column("title","id") 与 column("title,class_number","id")

column("title","id") 的结果

Array
(
    [2] => xxxx2
    [3] => xxxx3
)

column("title,class_number","id") 的结果

Array
(
    [2] => Array
        (
            [id] => 2
            [title] => xxxx2
            [class_number] => xxxxyyyy2
        )

    [3] => Array
        (
            [id] => 3
            [title] => xxxx3
            [class_number] => xxxxyyyy3
        )

)

column("id,title","id") 的结果

Array
(
    [2] => xxxx2
    [3] => xxxx3
)