Postgres 13

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CREATE TABLE influencer_follower_daily (influencer_id int, date date, followers int);
INSERT INTO influencer_follower_daily VALUES
  (1, '2020-05-29', 7361)
, (1, '2020-05-28', 7234)

, (2, '2020-05-29',  82)
, (2, '2020-05-28',  85)

, (3, '2020-05-29', 3434)
, (3, '2020-05-28', 2988)
, (3, '2020-05-27', 2765);
7 rows affected

-- with array_agg() once
SELECT influencer_id, arr[array_upper(arr, 1)] - arr[1]
FROM  (
   SELECT influencer_id, array_agg(followers) AS arr
   FROM  (
      SELECT influencer_id, followers
      FROM   influencer_follower_daily
      WHERE  date >= '2020-05-23'
      AND    date <  '2020-05-30'
      ORDER  BY influencer_id, date
      ) sub1
   GROUP  BY influencer_id
   ) sub2;


influencer_id
?column?


1
127

2
-3

3
669


-- with DISTNCT ON and last_value()
SELECT DISTINCT ON (influencer_id)
       influencer_id
     , last_value(followers) OVER (PARTITION BY influencer_id ORDER BY date
                                   ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) - followers AS growth
FROM   influencer_follower_daily
WHERE  date >= '2020-05-23'
AND    date <  '2020-05-30'
ORDER  BY influencer_id, date;


influencer_id
growth


1
127

2
-3

3
669


CREATE OR REPLACE FUNCTION f_growth(anyarray)
  RETURNS anyelement LANGUAGE SQL IMMUTABLE STRICT PARALLEL SAFE AS
'SELECT $1[array_upper($1, 1)] - $1[1]';

CREATE OR REPLACE AGGREGATE growth(anyelement) (
   SFUNC     = array_append
 , STYPE     = anyarray
 , FINALFUNC = f_growth
 , PARALLEL  = SAFE
);
SELECT influencer_id, growth(followers)
FROM  (
   SELECT influencer_id, followers
   FROM   influencer_follower_daily 
   WHERE  date >= '2020-05-23'
   AND    date <  '2020-05-30'
   ORDER  BY influencer_id, date
   ) z
GROUP  BY influencer_id
ORDER  BY influencer_id;


influencer_id
growth


1
127

2
-3

3
669


SELECT influencer_id, growth(followers ORDER BY date)
FROM   influencer_follower_daily 
WHERE  date >= '2020-05-23'
AND    date <  '2020-05-30'
GROUP  BY 1
ORDER  BY 1;


influencer_id
growth


1
127

2
-3

3
669