mysql where clause to show banner based on device

Question

I have a mysql table "banners" with a column "device" in it

the column "device" is NULL if the banner is for desktop, and is "ios" if the banner is for iphone/ipad device.

now i'm working on a script to show the banner based on device..

for example if someone visit the website with a desktop the query is:

SELECT id FROM banners WHERE 1 ORDER BY RAND() LIMIT 1

and if i visit the website with an iphone the query change:

SELECT id FROM banners WHERE device LIKE '%ios%' ORDER BY RAND() LIMIT 1

Everything works fine, except that if i have 0 banners for IOS i would to show a DESKTOP banner instead of nothing, but i don't know how to change the query..

for example if i add a OR clause the priority of IOS remain???

SELECT id FROM banners WHERE ( device LIKE '%ios%' OR device ='') ORDER BY RAND() LIMIT 1

Answer 1

 select id from banners A ,(select count(*) AS cnt  from banners  where device LIKE '%ios%') B

 WHERE 

 device LIKE (case when B. cnt = 0 then '%%' else '%ios%' end) 

 ORDER BY RAND() LIMIT 1

Answer 2

There is a difference between a database NULL and the empty string, which is what I think your stumbling on here. Try this:

SELECT id FROM banners WHERE ( device LIKE '%ios%' OR device IS NULL) ORDER BY RAND() LIMIT 1

See this bit of the MySQL website for information on working with NULL values in MySQL