Php 从三个标记中找出两个最大标记的平均值

我希望从三个系列的分数中平均得到两个最高的分数

以下是我尝试的代码：

if(isset($_POST['find'])){
    $a=$_POST['a'];
    $ins=mysql_query("SELECT * 
                      FROM test1
                      WHERE studid ='$a'
                      ORDER BY mark DESC 
                      LIMIT 2");
    while($r=mysql_fetch_row($ins)){
        echo $r[2];
    }
}

查询应为：

  -- studid=1 top two average marks
  SELECT AVG(A.mark) avgTop2Marks FROM
  (SELECT mark
  FROM test1
  WHERE studid=1
  ORDER BY mark DESC 
  LIMIT 2) A;

  -- studid=1 distinct top two average marks
  SELECT AVG(A.mark) avgDistinctTop2Marks FROM
  (SELECT DISTINCT mark
  FROM test1
  WHERE studid=1
  ORDER BY mark DESC 
  LIMIT 2) A;

---

这里有一个

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