RAND() In WHERE clause in MYSQL

I've found this Blog post that shows a way to grab a random rows from a table : http://www.rndblog.com/how-to-select-random-rows-in-mysql/

I used it in a heavy delete query that I was trying to LIMIT and it worked, while also being quite fast.

DELETE table1 FROM table1 
    JOIN table2 ON table2.id = table1.salesperson_id 
    JOIN table3 ON table3.id = table2.user_id
    JOIN table4 ON table3.office_id = table4.id
WHERE table1.type = "Snapshot" 
    AND table4.id = 113 OR table4.parent_id =113
    AND RAND()<=0.001;

I don't understand how does this works. I tried googling extensively, but I found nothing regarding a RAND() in the WHERE clause that's used in this way. Neither did the docs give anything about this.

Thank you in advance.

P.S. I'm Using MYSQL 5.5

The expression in your WHERE clause is evaluated for every row generated by your joins.

Every time the expression calls RAND(), that function returns a different random floating-point value between 0 and 1. If this random value is equal to or less than 0.001, then that term is true. So it will be true approximately 1 out of 1000 rows, randomly. Those will be the rows it will delete. Whereas the other 999 out of 1000 rows it will skip deleting.

By the way, I would guess you want the WHERE clause to have some parentheses to enforce operator precedence:

WHERE table1.type = "Snapshot" 
    AND (table4.id = 113 OR table4.parent_id = 113)
    AND RAND()<=0.001;

Otherwise AND binds more strongly than OR, so without these parentheses, the expression will work as if you had written it this way:

WHERE (table1.type = "Snapshot" 
    AND table4.id = 113) OR (table4.parent_id =113
    AND RAND()<=0.001);

That means all the rows with type='Snapshot' AND id=113 will be deleted, whereas 1/1000 rows with parent_id=113 will be deleted, including some rows with type other than 'Snapshot'.

That's probably not what you want. You should review boolean algebra and operator precedence. See https://dev.mysql.com/doc/refman/5.7/en/operator-precedence.html