Postgres 13

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CREATE TABLE mytable
    (item varchar(5), size varchar(6), cnt int)
;
    
INSERT INTO mytable VALUES
    ('A', NULL, 20),
    ('A', 'M', 5),
    ('A', 'XL', 10),
    ('B', NULL, 12),
    ('B', 'M', 8);
    
SELECT * FROM mytable;
5 rows affected



item
size
cnt


A
null
20

A
M
5

A
XL
10

B
null
12

B
M
8


SELECT
    *,
    SUM(cnt) FILTER (WHERE size IS NULL) OVER (PARTITION BY item),
    COUNT(cnt) FILTER (WHERE size IS NOT NULL) OVER (PARTITION BY item)
FROM
    mytable


item
size
cnt
sum
count


A
null
20
20
2

A
M
5
20
2

A
XL
10
20
2

B
null
12
12
1

B
M
8
12
1


SELECT 
    item,
    size,
    cnt,
    (sum / count)
FROM (
    SELECT
        *,
        SUM(cnt) FILTER (WHERE size IS NULL) OVER (PARTITION BY item),
        COUNT(cnt) FILTER (WHERE size IS NOT NULL) OVER (PARTITION BY item)
    FROM
        mytable
) s


item
size
cnt
?column?


A
null
20
10

A
M
5
10

A
XL
10
10

B
null
12
12

B
M
8
12


SELECT 
    item,
    size,
    cnt + (sum / count)
FROM (
    SELECT
        *,
        SUM(cnt) FILTER (WHERE size IS NULL) OVER (PARTITION BY item),
        COUNT(cnt) FILTER (WHERE size IS NOT NULL) OVER (PARTITION BY item)
    FROM
        mytable
) s


item
size
?column?


A
null
30

A
M
15

A
XL
20

B
null
24

B
M
20


SELECT 
    item,
    size,
    cnt + (sum / count)
FROM (
    SELECT
        *,
        SUM(cnt) FILTER (WHERE size IS NULL) OVER (PARTITION BY item),
        COUNT(cnt) FILTER (WHERE size IS NOT NULL) OVER (PARTITION BY item)
    FROM
        mytable
) s
WHERE size IS NOT NULL


item
size
?column?


A
M
15

A
XL
20

B
M
20