db_update add value to field I want use db_update to update a field,instead of update a field with new value (replace) , I want add a value to last value, 
I know I can do it with mysql command  

UPDATE `databasename`.`tablename` SET fieldB = fieldB + 1 WHERE fieldA='2';


but I use below code, It is not add to credit field, only replace it with $credit value, 
anybody know where is my problem and how can I achieve this job?

$cr_update_no = db_update('rah_cc_credit') // Table name no longer needs {}
                -&gt;fields(array('uid' =&gt; $user-&gt;uid,'update_date'=&gt;REQUEST_TIME,'last_code'=&gt;$code ,'credit' =&gt; 'credit'+ $credit))
                -&gt;condition('uid', $user-&gt;uid, '=')
                -&gt;execute();
The problem is way I used to update,
for this job drupal has syntax, according to  Update query with a dynamic expression in Drupal 7?
I found my solution 

$cr_update_no= db_update('rah_cc_credit') // Table name no longer needs {}
                -&gt;fields(array('uid' =&gt; $user-&gt;uid,'update_date'=&gt;REQUEST_TIME,'last_code'=&gt;$code ))
                -&gt;expression('credit', 'credit + :amount', array(':amount' =&gt; $credit))
                -&gt;condition('uid', $user-&gt;uid, '=')
                -&gt;execute();


I submit this answer maybe helpful for another in future.