FIND_IN_SET() vs IN()

attachedCompanyIDs is one big string, so mysql try to find company in this its cast to integer

when you use where in

so if comapnyid = 1 :

companyID IN ('1,2,3')

this is return true

but if the number 1 is not in the first place

 companyID IN ('2,3,1')

its return false

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)

In PostgreSQL, you could cast the string into array (or store it as an array in the first place):

SELECT  name
FROM    orders
JOIN    company
ON      companyID = ANY (('{' | attachedCompanyIDs | '}')::INT[])
WHERE   orderID = 1

and this would even use an index on companyID.

Unfortunately, this does not work in MySQL since the latter does not support arrays.

You may find this article interesting (see #2):

• 10 things in MySQL (that won’t work as expected)

Update:

If there is some reasonable limit on the number of values in the comma separated lists (say, no more than 5), so you can try to use this query:

SELECT  name
FROM    orders
CROSS JOIN
        (
        SELECT  1 AS pos
        UNION ALL
        SELECT  2 AS pos
        UNION ALL
        SELECT  3 AS pos
        UNION ALL
        SELECT  4 AS pos
        UNION ALL
        SELECT  5 AS pos
        ) q
JOIN    company
ON      companyID = CAST(NULLIF(SUBSTRING_INDEX(attachedCompanyIDs, ',', -pos), SUBSTRING_INDEX(attachedCompanyIDs, ',', 1 - pos)) AS UNSIGNED)

To get the all related companies name, not based on particular Id.

SELECT 
    (SELECT GROUP_CONCAT(cmp.cmpny_name) 
    FROM company cmp 
    WHERE FIND_IN_SET(cmp.CompanyID, odr.attachedCompanyIDs)
    ) AS COMPANIES
FROM orders odr