that alias rnd must be included in select clause. Otherwise changes the omptimizer the filter predicat from RND<0.1 to DBMS_RANDOM.VALUE()<0.1. In that case dbms_random.value will be executed only once.

As mentioned in answer @JonHeller, the best solution remains the pl/sql code block because it allows to avoid full table scan. Here is my suggestion:

create or replace type rowidListType is table of varchar(18);  
/
create or replace procedure updateRandomly (prefix varchar2 := '*') is
    rowidList rowidListType;  
begin  
    select rowidtochar (rowid) bulk collect into rowidList
    from t1 sample(15)
    ;
    update t1 set item=prefix||item 
    where exists (
        select 1 from table (rowidList) t2
        where chartorowid(t2.column_value) = t1.rowid
    );
    dbms_output.put_line ('updated '||sql%rowcount||' rows.'); 
end;
/
begin  updateRandomly; end;
/ 

Elapsed: 00:00:00.293
updated 14892 rows.