Question

I have two tables(table_a and table_b) combined with a UNION.

How can I check if record ist table_a.

I tried this, but not working:

$res=mysql_query("(SELECT id, added FROM table_a WHERE user_id = '".$uid."') 
UNION 
(SELECT id as comments, added FROM table_b WHERE user_id = '".$uid."') ORDER BY added DESC LIMIT 50");

if(!empty($rows["comments"])) // not working

Answer 1

It’s not working because you forgot to read the result-set:

while ($row = mysql_fetch_assoc($res)) {
// here you read $rows["comments"] etc
...

}

Important:
Please don’t use mysql_* functions, it’s deprecated (see red box) and vulnerable to sql-injection.
Use PDO or MySQLi.

Update:
If you want to know from which table the result “came” you can change your query to:

(SELECT id, added, 'A' as came_from FROM table_a WHERE user_id = '".$uid."') 
UNION 
(SELECT id as comments, added, 'B' as came_from FROM table_b WHERE user_id = '".$uid."') ORDER BY added DESC LIMIT 50

Then check the value of: $rows["came_from "]