小编Was*_*our的帖子

我需要编写一个SQL查询,显示学生平均大于55的课程名称

表格:学生,学分,课程,部门

我做了什么 :

SQL> SELECT COURSE_NAME
  2  FROM COURSES
  3  where
  4  (select avg(grade) 
  5  from grades,courses
  6  where   
  7  grades.course_id=courses.course_id)>55;

结果很糟糕!(它显示所有课程)

表格:

 create table DEPARTMENTS
(
DEPARTMENT_ID char(2),
NAME varchar2(20),
HEAD varchar2(20));

 create table COURSES
 (
COURSE_ID char(10), 
COURSE_NAME varchar2(20),
TYPE char(6),
POINTS number(2),
DEPARTMENT_ID char(2));

create table GRADES
(
STUDENT_ID number(3),
COURSE_ID char(10), 
SEMESTER varchar2(10),
TERM char(1), 
GRADE number(3),
GRADE_SEM number(3));

create table STUDENTS   
(
STUDENT_ID number(3),
NAME char(15),
CITY char(15));

我有这个方法从ArrayList 中删除特定的对象P.

这是我的代码:

public void removeProduct(Product p) throws StockException{
        int flag=0;
        for(Product i:StockProducts)
            if(p.getCode()==i.getCode()){
                this.StockProducts.remove(p);
                flag=1;
            }

        if(flag==0){
                StockException e = new StockException("could not remove the Product , PRODUCT IS NOT IN THE STOCK:  ", p);
        throw e;
          }
    }

错误:

Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
    at java.util.ArrayList$Itr.next(Unknown Source)
    at Stock.removeProduct(Stock.java:29)
    at Test.main(Test.java:18)

如果您需要有关我的代码的更多信息,请告诉我

添加方法

public void addProduct(Product p) throws StockException{
        for(Product i:StockProducts)
            if(p.getCode()==i.getCode()){
                        StockException e = new StockException("could not add the Product , CODE ALREADY …