Mysql：'由Rand（）限制4的订单'？

Question

i need to extract random 4 items from latest news database.

since this table has a lot of rows. is it wise to do so with order by RAND() or its waste of resources ?-since it will go thro the whole table and reorder it ?

is there a better way to do it ? extract random featured items from table with hundreds of rows ?

Example:

$l=$database->query("SELECT car,price,thumbnail FROM cardb WHERE type='new' order by RAND() LIMIT 4");
foreach($l as $l){print_r($l);}

Thankx for taking time to answer }

Answer 1

Well... I would do this in two queries. Get random row numbers:

SELECT
    floor(count * rand()) r1,
    floor(count * rand()) r2,
    floor(count * rand()) r3,
    floor(count * rand()) r4
FROM
    (SELECT count(*) count FROM cardb WHERE type = 'new') c

And get rows:

SELECT car, price, thumbnail FROM cardb WHERE type = 'new' LIMIT {r1}, 1
UNION ALL
SELECT car, price, thumbnail FROM cardb WHERE type = 'new' LIMIT {r2}, 1
UNION ALL
SELECT car, price, thumbnail FROM cardb WHERE type = 'new' LIMIT {r3}, 1
UNION ALL
SELECT car, price, thumbnail FROM cardb WHERE type = 'new' LIMIT {r4}, 1