php and database

I want to create an auto action such that a field is read from database, some manipulations are performed on it using php and then it is reassigned to some other field. How can i do that?

Just do a Code action with method set to PHP and use Joomla/CB API to do whatever you need. If you were more specific as to your needs you could actually just use a Query action and use database functions to alter the field.

How do I pass values from a php Code action to a Query Code Action and vice-versa?

You don't. They can't communicate with one another. If you need queries and PHP then you need to use a Code action with PHP method and use Joomla or CB API to perform the query using PHP.

Is there a tutorial about use of CB API to perform the query using php.If not, please give a brief example of how to write, say an update query. Do I need to connect to database.
I have tried that before also but did not succeed with it.

To be more specific: I want to run a query and find out the total number of rows affected by it(say x). Then I want to run another query with LIMIT x.

Is there a tutorial about use of CB API to perform the query using php

No.

please give a brief example of how to write, say an update query.

Below is an example of CB API to perform a query.

Replace "YOUR QUERY HERE" with your database query of course.

Do I need to connect to database.

No.

To be more specific: I want to run a query and find out the total number of rows affected by it(say x). Then I want to run another query with LIMIT x.

Being PHP you can do whatever you want, but based off your needs you should be able to just use a Query action with the LIMIT being a subquery.