在 SQL 中选择最大值的问题答案

【问题标题】：problems selecting max of an avg in SQL在 SQL 中选择最大值的问题
【发布时间】：2018-04-14 16:43:24
【问题描述】：

我不明白为什么我在 SQL 中选择具有 max avg 的记录时遇到了一个愚蠢的问题。

有问题的表是由这个查询生成的：

   SELECT *
   FROM (SELECT MATRDOC AS TEACHERID
         FROM DOCENTE
         WHERE MATRDOC NOT IN (SELECT MATRDOC
                               FROM CORSO
                               WHERE AREA <> 'BASI DI DATI')) DOC,
        (SELECT C.CODCORSO AS COURSEID, 
                MATRDOC AS TEACHERID, 
                AVG(NUMSTUDENTI) AS GVA
        FROM CORSO C, LEZIONE L
        WHERE C.CODCORSO = L.CODCORSO
        GROUP BY C.CODCORSO, MATRDOC) L
   WHERE DOC.TEACHERID = L.TEACHERID;

这个输出：

TEACHERID   COURSEID    GVA
S12346      C9          40.3333
S12346      C3          213
S12346      C11         88
S12348      C7          123
S12348      C5          79
S12348      C13         82.5

现在我想获取最大'GVA'的TEACHERID和COURSEID，所以我在查询末尾添加了：

   GROUP BY TEACHERID
   HAVING MAX(GVA)

但它不起作用，我获得了这些值：

TEACHERID   COURSEID    GVA
S12346      C9          40.3333
S12348      C7          123

【问题讨论】：

你想要什么结果？你用的是什么数据库？
每个teacherid是1行还是1行？
每个teacherid 1行，我在底部做了一点解释：stackoverflow.com/a/49833871/2354957

标签： mysql group-by max average

【解决方案1】：

平均学生人数是“GVA”，这似乎很奇怪，但没关系。

在 ANSI SQL 中，您可以使用 fetch first 子句：

SELECT C.CODCORSO AS COURSEID, MATRDOC AS TEACHERID, AVG(NUMSTUDENTI) AS GVA
FROM CORSO C JOIN
     LEZIONE L
     ON C.CODCORSO = L.CODCORSO
GROUP BY C.CODCORSO, MATRDOC
ORDER BY GVA DESC
FETCH FIRST 1 ROW ONLY;

并非所有数据库都支持FETCH FIRST。常见的替代品包括SELECT TOP 和LIMIT。

编辑：

如果您想要每位教师的最大值，请使用ROW_NUMBER()：

SELECT *
FROM (SELECT C.CODCORSO AS COURSEID, MATRDOC AS TEACHERID, AVG(NUMSTUDENTI) AS GVA,
             ROW_NUMBER() OVER (PARTITION BY TEACHERID ORDER BY AVG(NUMSTUDENTI) DESC) as seqnum
      FROM CORSO C JOIN
           LEZIONE L
           ON C.CODCORSO = L.CODCORSO
      GROUP BY C.CODCORSO, MATRDOC
     ) cl
WHERE seqnum = 1
ORDER BY GVA DESC;

这都是 ANSI 标准语法，包括正确使用 JOIN。

【讨论】：

【解决方案2】：

如果我正确理解了您的问题，我认为这应该可行

   SELECT teacherid, courseid, max(gva) ------*****changed
   FROM (SELECT MATRDOC AS TEACHERID
         FROM DOCENTE
         WHERE MATRDOC NOT IN (SELECT MATRDOC
                               FROM CORSO
                               WHERE AREA <> 'BASI DI DATI')) DOC,
        (SELECT C.CODCORSO AS COURSEID, 
                MATRDOC AS TEACHERID, 
                AVG(NUMSTUDENTI) AS GVA
        FROM CORSO C, LEZIONE L
        WHERE C.CODCORSO = L.CODCORSO
        GROUP BY C.CODCORSO, MATRDOC) L
   WHERE DOC.TEACHERID = L.TEACHERID
   group by teacherid;  -----*********added

【讨论】：