0. 源数组

Array
(
    [0] => Array
        (
            [id] => 1
            [name] => 学生1
            [school] => 学校1
            [class] => 班级1
        )

    [1] => Array
        (
            [id] => 4
            [name] => 学生4
            [school] => 学校2
            [class] => 班级2
        )

    [2] => Array
        (
            [id] => 2
            [name] => 学生2
            [school] => 学校1
            [class] => 班级2
        )

    [3] => Array
        (
            [id] => 3
            [name] => 学生3
            [school] => 学校2
            [class] => 班级1
        )

    [4] => Array
        (
            [id] => 5
            [name] => 学生5
            [school] => 学校2
            [class] => 班级3
        )

)

1. 提取name字段

Array
(
    [0] => 学生1
    [1] => 学生4
    [2] => 学生2
    [3] => 学生3
    [4] => 学生5
)

2. 提取空字段（数据项为null）

array(5) {
  [0]=>
  NULL
  [1]=>
  NULL
  [2]=>
  NULL
  [3]=>
  NULL
  [4]=>
  NULL
}

3. 提取空字段（数据项为array）

array(5) {
  [0]=>
  array(0) {
  }
  [1]=>
  array(0) {
  }
  [2]=>
  array(0) {
  }
  [3]=>
  array(0) {
  }
  [4]=>
  array(0) {
  }
}

4. 提取多个指定字段，以id为主键

Array
(
    [1] => Array
        (
            [id] => 1
            [name] => 学生1
            [school] => 学校1
        )

    [4] => Array
        (
            [id] => 4
            [name] => 学生4
            [school] => 学校2
        )

    [2] => Array
        (
            [id] => 2
            [name] => 学生2
            [school] => 学校1
        )

    [3] => Array
        (
            [id] => 3
            [name] => 学生3
            [school] => 学校2
        )

    [5] => Array
        (
            [id] => 5
            [name] => 学生5
            [school] => 学校2
        )

)

5. 指定两个字段拼接为主键，提取所有字段

Array
(
    [班级1学生1] => Array
        (
            [id] => 1
            [name] => 学生1
            [school] => 学校1
            [class] => 班级1
        )

    [班级2学生4] => Array
        (
            [id] => 4
            [name] => 学生4
            [school] => 学校2
            [class] => 班级2
        )

    [班级2学生2] => Array
        (
            [id] => 2
            [name] => 学生2
            [school] => 学校1
            [class] => 班级2
        )

    [班级1学生3] => Array
        (
            [id] => 3
            [name] => 学生3
            [school] => 学校2
            [class] => 班级1
        )

    [班级3学生5] => Array
        (
            [id] => 5
            [name] => 学生5
            [school] => 学校2
            [class] => 班级3
        )

)

6. 以school为分组，以id为主键，提取name字段

Array
(
    [学校1] => Array
        (
            [1] => Array
                (
                    [name] => 学生1
                )

            [2] => Array
                (
                    [name] => 学生2
                )

        )

    [学校2] => Array
        (
            [4] => Array
                (
                    [name] => 学生4
                )

            [3] => Array
                (
                    [name] => 学生3
                )

            [5] => Array
                (
                    [name] => 学生5
                )

        )

)

7. 以school和class为分组，以id为主键，提取name字段

Array
(
    [学校1] => Array
        (
            [班级1] => Array
                (
                    [1] => Array
                        (
                            [name] => 学生1
                        )

                )

            [班级2] => Array
                (
                    [2] => Array
                        (
                            [name] => 学生2
                        )

                )

        )

    [学校2] => Array
        (
            [班级2] => Array
                (
                    [4] => Array
                        (
                            [name] => 学生4
                        )

                )

            [班级1] => Array
                (
                    [3] => Array
                        (
                            [name] => 学生3
                        )

                )

            [班级3] => Array
                (
                    [5] => Array
                        (
                            [name] => 学生5
                        )

                )

        )

)

8. 无限级重组演示（数据源比较单薄，暂且这么演示）

Array
(
    [学校1] => Array
        (
            [班级1] => Array
                (
                    [学校1] => Array
                        (
                            [班级1] => Array
                                (
                                    [学校1] => Array
                                        (
                                            [班级1] => Array
                                                (
                                                    [1] => Array
                                                        (
                                                            [name] => 学生1
                                                        )

                                                )

                                        )

                                )

                        )

                )

            [班级2] => Array
                (
                    [学校1] => Array
                        (
                            [班级2] => Array
                                (
                                    [学校1] => Array
                                        (
                                            [班级2] => Array
                                                (
                                                    [2] => Array
                                                        (
                                                            [name] => 学生2
                                                        )

                                                )

                                        )

                                )

                        )

                )

        )

    [学校2] => Array
        (
            [班级2] => Array
                (
                    [学校2] => Array
                        (
                            [班级2] => Array
                                (
                                    [学校2] => Array
                                        (
                                            [班级2] => Array
                                                (
                                                    [4] => Array
                                                        (
                                                            [name] => 学生4
                                                        )

                                                )

                                        )

                                )

                        )

                )

            [班级1] => Array
                (
                    [学校2] => Array
                        (
                            [班级1] => Array
                                (
                                    [学校2] => Array
                                        (
                                            [班级1] => Array
                                                (
                                                    [3] => Array
                                                        (
                                                            [name] => 学生3
                                                        )

                                                )

                                        )

                                )

                        )

                )

            [班级3] => Array
                (
                    [学校2] => Array
                        (
                            [班级3] => Array
                                (
                                    [学校2] => Array
                                        (
                                            [班级3] => Array
                                                (
                                                    [5] => Array
                                                        (
                                                            [name] => 学生5
                                                        )

                                                )

                                        )

                                )

                        )

                )

        )

)