Mysql : FIND_IN_SET() vs IN()

Orders has this structure:

OrderID     |     attachedCompanyIDs
------------------------------------
   1                     1,2,3
   2                     2,4

And Company has this structure:

CompanyID      |        name
--------------------------------------
    1                 Company 1
    2                 Another Company
    3                 StackOverflow
    4                 Nothing

To get an order's companies names, 

I can do a query as such: SELECT name FROM orders,company WHERE orderID = 1 AND FIND_IN_SET(companyID, attachedCompanyIDs) 

That query works fine, but the following query does not.

 SELECT name FROM orders,company WHERE orderID = 1 AND companyID IN (attachedCompanyIDs) 

Why does the first query work but not the second one?

Explanation :

SELECT  name
FROM    orders,company
WHERE   orderID = 1
        AND companyID IN (attachedCompanyIDs)

attachedCompanyIDs is a scalar value which is cast into INT (type of companyID).

The cast only returns numbers up to the first non-digit (a comma in your case).

Thus,

companyID IN ('1,2,3') ≡ companyID IN (CAST('1,2,3' AS INT)) ≡ companyID IN (1)