how to replace user's names with random names? How can I use SQL only on mysql DB to replace first and last names of real users in users table using a random name from two related tables: random_first_names and random_last_names. Our users table contains over 250K records and each of the rundome tables contain over 5000 names that should be picked at random for each record in users table. Is it possible to achieve with SQL only? 

[UPDATE]

I solved the problem based on Rick's input and shared the full SQL script below.
If you decide that the Update is too slow, I suggest the following will be about 1000 times as fast.

Loop (can be done in a Stored Proc)...


[Re]Create a table with randomly ordered set first_names, with a `PRIMARY KEY of 1..5000.  Ditto for last_names (a second table).
Multi-table UPDATE the 'next' 5000 rows joined to the two random tables.  Use ON Users.id % 5000 = RandomFirstNames.id (etc)


End Loop

Shuffling the table (step 1 of the loop) is something like

CREATE TABLE RandomFirstNames (
    id SMALLINT UNSIGNED AUTO_INCREMENT,
    first_name VARCHAR(...),
    PRIMARY KEY(id) )
SELECT first_name FROM FirstNames ORDER BY RAND();


After OP's UPDATE

Don't do 

SELECT count(id) INTO count_names FROM _RandomFirstNames;


Instead, do this once:

SELECT @mask_ct := COUNT(*) FROM _masked_names;


and use @mask_ct instead of count_names;

As for the skipped ids, CREATE TABLE _RandomFirstNames without an id, then ALTER TABLE _RandomFirstNames ADD id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY; to get the ids.  This should give you ids without gaps (unless you are on a multi-Master cluster of any kind).