是否可以仅对一个变量使用 DISTINCT 语句？

Question

我只想要 'yearLevel' 上的 SELECT DISTINCT 语句（因为我不想只在 yearLevel 上出现任何重复）

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例如，我只想要表中的一条 'yearLevel' 12 记录，并且它是具有最低 'totalSeconds' 的记录

code

$sql = "SELECT firstName, lastName, yearLevel, totalSeconds 

FROM studentInformation
  JOIN record
    ON studentInformation.studentId = record.student_Id

    ORDER BY totalSeconds ASC 
    LIMIT  1  "; 

这样可以吗-

$sql = "SELECT firstName, lastName, DISTINCT yearLevel, totalSeconds 

    FROM studentInformation
      JOIN record
        ON studentInformation.studentId = record.student_Id

        ORDER BY totalSeconds ASC 
        LIMIT  1  "; 

Answer 1

如果“firstName”和“lastName”在表“studentInformation”中，并且“yearLevel”和“totalSeconds”在表“record”中，则此查询应该可以工作。它使用相关子查询。我没有对此进行测试；如果它不起作用，请告诉我。

    SELECT a.firstName, a.lastName, b.yearLevel, b.totalSeconds
      FROM studentInformation a
INNER JOIN record b ON a.studentId = b.studentId
     WHERE b.totalSeconds = ( SELECT min(totalSeconds)
                                FROM record
                               WHERE yearLevel = b.yearLevel )
  ORDER BY b.totalSeconds ASC

假设只有“totalSeconds”在表“记录”中，这也可以工作。

    SELECT a.firstName, a.lastName, a.yearLevel, b.totalSeconds
      FROM studentInformation a
INNER JOIN record b ON a.studentId = b.studentId
     WHERE b.totalSeconds = ( SELECT MIN(d.totalSeconds)
                                FROM studentInformation c
                          INNER JOIN record d ON c.studentId = d.studentId
                               WHERE c.yearLevel = a.yearLevel )
  ORDER BY b.totalSeconds ASC