banner rotation

I'm working on a banner rotation script for my new site but I'm not sure what would be the best way to rotate the banners in the pool. All the banners are stored in a database, I was thinking about adding an extra field to mark the last displayed banner (or the next one to be displayed) But since the users of the site can submit/delete their own banners this could cause a problem (when the marked record is deleted)
How would you implement the rotation of a banner pool?

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Do you want to equally display the banners?

for starters, yes

Looks like I've nailed it: I'm counting the number of records in the banner pool and create a random number between 1 and the number of banners, thats the banner number that will be displayed...
Do you think this is a good way of doing this?

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You could do this by using ORDER BY RAND(), but it still wont give the same impressions to every banner.

The best way is to add a field which will keep the count of impressions (eg. impr) and do something like :

SELECT id, .. .... . . .. .. . . . . . FROM banners ORDER BY impr ASC LIMIT 1

And then after you show the banner execute something like :

UPDATE LOW_PRIOTITY banners SET impr = impr + 1 WHERE id = $id LIMIT 1

You could do this by using ORDER BY RAND(), but it still wont give the same impressions to every banner.

The best way is to add a field which will keep the count of impressions (eg. impr) and do something like :

SELECT id, .. .... . . .. .. . . . . . FROM banners ORDER BY impr ASC LIMIT 1

And then after you show the banner execute something like :

UPDATE LOW_PRIOTITY banners SET impr = impr + 1 WHERE id = $id LIMIT 1

Interesting. I will do that too.

Thanks

I'll try that, I already keeping a 'views counter' on the banners and I did notice that the random function does not show the different banners the same number of times, although it should in the long run...
A problem with your sollution would be that if the banners have been shown a lot of times already, a newly added banner will be shown exclusively untill its count is up to the same number, but I guess this can be solved by setting the counter to the same value as the other banners when adding a new banner.

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Yes because otherwise the solution would be very costly to your server's CPU usage.

Except if you also use a php array on the session to know what banners you have shown to the user (so you want serve the same banner twice)

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