How to update data by select random row value from another table - oracle

Question

I have three tables A, B, C, and I want to randomly take a row from the col_b column of the table B then update it to the table A. Table C is the subtable of Table B, which is used to filter the data of Table B.
Here is my sql statement:
update a a
set a.col_a_b =
(select t.col_b
from (select a1.col_a, b1.col_b, a1.rn_var
-- the number 6 is because I only have 6 rows of data,
-- and the real situation should be the total number of conditions in table b
from (select a0.col_a, TRUNC(dbms_random.value(1, 6)) rn_var
from a a0) a1
left join (select b.col_b, rownum rn
from b b
where exists (select 1
from c c
where b.id = c.col_b_id
and c.col_c = 'c1')) b1
on a1.rn_var = b1.rn) t
where t.col_a = a.col_a);
I found a strange phenomenon:
If I remove a1.rn_var (line from (select a1.col_a, b1.col_b, a1.rn_var), it doesn't work as my expected
On the basis of the above, if I replace exists with left join (or join), the result is the same
If I reomve both a1.rn_var and exists, it will work fine.
I know there may be a better way to implement it, but who can tell me why?
Update:
Actually, it is caused by this sql:
select a1.col_a, b1.col_b -- remove a1.rn_var
from (select a0.col_a, TRUNC(dbms_random.value(1, 6)) rn_var from a a0) a1
left join (select b.col_b, rownum rn
from b b
where exists (select 1
from c c
where b.id = c.col_b_id
and c.col_c = 'c1')) b1
on a1.rn_var = b1.rn
-- this is for better display of results
where a1.col_a = 'a1';
In the above sql, I may get multiple rows of data or column b1.col_b is empty, as shown below:
a1 b1
a1 b2
a1 b4
------------------------------------------------
a1 -- here is null
In addition, each value of column a1.col_a is the same, I mean, if value a1 has multiple rows, then value a2 (and so on) has the same result, like this:
a1 b2
a1 b4
a1 b5
a2 b2
a2 b4
a2 b5
...

Answer 1

You can use a random number and order by that random number to get random records.
I prefer using the following technique:
UPDATE A A
SET
A.COL_A_B = (
SELECT
COL_B
FROM
(
SELECT
COL_B,
TRUNC(DBMS_RANDOM.VALUE(1, COUNT(1) OVER())) RANDOM_NUMBER --GENERATES RANDOM NUMBER
FROM
(
SELECT DISTINCT
B.COL_B -- FETCHING DISTINCT RESULT
FROM
B B
-- EXISTS IS CONVERTED INTO JOIN
JOIN C C ON ( B.ID = C.COL_B_ID
AND C.COL_C = 'c1' )
)
ORDER BY
RANDOM_NUMBER -- ORDERING IS DONE BY RANDOM NUMBER
FETCH FIRST ROWS ONLY -- FETCHING ONLY FIRST ROW FROM ORDERED RECORDS
)
)
Cheers!!