vBulletin SQL query syntax explanation needed

[nirvana43]

Hello
I've been looking around but couldn't find the exact difference between
$db->query_read()
$db->query_write()
$db->query_first()

$db->query_read() is i think only for getting something outta tables and $db->query_write() is for putting something in database.. but whats about $db->query_first()

I would like to know sql performance in above functions for UPDATE, SELECT, ALTER, DROP, INSERT INTO etc.
Which functions (outta above 3) i'm suppose to use for performing those operations.
Also, is there any more vBulletin inbuilt functions like those i should be knowing of.

Regards