Better choice instead of functions for multiple inserts - mysql

Question

in my sql database I have a procedure that needs to insert in a table tons of randomly generated records.
Something like this:
insert into table_AAA
SELECT
, round(X + rand() * 10 - rand() * 10 )
, round(Y + rand() * 10 - rand() * 10 )
FROM db_numbers d /*very big table containing just 1 column with numbers from 1 to 1M*/
limit 100000;
it takes 3 seconds, sound reasonable compared to other procedure of my servers.
Then I had to make that if the result of the round () is >=0
To do it I made a function:
CREATE FUNCTION `fn_normalize`(`p_value` INT) RETURNS int(11)
BEGIN
declare v_output INT;
IF p_value < 0 THEN
SET v_output = 0 ;
ELSE SET v_output = p_value ;
END IF;
RETURN (v_output);
END
and my insert became:
insert into table_AAA
SELECT
, fn_normalize(round(X + rand() * 10 - rand() * 10 ))
, fn_normalize(round(Y + rand() * 10 - rand() * 10 ))
FROM db_numbers d /*very big table containing just 1 column with numbers from 1 to 1M*/
limit 100000;
very slow, 10x the original one, probably because the function works on each value individually
I thought to use CASE WHEN:
insert into table_AAA
SELECT
, case when(round(X + rand() * 10 - rand() * 10 )) < 0 then 0 else (round(X + rand() * 10 - rand() * 10 )) end
, case when(round(Y + rand() * 10 - rand() * 10 )) < 0 then 0 else (round(X + rand() * 10 - rand() * 10 )) end
FROM db_numbers d /*very big table containing just 1 column with numbers from 1 to 1M*/
limit 100000;
But the else will re-run the rand function, so I cannot be sure is a positive number. Using rand(x) is not an option, because I need the most randomness of values.
Make an update after the insert is even worst.
Am I missing some obvious alternative?
Thanks a lot

Answer 1

YOu can use a subquery:
insert into table_AAA
SELECT CASE WHEN v.x<0 THEN 0 ELSE v.x END,
CASE WHEN v.y<0 THEN 0 ELSE v.y END,
FROM (
SELECT
round(X + rand() * 10 - rand() * 10 ) AS x
, round(Y + rand() * 10 - rand() * 10 ) AS y
FROM db_numbers d /*very big table containing just 1 column with numbers from 1 to 1M*/
limit 100000) v;

Answer 2

Much simpler (and faster) function body:
RETURN GREATEST(0, p_value);
Or, applying that to davide's solution:
insert into table_AAA
SELECT
GREATEST(0, round(X + rand() * 10 - rand() * 10 )) AS x,
GREATEST(0, round(Y + rand() * 10 - rand() * 10 )) AS y
FROM db_numbers d /* table of numbers from 1 to 1M*/
limit 100000) v;