PostGres- How to loop in Table to update every record in Table

I am working on complicated problem, but I will simplify it to this problem.

I have two tables

A [ID, column1, column2, column3]
B [ID, column1, column2, column3, column4]

and i want to update the third:

C [ID, column1, column2]

I am writing a query to

Select A.column1, B.column1 from A,B where  (A.column1 -B.column1)=1 limit 1;

Now I want to get all the points that

insert C(column1, column2)
Select A.column1, secElement from (Select A.column1 firstElement, B.column1 secElement from A,B where  (A.column3 -B.column3)=1 limit 1) as ex, A where A.column1=ex.firstElement limit 5;

But I am getting just one row.

I aspire to get a result like that.

• Loop on A

• for every element in A, I should find its peer.

But I couldn't find a way to do the condition. Take the element, look for its peer then go to the other element.

A.column1 | secElement

----------+-------------
25033419 | 244959
25033420 | 244960
25033421 | 244961
25033422 | 244962
25033424 | 244963 (5 rows)

I can express it algorithmically :

Foreach (element e in A)   //Global Query 
  Find_Peer(e)             // Query 1

UPDATE: I'm not in syntax. I am trying to get adapted to it. but it should be something like that.

CREATE OR REPLACE FUNCTION PointPeer(elem integer) RETURNS text AS $$
Select A.column1, B.column1 from A,B where  (A.column1 -B.column1)=1 where  A.ID=elem limit 1::text
$$ LANGUAGE sql;

Select A.ID, PointPeer(A.ID) from A limit 5;

I still struggling with the Syntax.