Multiple small queries vs a single long query. Which one is more efficient?

Question

Which one the below approaches is more efficient:

1. Single longer query:

   "SELECT COUNT( * ) AS num
   FROM (
   
       SELECT users.UID, cnumber
       FROM users
       JOIN identity ON users.UID = identity.UID
       WHERE  'abc'
       IN (google, facebook, twitter, linkedin)
   
   ) AS userfound
   JOIN jobs ON userfound.cnumber = jobs.cnumber
   WHERE JID = 24";
   
   
   if(resultfromquery == 1)
       //Some code here
   else
       //Some code here
   

2. Break the longer query into multiple single table queries:

   uid = "SELECT UID FROM identity WHERE 'abc' IN (google, facebook, twitter, linkedin)";
   cnumber_from_usertable = "SELECT cnumber FROM users WHERE UID = 'uid'";
   cnumber_from_jobtable = "SELECT cnumber FROM jobs WHERE JID = 24";
   
   if(cnumber_from_usertable == cnumber_from_jobtable)
       //Some code here
   else
       //Some code here
   

Answer 1

measure the microtime difference ;-) I would go with the single query though, less connections, less wait time and it's designed to handle those kind of things.

http://www.phpjabbers.com/measuring-php-page-load-time-php17.html

<?php
$time = microtime();
$time = explode(' ', $time);
$time = $time[1] + $time[0];
$start = $time;
?>
"SELECT COUNT( * ) AS
FROM (

    SELECT users.UID, cnumber
    FROM users
    JOIN identity ON users.UID = identity.UID
    WHERE  'abc'
    IN (google, facebook, twitter, linkedin)

) AS userfound
JOIN jobs ON userfound.cnumber = jobs.cnumber
WHERE JID = 24";


if(resultfromquery == 1)
    //Some code here
else
    //Some code here

<?php
$time = microtime();
$time = explode(' ', $time);
$time = $time[1] + $time[0];
$finish = $time;
$total_time = round(($finish - $start), 4);
echo 'Page generated in '.$total_time.' seconds.';
?>

Answer 2

Actually I have somewhat different view on this. MYSQL support subqueries and one of the reasons being there is breaking the larger query into smaller. If you are using INNODB engine and are using the parameters correctly for example tmp table, heap table, bufferpool, or query cache then you may get better results then smaller queries. MYSQL was designed to execute alot of small queries so it would be better to have more small queries then a larger query.

If a larger query was a better option then we did not had the need to create tool like Infinidb and INfobright that uses column based structure to run large queries, where as MYSQL is row based.

So think about it, small queries would result better then a large single query and overhead is not of queries it is of connections and again MYSQL is very cheap with connections so no worries their either. BUt if your MYSQL is creating threads then you have nothing to worry about.

In short small few queries vs giant large query would be my choice.