SQL count with join - sql

Question

Total novice. Trying this again.
2 Tables Biz and Users
Business has IdNum, created_at, account_type, business_name
Users has IdNum, country, first_name, last_name
Question: How many total businesses are from Japan?
I know I need to use inner join.

Answer 1

I made some assumptions and created this example.
First I created two tables, db_users and db_partners and
inserted some sample data. I am assuming "Users" are
sales managers for the "Partners" and that each partner
is assigned one user. Users can have multiple partners.
It is strange that "Country" is an attribute of Users and
not Partners, but that was how I interpreted the example.
MariaDB [test_time]> create table db_users (
UserID int unsigned not null auto_increment primary key,
UserName varchar(20),
Country varchar(8)
);
MariaDB [test_time]> create table db_partners (
PartnerID int unsigned not null auto_increment primary key,
PartnerName varchar(20),
Created datetime,
Size int unsigned,
UserID int unsigned
);
MariaDB [test_time]> insert into db_users
(UserName,Country)
values
('Abel','CA'),
('Baker','CA'),
('Charlie','JP'),
('Donald','JP'),
('Edgar','JP')
;
MariaDB [test_time]> insert into db_partners
(PartnerName,Created,Size,UserID)
values
('Kraft',now(),45,1),
('Ford',now(),66,2),
('Hortons',now(),22,1),
('Kroger',now(),15,4)
;
Then I selected the partners where the associated user was in CA:
MariaDB [test_time]> select
UserName,PartnerName
from
db_users join db_partners using (UserID)
where
Country='CA'
;
+----------+-------------+
| UserName | PartnerName |
+----------+-------------+
| Abel | Kraft |
| Abel | Hortons |
| Baker | Ford |
+----------+-------------+
3 rows in set (0.00 sec)
MariaDB [test_time]> select
count(Country)
from
db_users join db_partners using (UserID)
where
Country='CA'
;
+----------------+
| count(Country) |
+----------------+
| 3 |
+----------------+
1 row in set (0.00 sec)
I am not sure this is what you wanted. If not, please clarify your
question.

Answer 2

If IdNum is your foreign key (join condition between your tables) just use
SELECT count(*)
FROM Business, Users
WHERE Business.IdNum = Users.IdNum
AND Business.country = 'Japan';

Answer 3

If you wanted the total count of all businesses from Canada (assuming you can join from the ID field in Users to the ID field in Biz), counting each distinct business only once-
select count(distinct u.country) --will only count unique entries once
from Biz b
inner join Users u
on b.ID = u.ID
where u.country = 'Japan'
If you wanted a count of all rows, instead of the unique rows-
select count(u.country) --will count all entries
from Biz b
inner join Users u
on b.ID = u.ID
where u.country = 'Japan'