How to select random ID's from database excluding deleted ones?

Question

I have a "banners" table which has image names and paths to image files that will be randomly viewed each time visitor navigates to another page.

banners table consists of banner_id (auto_increment, unique, primary, tinyint), banner_name (varchar) and banner_path (varchar) fields.

Banners table will be editable via a control panel. New banners will be ADDED, some banner will be DELETED after a while, and may be UPDATED. General CRUD operations, you know...

Now... Because of my goal is showing banners randomly, I will need a random number generator function which gives ability to exclude specific ones.

To be more clear,

Let's say my table looks like this:

banner_id     banner_name     banner_path
---------     ------------    ------------
1             First Banner    first_banner.jpg
2             Second Banner   second_banner.jpg
3             Third Banner    third_banner.jpg

I can get random ID by using PHP function like this easily: mt_random(1, 3);

But wait. What if I delete one of these banners?

banner_id     banner_name     banner_path
---------     ------------    ------------
1             First Banner    first_banner.jpg
3             Third Banner    third_banner.jpg

In this case, as random output becomes "2", what will happen? There is no a "2" banner_id'd row?? So I must exclude DELETED id's from random generator number range. Will this be the best practise? If so, how can I do that?

I'm completly open for any new ideas which will help me to do thing I want to do.

Please help me to figure out this problem...

Thanks

Thanks.

Answer 1

SELECT FLOOR(RAND() * COUNT(*)) INTO @offset FROM banners;
SELECT * FROM banners LIMIT @offset, 1;

Answer 2

Why not just select a random row instead of using PHP to select a random ID to show?

SELECT * FROM `banners` ORDER BY RAND() LIMIT 0,1;

And if you have an array of ID's to exclude

SELECT * FROM `banners` 
WHERE `banner_id` NOT IN (/* array values*/) 
ORDER BY RAND() LIMIT 0,1;

Answer 3

Assuming you can live with the small race window, you could do

SELECT FLOOR(RAND()*COUNT(*)) AS bannercount FROM banners;

and fetch this into $bannercount , next run

SELECT * FROM banners ORDER BY banner_id LIMIT $bannercount,1

Answer 4

您将根据可用ID创建一个数组，并从该数组生成随机数