how can I join multiple columns on two table in sql

Question

I have two table like below

create table test_user (
        id INT AUTO_INCREMENT PRIMARY KEY,
        name varchar(64)
);

create table test_info(
        id INT AUTO_INCREMENT PRIMARY KEY,
        sellerId INT,
        buyerId INT,
        x varchar,
        y varchar,
        z varchar
);

how can I get all the information in table test_info and the name of the buyer AND the name of seller from the test_user table in the same statement?

select * from test_table join test_user on sellerId = test_user.id;
or 
select * from test_table join test_user on buyerId = test_user.id;

would just give me a seller or buyer but i would like to get the result for both

Answer 1

You must join 2 copies of test_user to test_info :

SELECT ti.*, s.name seller_name, b.name buyer_name
FROM test_info ti
INNER JOIN test_user s ON ti.sellerId = s.id
INNER JOIN test_user b ON ti.buyerId = b.id;

The 1st copy of test_user is used to return the seller's name and the 2nd to return the buyer's name.

Answer 2

In addition to the other answer, you can also try subquerys on SELECT:

SELECT t.*, (SELECT name FROM test_user WHERE id=t.sellerId) sellerName, (SELECT name FROM test_user WHERE id=t.buyerId) buyerName
FROM test_info t;

In this case, I think this would be less efficient, but it's an useful resource that you can use in the future