是否可以只对一个变量使用 DISTINCT 语句？

Question

我只想在 'yearLevel' 上使用 SELECT DISTINCT 语句（因为我不希望只在 yearLevel 上有任何重复项）

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例如，我只想要 'yearLevel' 12 条记录在 table 中，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  ";

是否可以这样做 -

$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

前提是"firstName"和"lastName"在table"studentInformation"，"yearLevel"和"totalSeconds"在[=24] =] "record" 这个查询应该有效。它使用相关子查询。我没有测试这个；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 table "record" 中，这也可行。

    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

是否可以只对一个变量使用 DISTINCT 语句？

Is it possible to have a DISTICT statement only on one varible?

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mysql

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distinct