How to get 2 value from same id and same column? - response (3) by dnoeth

For your example (up to two rows per ID) there's a simple solution:

SELECT ID, Customer, 
  MIN(Book) || COALESCE(',' || MAX(Book), ''), SUM(Price)
FROM tab
GROUP BY 1, 2

But it's probably more complicated and i don't have a clue what you actually want.
 
Dieter