Oracle SQL: Calculate value using previous row
<p>I have a table with two columns: Cur_value and  Difference</p>

<pre><code>       Cur_value        Difference
(-3)       3    
(-2)       4    
(-1)       5    
(1)                        1
(2)                       -2
(3)                        3
(4)     
(5)     
(6)     
(7)     
</code></pre>

<p>Now I want to caculate value of Cur_value from row (1) to row (7) so that  </p>

<p>cur_value(t) = cur_value(t-3) + dif(t). </p>

<p>It means my result is: </p>

<pre><code>           Value        Difference
(-3)           3    
(-2)           4    
(-1)           5    
(1)        3 + 1           1
(2)        4 - 2          -2
(3)        5 + 3           3
(4)    3 + 1 + 2           2
(5)    4 - 2 + 1           1
(6)    5 + 3 - 2          -2
(7) 3 + 1 + 2 - 1         -1
</code></pre>

<p>. 
My question is: how to do this action by single Oracle SQL statements?</p>