Where are join practice questions? Joins are very complicated. I've been trying to fully understand them for years. The only way to do this is practice. I thought this was a good website to practice but most of the questions have many answers that return the same results, since the data is limited. Like this question's answer uses a full outer join. But I get the same result with this query 

select c.cust_name, c.city, o.ord_no, o.ord_date, o.purch_amt
from orders as o 
join customer as c on o.customer_id=c.customer_id


So there's no way I can tell that either both answers are correct, or my answer would produce bad results with different data. 

I'm looking for SQL join practice problems where I'll know if my answers are wrong. SQL version doesn't matter to me.
https://www.hackerrank.com/domains/sql/join

https://www.hackerrank.com/domains/sql/advanced-join

As far as I remember, the solution you give is evaluated on different datasets.

Also you can choose which RDBMS to use: DB2, SQL Server, MySQL, Oracle (unfortunately no PostgreSQL).