小编Lyn*_*ynx的帖子

我创建了这两个实体来证明我的问题:

OwnerEntity.java:

@Entity
public class OwnerEntity {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Size(min = 1)
    @OneToMany(mappedBy = "ownerEntity", cascade = CascadeType.ALL)
    private Set<ChildEntity> childEntities = new HashSet<>();
}

ChildEntity.java:

@Entity
public class ChildEntity {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    @NotNull
    @ManyToOne(optional = false)
    private OwnerEntity ownerEntity;

    public ChildEntity() {
    }

    public Long getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    } …

我已经阅读过ORACLE中带有视图的DML操作规则.他们中的大多数似乎对我很清楚,但我对聚合功能感到困惑.

例如,我有两个表.

               EMPLOYEES
??????????????????????????????????????
?   emp_id  ?   salary    ?  dept_id ?
??????????????????????????????????????
?   2134    ?   2200      ?    10    ?
?   2327    ?   3100      ?    10    ?
?   2428    ?   4100      ?    20    ?
?   2637    ?   1700      ?    30    ?
??????????????????????????????????????
      \ | /
       \|/
        |
        |    DEPARTMENTS
??????????????????????????????????????
? dept_id   ?   dname     ? location ?
??????????????????????????????????????
?    10     ?             ?          ?
?    20     ?             ?          ?
?    30     ?             ?          ?
??????????????????????????????????????

我想要一个按部门显示员工平均工资的视图.所以,我运行这个SQL:

CREATE OR REPLACE VIEW dept_sals AS …