Mysql 在SQL中选择平均值的最大值时出现问题

我不知道为什么在SQL中用max avg选择记录时遇到了一个愚蠢的问题

此查询将生成有问题的表：

   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;

现在，我想用max'GVA'获取TEACHERID和COURSEID，因此我在查询的末尾添加了：

   GROUP BY TEACHERID
   HAVING MAX(GVA)

但它不起作用，我得到以下值：

TEACHERID   COURSEID    GVA
S12346      C9          40.3333
S12348      C7          123

---

学生的平均人数是GVA，这似乎很奇怪，但还行

在ANSI SQL中，可以使用fetch first子句：

并非所有数据库都支持先获取。常见的备选方案包括选择顶部和限制

编辑：

如果您想要每个教师的最大值，请使用行号：

---

这是所有ANSI标准语法，包括正确使用JOIN。

如果我正确理解了您的问题，我认为这应该是可行的

   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

---

你想要什么结果？您使用的是什么数据库？您希望每个教师ID有一行还是一行？每个教师ID有一行，我在底部做了一点解释：

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;

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;

   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