Add missing values in each group using presto or mysql

I have a following table :

prod_name - quantity - group
tv        - 30  - base 
microwave - 10  - base 
watch     - 5   - base  
phone     - 25  - base 
washer    - 15  - base 
dryer.    - 14  - base  
microwave - 7   - inventory_2021 
phone     - 16  - inventory_2021  
tv        - 30  - inventory_2022 
watch     - 5   - inventory_2022 
phone     - 25  - inventory_2022 

I want to fill/ enter missing records from base(group) to all other groups

Expected output :

tv        - 30  - base 
microwave - 10  - base 
watch     - 5   - base 
phone     - 25  - base 
washer    - 15  - base 
dryer.    - 14  - base 
tv        - 30  - inventory_2021 
microwave - 7   - inventory_2021 
watch     - 5   - inventory_2021 
phone     - 16  - inventory_2021 
washer    - 15  - inventory_2021 
dryer.    - 14  - inventory_2021 
tv        - 30  - inventory_2022 
microwave - 10  - inventory_2022 
watch     - 5   - inventory_2022 
phone     - 25  - inventory_2022 
washer    - 15  - inventory_2022 
dryer.    - 14  - inventory_2022 

So in the output I have same number of entries in each group and total number of entries should match the base group.

Use CROSS JOIN to create all the missing rows. Then you can LEFT JOIN this with the original table to fill in the gaps.

WITH all_prods_and_groups AS (
    SELECT p1.prod_name, p1.quantity, p2.group
    FROM products AS p1
    CROSS JOIN (
        SELECT DISTINCT `group`
        FROM products
    ) AS p2
    WHERE p1.group = 'base'
)
SELECT t1.prod_name, 
    IFNULL(t2.quantity, t1.quantity) AS quantity, 
    IFNULL(t2.group, t1.group) AS `group`
FROM all_prods_and_groups AS t1
LEFT JOIN products AS t2 ON t1.prod_name = t2.prod_name AND t1.group = t2.group

DEMO

MySQL and Presto/Trino are two a bit SQL different dialects. The basic idea is the same - build the "base" cartesian product of products and groups. If you are sure that all products are present in base you can go simply with the following for Presto/Trino:

-- sample data
WITH dataset(prod_name, quantity, "group") as (
    values ('tv'       , 30, 'base'),
        ('microwave', 10, 'base'),
        ('watch'    , 5 , 'base'),
        ('phone'    , 25, 'base'),
        ('washer'   , 15, 'base'),
        ('dryer'   , 14, 'base'),
        ('microwave', 7 , 'inventory_2021'),
        ('phone'    , 16, 'inventory_2021'),
        ('tv'       , 30, 'inventory_2022'),
        ('watch'    , 5 , 'inventory_2022'),
        ('phone'    , 25, 'inventory_2022')
),
-- query parts
base_products AS (
    SELECT prod_name, quantity
    FROM dataset AS p1
    WHERE "group" = 'base' -- select all base products
),
all_groups as (
    SELECT distinct "group" grp
    FROM dataset
),
base_all as(
    SELECT b.prod_name, b.quantity, grp -- build base lookup table
    FROM base_products b
    CROSS JOIN all_groups
)

select b.prod_name,
    coalesce(d.quantity, b.quantity) quantity, -- use "original" quantity
    b.grp "group"
from base_all b
left join dataset d on b.prod_name = d.prod_name and b.grp = d."group"
;

Output: