PostgreSQL 9.6使用基于列总和的case-when子句

Question

Here is create and insert script:

CREATE TABLE numbers (
  id integer NOT NULL,
  number1 integer NOT NULL
);

INSERT INTO numbers (id, number1) VALUES
  ('1', '1'),
  ('2', '1'),
  ('1', '2'),
  ('1', '3');

if the sum of number1 column is even return its average, other wise return 0. So it should return [id:1, number1:0] for above example

I have tried this:

select id,
  case
    when sum(number1) %2=1 then 0 else avg(number1)
  end as number1
from numbers

it works fine with mySql but not for PostgreSQL (I used http://sqlfiddle.com to try it out). error message in PostgreSQL is

 ERROR: column "numbers.id" must appear in the GROUP BY clause or be used in an aggregate function

Answer 1

Why the expected value of id is 1 and not 2 ? Is this defined by the query in any way? MySql does not care about it, in contrary to Postgres (and other well known RDBMs).

If the value of id is negligible:

select
    case
        when sum(number1) %2 = 1 then 0 
        else avg(number1)
    end as number1
from numbers;

 number1 
---------
       0
(1 row) 

If you want to get eg the smallest value of id :

select
    min(id) as id,
    case
        when sum(number1) %2 = 1 then 0 
        else avg(number1)
    end as number1
from numbers;

 id | number1 
----+---------
  1 |       0
(1 row)