How to compute the sum of multiple columns in PostgreSQL

Question

SELECT COALESCE(col1,0) + COALESCE(col2,0)
FROM yourtable

It depends on how you'd like to sum the values. If I read your question correctly, you are looking for the second SELECT from this example:

template1=# SELECT * FROM yourtable ;
 a | b 
---+---
 1 | 2
 4 | 5
(2 rows)

template1=# SELECT a + b FROM yourtable ;
 ?column? 
----------
        3
        9
(2 rows)

template1=# SELECT SUM( a ), SUM( b ) FROM yourtable ;
 sum | sum 
-----+-----
   5 |   7
(1 row)

template1=# SELECT SUM( a + b ) FROM yourtable ;
 sum 
-----
  12
(1 row)

template1=# 

Combined the current answers and used this to get total SUM:

SELECT SUM(COALESCE(col1,0) + COALESCE(col2,0)) FROM yourtable;