如何使用SQL连接3个表（1个查找）

Question

I am trying to join 3 different tables but having difficulty following the logic in other tutorials.

table_1(user_ID, seller_country_ID)
table_2(user_ID, users_country_ID)
table_3(country_ID, country_Name)

I am trying to create a query that returns sellers country name and the users country name. I can use a join to get the sellers country name, but my logic abilities stop there when it comes to getting the users country name. I have a WHERE clause that further restricts the records returned and that needs to be there.

SELECT tbl1.user_ID, tbl1.seller_country_ID, tbl3.country_Name
FROM table_1 AS tbl1
JOIN table_3 AS tbl3
ON tbl1.seller_country_ID = tbl3.country_ID
WHERE <sales_type> = 1

So, I've got tables 1 and 3, but not sure how to incorporate table 2 to get my results.

Results I am seeking are:

user ID, user_country, seller_country
010101, USA, CANADA

Answer 1

You have to use country look up table join with seller_id and user_id separetly two times, to get user and seller country

create table table_1(user_ID int, seller_country_ID int)
create table table_2(user_ID int, users_country_ID int)
create table table_3(country_ID int, country_Name varchar(50))


insert into table_1 values(1, 100)
insert into table_1 values(2, 101)

insert into table_2 values(1, 200)
insert into table_2 values(2, 201)


insert into table_3 values(100, 'USA')
insert into table_3 values(101, 'China')
insert into table_3 values(200, 'CANADA')
insert into table_3 values(201, 'Japan')

Select table_1.user_ID, uc.country_Name "User Contry", sc.country_Name "Seller Country"
FROM table_1 INNER JOIN table_2 ON table_1.user_ID= table_2.user_ID
INNER JOIN table_3 uc ON table_2.users_country_ID= uc.country_ID
INNER JOIN table_3 sc ON table_1.seller_country_ID= sc.country_ID

OUTPUT

user_ID   User Contry     Seller Country
1        CANADA            USA
2        Japan            China

DEMO SQL FIDDLE

Answer 2

You could do this and join table_3 in twice. Once to get users country and another time to get sellers country.

SELECT
    tbl1.user_id
    , tbl3u.country_name
    , tbl3s.country_name
FROM
    table_1 tbl1
    JOIN table_2 tbl2 ON tbl1.user_id = tbl2.user_id
    JOIN table_3 tbl3s ON tbl3s.country_ID = tbl1.seller_country_id
    JOIN table_3 tbl3u ON tbl3u.country_id = tbl2.users_country_id
WHERE <sales_type> = 1