Selecting a Random Row in Oracle

问题:

I need to randomly select values from one table, e.g. tableA.a_id which is a VARCHAR2, and use the value to insert into another table. For instance, assuming three columns needs to be inserted into 100 rows of tableX (a sequence number, a random number between 100 and 999, and values of tableA.a_id):

insert into tableX
select
    rownum,
    dbms_random.value(100,999), 0),
    (select a_id from 
    (
      SELECT a_id 
      FROM tableA
      ORDER BY dbms_random.value
    )
    where rownum = 1)
from
   (select level from dual connect by level <= 100);

However, rather than picking a random row from tableA.a_id for each row, it selects the same value for all the rows, e.g.:

1 129 A-ID-48
2 849 A-ID-48
3 367 A-ID-48

However, if I execute the subquery repeatedly, I get a new value each time (for obvious reason), e.g.:

select a_id from 
    (
      SELECT a_id 
      FROM tableA
      ORDER BY dbms_random.value
    )
where rownum = 1;

Result would be after each execution:

A-ID-7
A-ID-48
A-ID-74

How do I alter the original query, or come up with a new one for that matter, that would insertion of random rows from tableA's a_id column for each insert row into the destination table? Desire outcome:

1 129 A-ID-7
2 849 A-ID-48
3 367 A-ID-74

Update 1

Based on mathguy answer, I updated the query for a single table selection:

insert into tableX
select
    rownum,
    round(dbms_random.value(100,999), 0),
    a_id
from
    (
      select 
        round(dbms_random.value(1, (select count(*) from tableA)), 0) tableX_rand_num
      from tableX
    ) x
join 
    (
      select
        a_id, 
        dbms_random.value() rnd,
        rownum tableA_rownum
      from tableA
      order by rnd
    ) a
on x.tableX_rand_num = a.tableA_rownum
where rownum <= 100;

LIMITATION: The number of inserted rows using this method is not going to be independent of number records available in the parent table (tableX). In other word, you can only insert as many as records as the total rows available in tableX. e.g. if tableX has 200 records, and you wish to insert 1000, the query above would only allow you to insert up to 200 rows.

回答1:

Make the inner query:

select a_id, dbms_random.value() rnd from tableA order by rnd

and then in the outer query select 100 rows in one shot, with rownum <= 100.

Like so:

insert into tableX
select
    rownum,
    round(dbms_random.value(100,999), 0),
    a_id
from
    (
      SELECT a_id, dbms_random.value() rnd
      FROM tableA
      ORDER BY rnd
    )
where rownum <= 100;