哪种使用查询的方式会有效？

Question

i have two tables hsc,sslc hsc:

id         name           class   section
1          karna            1        a
2          bavi             2        b
3          chidu            3        c

sslc

id         name           class    section
1          ram             11        a2
2          sam             11        b1
3          guna            14        c2

note:class which is present in hcs wont be there in sslc. i should pass only id and class as a condition to get the result using php but i wont pass the table name.so i am using using union query

$id=$_REQUEST['id'];
$class=$_REQUEST['class'];

$sql="select t1.* (select id,name,class,section from hsc union select  id,name,class,section from sslc)t1 where id=$id and class=$class;"//this runs successfully 
$StudentArray = $Cobj->union($sql);

OR

$sql="select * from hsc where id=$id and class=$class;"
$StudentArray = $Cobj->union($sql);
if(count($StudentArray)>0){
$table="hsc";}
else{
$table="sslc";}
$sql="select * from $table where id=$id and class=$class";
$StudentArray = $Cobj->union($sql);

both query result the same output . but will is an effective way?

Answer 1

First one is effective way
A single call will always be faster than several calls for the same data.Simply the network turn around and latency alone is a component, but also the start up and tear down of the SQL processing will have some impact.