DERIVEDTBL关键字的作用？

[treeforest]

我在分组中遇到一个问题：
我有两个表格，一个学生student(sno,sname,sex,sage)，一个选课表sc(sno,cno,grade),其中，sno为学号，cno为课程号,grade为成绩。下面我要查询女生的平均成绩，所用语句为：
SELECT AVG(Grade) AS Girl_Grade FROM
(SELECT Student.Sno, Sname, Cno, Grade, sex
FROM Student, SC
WHERE (Student.Sno = sc.Sno)) GROUP BY Sex
HAVING (sex = '女')

这条语句不对，提示group 附近有语法错误，但是如果这样加入DERIVEDTBL 就可以了，
SELECT AVG(Grade) AS Girl_Grade FROM
(SELECT Student.Sno, Sname, Cno, Grade, sex
FROM Student, SC
WHERE (Student.Sno = sc.Sno)) DERIVEDTBL GROUP BY Sex
HAVING (sex = '女')

请问DERIVEDTBL关键字的作用？

第二个问题：
SELECT AVG(Grade) AS Girl_Grade FROM
(SELECT Student.Sno, Sname, Cno, Grade, sex
FROM Student, SC
WHERE ((Student.Sno = sc.Sno) and sex = '女') )
这个语句不对，提示最后一个括号附近有错误，这是为什么？

谢谢！！

[txlicenhe]

1:是关键字吗？只是一个别名吧？
(SELECT Student.Sno, Sname, Cno, Grade, sex
FROM Student, SC
WHERE (Student.Sno = sc.Sno)) 的一个别名
2:
SELECT AVG(Grade) AS Girl_Grade FROM
(SELECT Student.Sno, Sname, Cno, Grade, sex
FROM Student, SC
WHERE ((Student.Sno = sc.Sno) and sex = '女') ) temp
要在最后给这个查询语句 (SELECT Student.Sno, Sname, Cno, Grade, sex
FROM Student, SC
WHERE ((Student.Sno = sc.Sno) and sex = '女') )
加一个别名

[yujohny]

一、为子查询加个别名就可以了
SELECT AVG(Grade) AS Girl_Grade FROM
(SELECT Student.Sno, Sname, Cno, Grade, sex
FROM Student, SC
WHERE (Student.Sno = sc.Sno)) A GROUP BY Sex
HAVING (sex = '女')