Three columns - get MAX with SQL

Question

I have table:

Rating:

id | one | two | three
1  | 12  | 3   | 7
2  | 11  | 30  | 3
3  | 8   | 14  | 4

How can i get with SQL MAX values from these fields (one, two, three)? For this example this is 30.

Answer 1

In MySQL you can use the GREATEST Function:

SELECT  MAX(GREATEST(one, two, three))
FROM    T;

Example on SQL Fiddle

Answer 2

SELECT MAX(field) FROM (
    SELECT one AS field FROM table
    UNION 
    SELECT two AS field FROM table
    UNION 
    SELECT three AS field FROM table
) AS t