java 我什么时候应该在 JPA 中使用 @JoinColumn 或 @JoinTable？
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When Should I Use @JoinColumn or @JoinTable with JPA?
提问by Sarah Szabo
@JoinColumngives an Entitya foreign key to another Entitywhereas @JoinTablewill list the relationship between all relationships between EntityA and EntityB. As far as I can tell, they both appear to do similar things. When should I use one or the other?
@JoinColumn给Entity另一个外键，Entity而@JoinTable将列出EntityA 和EntityB之间所有关系之间的关系。据我所知，它们似乎都在做类似的事情。我什么时候应该使用其中一种？
回答by Vlad Mihalcea
Let's say you have an entity Awhich has a @ManyToOneassociation ot an entity B
假设您有一个与实体A有@ManyToOne关联的实体B
@JoinColumnwill define the target table Foreign Key (e.g B_ID) while using the target Entity table (e.g. B).
@JoinColumn将B_ID在使用目标实体表（例如B）时定义目标表外键（例如）。
@Entity
public class A {
private Long id;
@ManyToOne
@JoinColumn(name="B_ID")
private B b;
}
@JoinTablewill use a separate table to hold the relationship between Aand B.
@JoinTable将使用一个单独的表来保存之间的关系A和B。
@Entity
public class A {
private Long id;
@ManyToOne
@JoinTable(
name = "A_B",
joinColumns = @JoinColumn(name = "B_ID"),
inverseJoinColumns = @JoinColumn(name = "A_ID")
)
private B b;
}
This time neither Anor Bcontain any Foreign Key, because there's a separate table (e.g. A_B) to hold the association between Aand B.
这一次没有A，也不B含有任何外键，因为有一个单独的表（例如A_B）持有之间的关联A和B。
回答by Rabin Pantha
@JoinTable stores the id of both the table into a separate table while @JoinColumn stores id of the another table in a new column.
@JoinTable 将两个表的 id 存储到一个单独的表中，而 @JoinColumn 将另一个表的 id 存储在一个新列中。
@JoinTable : This is the default type. Use this when you you need a more normalized database. ie. to reduce redundancy.
@JoinTable ：这是默认类型。当您需要更规范化的数据库时使用此选项。IE。以减少冗余。
@JoinColumn : Use this for better performance as it does not need to join extra table.
@JoinColumn ：使用它可以获得更好的性能，因为它不需要加入额外的表。
回答by mhrsalehi
one important difference: @JoinColumnalways depends upon the contextit is used:
一个重要的区别：@JoinColumn总是 取决于它使用的上下文：
- If the join is for a OneToOne or ManyToOne mapping using a foreign key mapping strategy, the foreign key column is in the table of the
source entity or embeddable.- If the join is for a unidirectional OneToMany mapping using a foreign key mapping strategy, the foreign key is in the table of the target entity.
- If the join is for a ManyToMany mapping or for a OneToOne or bidirectional ManyToOne/OneToMany mapping using a join table, the
foreign key is in a join table.- If the join is for an element collection, the foreign key is in a collection table.
- 如果连接用于使用外键映射策略的 OneToOne 或 ManyToOne 映射，则外键列位于
源实体的表中或可嵌入。- 如果联接用于使用外键映射策略的单向 OneToMany 映射，则外键位于目标实体的表中。
- 如果连接用于多对多映射或使用连接表的单对一或双向多对一/一对多映射，则
外键位于连接表中。- 如果联接用于元素集合，则外键在集合表中。
