SQL查找教师课程查询

Question

我正在尝试让老师FIRSTNAME，LASTNAME和他获得的课程数量。甲骨文数据库。 CLASS表包含以下列：CLASSID、TEACHERID、CLASSNAME当前代码：

SELECT DISTINCT FIRSTNAME, LASTNAME, COUNT(TEACHERID)
FROM PERSON 
INNER JOIN TEACHER T ON PERSON.PERSONID = T.TEACHERID 
INNER JOIN CLASS C ON T.TEACHERID = C.TEACHERID
WHERE T.TEACHERID = C.TEACHERID;

我的错误在哪里？

Answer 1

听起来你需要一个GroupBy：

SELECT FIRSTNAME, LASTNAME, COUNT(T.TEACHERID)
FROM PERSON 
INNER JOIN TEACHER T ON PERSON.PERSONID = T.TEACHERID 
INNER JOIN CLASS C ON T.TEACHERID = C.TEACHERID
GROUP BY LASTNAME,FIRSTNAME

顺便说一句，我最初删除了这个答案，因为这不是进行分组的好方法。 相反，我更喜欢按TEACHERID分组，然后重新连接以获取名称，而不是按名称分组。

我认为这是一个更好的方法：

SELECT FIRSTNAME, LASTNAME, C.NUMCLASSES
FROM PERSON 
INNER JOIN TEACHER T ON PERSON.PERSONID = T.TEACHERID 
INNER JOIN (SELECT TEACHERID, COUNT(CLASSID) AS NUMCLASSES FROM CLASS GROUP BY TEACHERID) C
ON C.TEACHERID=T.TEACHERID

Answer 2

WHERE 子句

后面缺少 GROUP BY 子句

按名字、姓氏分组 --除聚合函数(如 COUNT(中的列之外的所有选定列

Answer 3

也许你可以试试

SELECT FIRSTNAME, LASTNAME, 
(select COUNT(C.TEACHERID) form CLASS C ON T.TEACHERID = C.TEACHERID)
as COURSECOUNT
FROM PERSON 
INNER JOIN TEACHER T ON PERSON.PERSONID = T.TEACHERID