Postgres 13

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CREATE TABLE my_table (
    id integer,
    username character varying NOT NULL,
    startdate timestamptz NOT NULL,
    disposal numeric NOT NULL,
    CONSTRAINT my_table_pkey PRIMARY KEY (id)
) WITH (oids = false);

CREATE TABLE


INSERT INTO my_table (id, username, startdate, disposal) 
VALUES
(1, 'user1',    '2021-03-22 07:00:00+00',   1234),
(2, 'user2',    '2021-03-23 07:00:00+00',   123456),
(3, 'user3',    '2021-04-02 07:00:00+00',   987),
(4, 'user4',    '2021-04-12 07:00:00+00',   456);

INSERT 0 4


SELECT t.username,
       coalesce(m.disposal,0) as disposal
FROM (
  values('user1'),( 'user2'),('user3'),('user4')
) as t (username)
  left join my_table m 
    on t.username=m.username  
   and startdate BETWEEN '2021-03-01 07:00:00+00' AND '2021-03-31 07:00:00+00';


username
disposal


user1
1234

user2
123456

user3
0

user4
0



SELECT 4