MySQL错误的结果使用MIN（）函数

Question

问题：

在这个例子中，我们有来自学校的5名学生的成绩1.我们想知道哪个学生的成绩最低。

我们期待获得4号学生，但SQL返回学生1

有人能帮助我吗？ 提前致谢

表1：

CREATE TABLE `table1` (
  `school_id` int(11) unsigned NOT NULL,
  `student_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `grade` int(11) unsigned NOT NULL,
  PRIMARY KEY (`student_id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;

数据：

INSERT INTO `table1` (`school_id`, `student_id`, `grade`)
VALUES
    (1, 1, 20),
    (1, 2, 15),
    (1, 3, 18),
    (1, 4, 12),
    (1, 5, 15);

SQL查询：

SELECT t1.`school_id`, t1.`student_id`, MIN(t1.grade)
FROM table1 as t1
WHERE t1.`school_id`=1
GROUP BY t1.`school_id`;

PRINTSCREEN：

Answer 1

SELECT * FROM table1 ORDER BY grade LIMIT 1

如果你想要每所学校表现最差的学生，那就是......

SELECT x.* 
  FROM table1 x
  JOIN 
     ( SELECT school_id
            , MIN(grade) grade 
         FROM table1
        GROUP
           BY school_id
     ) y
    ON y.school_id = x.school_id
   AND y.grade = x.grade;

http://sqlfiddle.com/#!9/f44cb2/1

Answer 2

有了@ tadman的提示，我们提出了一个解决方案：

如果你遇到同样的问题，你可以找到它。

我们不明白为什么必须使用限制。如果我们取出限制行，我们会得到错误的结果

SELECT t2.`school_id`, t2.`student_id`, t2.grade
FROM 
    (
    SELECT t1.`school_id`, t1.`student_id`, t1.grade
    FROM table1 as t1
    WHERE t1.`school_id`=1
    ORDER BY t1.`grade` ASC
    limit 4294967295
    )
as t2
GROUP BY t2.`school_id`;

Answer 3

除非对您的问题有更多要求，否则我猜您只会：

select t1.school_id, t1.student_id, t1.grade
from table1 as t1,
(select school_id, min(grade) as grade from table1 group by school_id) as t2
where t1.school_id=t2.school_id
and t1.grade=t2.grade;