MySQL 8.0

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CREATE TABLE user (
  `username` VARCHAR(5),
  `colb` VARCHAR(1),
  `colc` VARCHAR(1),
  `etc` VARCHAR(1)
);

INSERT INTO user
  (`username`, `colb`, `colc`, `etc`)
VALUES
  ('user1', '*', '*', '*'),
  ('user2', '*', '*', '*'),
  ('user3', '*', '*', '*');

Records: 3  Duplicates: 0  Warnings: 0


CREATE TABLE purchase (
  `purchaseID` INTEGER,
  `storeID` INTEGER,
  `username` VARCHAR(5),
  `cost` VARCHAR(1)
);

INSERT INTO purchase
  (`purchaseID`, `storeID`, `username`, `cost`)
VALUES
  ('1', '1', 'user1', '*'),
  ('2', '1', 'user2', '*'),
  ('3', '5', 'user2', '*'),
  ('4', '3', 'user1', '*');

Records: 4  Duplicates: 0  Warnings: 0


CREATE TABLE store (
  `storeID` INTEGER,
  `storeName` VARCHAR(6),
  `etc` VARCHAR(1)
);

INSERT INTO store
  (`storeID`, `storeName`, `etc`)
VALUES
  ('1', 'store1', '*'),
  ('2', 'store2', '*'),
  ('3', 'store3', '*');

Records: 3  Duplicates: 0  Warnings: 0


with 
purchases as 
(
    select    storeID
             ,count(distinct username) as purchase
    from      purchase
    group by  storeID
),
users as
(
    select    count(*) as total_users
    from      user
)
select storeID
      ,storeName
      ,coalesce(purchase, 0)               as purchase
      ,total_users - coalesce(purchase, 0) as nonPurchases
      
from   store 
       left join   purchases using (storeID)
       cross join  users


storeID
storeName
purchase
nonPurchases


1
store1
2
1

2
store2
0
3

3
store3
1
2