title: Postgresql Update inside For Loop
body: <p>I'm new enough to postgresql, and I'm having issues updating a column of null values in a table using a for loop. The table i'm working on is huge so for brevity i'll give a smaller example which should get the point across. Take the following table</p>
  
  <pre><code>+----+----------+----------+
  | id  |  A  | B  |  C       |
  +----+----------+----------+
  | a   | 1   | 0  |  NULL    |
  | b   | 1   | 1  |  NULL    |
  | c   | 2   | 4  |  NULL    |
  | a   | 3   | 2  |  NULL    |
  | c   | 2   | 3  |  NULL    |
  | d   | 4   | 2  |  NULL    |
  +----+----------+----------+
  </code></pre>
  
  <p>I want to write a for loop which iterates through all of the rows and does some operation
  on the values in columns a and b and then inserts a new value in c. 
  For example,  where id = a , update table set C = A*B, or where id = d set C = A + B etc. This would then give me a table like  </p>
  
  <pre><code>+----+----------+----------+
  | id  |  A  | B  |  C       |
  +----+----------+----------+
  | a   | 1   | 0  |  0       |
  | b   | 1   | 1  |  NULL    |
  | c   | 2   | 4  |  NULL    |
  | a   | 3   | 2  |  6       |
  | c   | 2   | 3  |  NULL    |
  | d   | 4   | 2  |  6       |
  +----+----------+----------+ 
  </code></pre>
  
  <p>So ultimately I'd like to loop through all the rows of the table and update column C according to the value in the "id" column. The function I've written (which isn't giving any errors but also isn't updating anything either) looks like this...</p>
  
  <pre><code>-- DROP FUNCTION some_function();
  
  CREATE OR REPLACE FUNCTION some_function()
  RETURNS void AS
  $BODY$
  DECLARE
  --r integer; not too sure if this needs to be declared or not
  result int;
  
  BEGIN
  
  FOR r IN select * from 'table_name' 
  LOOP
  select(
  case
  when id = 'a'  THEN B*C
  when id = 'd'  THEN B+C
  end) 
  into result;
  
  update table set C = result 
  WHERE id = '';
  END LOOP;
  RETURN;
  
  END
  $BODY$
  LANGUAGE plpgsql
  </code></pre>
  
  <p>I'm sure there's something silly i'm missing, probably around what I'm, returning... void in this case. But as I only want to update existing rows should I need to return anything? There's probably easier ways of doing this than using a loop but I'd like to get it working using this method.
  If anyone could point me in the right direction or point out anything blatantly obvious that I'm doing wrong I'd much appreciate it. 
  Thanks in advance. </p>