Tables :: How To Merge Large Data From Two Different Tables

So I have two tables,

table1: (company_name, company_code, year_month, rating)
table2: (company_name, company_code, year_month, asset, debt, equity,...)

What I would like to do is to call up all the data in table1, and then call up (asset, debt, equity,...) from table2 where the company_code and year-month are equal. What do you think is the best way to do this? Keep in mind a few things:

1. I'm dealing with hundreds of thousands of data lines
2. I may need to get more data similar to table2 in the future and call it up in the same way

I thought I should create a primary key "company_code-year_month"for both tables, but that method doesn't seem like a good one, and if I were to have another table, will have to create the same primary key for that table as well?