Postgres 13

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CREATE TABLE t
    (call_id int, caller_id int, receiver_id int, call_duration int)
;
    
INSERT INTO t
    (call_id, caller_id, receiver_id, call_duration)
VALUES
    (1, 101, 102, 1),
    (2, 101, 103, 2),
    (3, 102, 103, 3),
    (4, 103, 104, 4)
;

CREATE TABLE



INSERT 0 4


select max(total_duration) * 1.0 / count(distinct v.id)
from (select t.*, sum(call_duration) over () as total_duration
      from t
     ) t cross join lateral
     (values (caller_id), (receiver_id)) v(id);


?column?


2.5000000000000000



SELECT 1


1.0

ERROR:  syntax error at or near "1.0"
LINE 1: 1.0
        ^