获取每个组SQL的第一行

Question

我有如下数据。我想从以下数据中获得每种性别的首选

subjectID <- c("1", "2", "1", "0", "1", "0", "0", "1", "0", "2", 
       "0", "0", "2", "2","2","1","2","1","0","2")
gender <- c("M", "M", "F", "M", "M", "F", "M", "M", "M", "F", 
    "M", "F", "M", "M", "F","M", "F", "M", "F",  "F")

 selection <- data.frame(subjectID, gender)
       subjectID <- c("1", "2", "0")
       subject <- c("Maths", "Music",  "English")

    subjects <- data.frame(subjectID, subject)

我尝试按如下降序映射选项：

 favourite <- sqldf("SELECT  a.gender, b.subject, COUNT(a.subjectID) as `no of selections`
              FROM selection a
              JOIN subjects b
              ON (a.subjectID  = b.subjectID )
              GROUP BY a.subjectID, a.gender
              ORDER BY a.gender, `no of selections` DESC
              ")

但是，我想获得下表，其中我是每种性别的首选：

gender <- c("F", "M")
subjects <- c("Music", "Maths")
mostfav <- data.frame(gender, subjects)

Answer 1

如果我理解正确，则可以在SQL中使用窗口函数：

SELECT gs.*
FROM (SELECT s.gender, su.subject, COUNT(*) as cnt,
             ROW_NUMBER() OVER (PARTITION BY s.gender ORDER BY COUNT(*) DESC) as seqnum
      FROM selection s JOIN
           subjects su
           ON su.subjectID  = s.subjectID
      GROUP BY s.gender, su.subject
     ) gs
WHERE seqnum = 1;

Answer 2

[如果您正在运行MySQL 8.0，则可以在子查询中使用RANK()对每个性别按主题计数对记录进行排名，并在外部查询中按组的顶部记录进行过滤（如果存在顶部关系，则[C0 ]保留它们）：

RANK()

在较早的版本中，没有窗口功能，一种选择是使用

SELECT gender, subject, no_of_selections
FROM (
    SELECT  
        se.gender, 
        su.subject, 
        COUNT(*) as no_of_selections,
        RANK() OVER(PARTITION BY se.gender ORDER BY COUNT(*) DESC) rn
    FROM selection se
    JOIN subjects su ON se.subjectID  = su.subjectID
    GROUP BY se.subjectID, se.gender, su.subject
) t
WHERE rn = 1
ORDER BY gender DESC

子句进行过滤，该子句返回按性别返回的最高计数：

having

注意：

• 我更改了表别名以使其更有意义

• 您应该在

  SELECT  
      se.gender, 
      su.subject, 
      COUNT(*) as no_of_selections
  FROM selection se
  JOIN subjects su ON se.subjectID  = su.subjectID
  GROUP BY se.subjectID, se.gender, su.subject
  HAVING COUNT(*) = (
      SELECT COUNT(*) 
      FROM selection se1
      WHERE se1.gender = se.gender
      GROUP BY se1.subjectID, se1.gender
      ORDER BY COUNT(*) DESC
      LIMIT 1
  )

  子句中使用subject列，以使您的查询在sql模式GROUP BY下运行，该模式默认情况下从MySQL 5.7开始启用]]]