php - 如何改进这个php mysql代码？

Question

使用这些代码，我回显了 regd 等于 $regd 的学生的 Rank。事实上，这是一个工作代码。但是，有 friend 告诉我，在mysql语句中，Distinct和Group By不应该一起使用。但是作为一个新手，我无法弄清楚如果不使用 Distinct 我将如何实现它，因为它不会返回没有 Distinct 的行。谁能建议我如何改进这些代码？

<?php 
mysql_select_db($database_dbconnect, $dbconnect);
$query_myrank = "SELECT Distinct regd, Name_of_exam, 
                        Name_of_Student, TOTALSCORE, Rank 
                 FROM (SELECT *, IF(@marks = (@marks := TOTALSCORE), 
                              @auto, @auto := @auto + 1) AS Rank 
                       FROM (SELECT Name_of_Student, regd, 
                                   Name_of_exam, SUM(Mark_score) AS TOTALSCORE 
                             FROM cixexam, (SELECT @auto := 0, 
                                                   @marks := 0) AS init 
                                            GROUP BY regd 
                             ORDER BY TOTALSCORE DESC) t) AS result 
                 HAVING (Name_of_exam='First Terminal Exam' OR 
                         Name_of_exam='First Term Test')";

$myrank = mysql_query($query_myrank, $dbconnect) or die(mysql_error());

$i = 0;
$j = 0;
$data = array();
while($row_myrank = mysql_fetch_assoc($myrank))
{
    $data[$i] = $row_myrank;
    if(isset($data[$i - 1]) 
           && $data[$i - 1]['TOTALSCORE'] == $data[$i]['TOTALSCORE'])
    {
       $data[$i]['Rank'] = $j;
    }else{
       $data[$i]['Rank'] = ++$j;
    }
       $i++;
}
foreach($data as $key => $value)
{
    if($value['regd'] == $regd)
    {
        echo $value['Rank'];
    }
}
?>    

Answer 1

Distinct 比 Group By 慢。您可以在不同时使用 Group By 和 Distinct 的情况下这样做，这可能是您想要实现的。

SELECT regd, Roll_no, Name_of_Student, Name_of_exam,
       TOTALSCORE, Rank 
FROM 
(
  SELECT t.*, IF(@p = TOTALSCORE, @n, @n := @n + 1) AS Rank, @p := TOTALSCORE 
    FROM
  (
    SELECT regd, Roll_no, Name_of_Student, Name_of_exam, 
            SUM(Mark_score) TOTALSCORE
    FROM cixexam, (SELECT @n := 0, @p := 0) n 
     WHERE (Name_of_exam='First Terminal Exam' OR Name_of_exam='First Term Test')
     GROUP BY regd 
     ORDER BY TOTALSCORE DESC
  ) t
) r