Question

I got an question about mysql. Is it possible to check if an row exists in an external table?
For example, I need to show recent uploaded images and I would like to show some userdata with it and show if the user is online.

My current query is:

SELECT `users`.`username`, `users`.`location`, `users`.`age`, `users`.`dateofbirth`
FROM (`images`)
JOIN `users` 
  ON `users`.`userid` = `images`.`userid`
WHERE `images`.`active` =  1
LIMIT 12 

But I’m missing the ‘is online’ part. Is it possible to use this in the select? For example if the row exists there would be a value in the select named online and it’s value = 1 and if user is not online (no row in table) the value needs to be 0. Is that possible?

Answer 1

Seems like you just need to use a LEFT JOIN on the other table and then a CASE to provide the value:

SELECT `users`.`username`, 
  `users`.`location`, 
  `users`.`age`, 
  `users`.`dateofbirth`,
  case when o.`userid` is null then 0 else 1 end UserOnline
FROM (`images`)
JOIN `users` 
  ON `users`.`userid` = `images`.`userid`
left join online_table o
  on `users`.`userid` = o.`userid`
WHERE `images`.`active` =  1
LIMIT 12