1、outer-join關鍵字(many-to-one的情況)

outer-join關鍵字有3個值，分別是true,false,auto,默認是auto。
true: 表示使用外連接抓取關聯的內容，這里的意思是當使用load(OrderLineItem.class,"id")時，Hibernate只生成一條SQL語句將OrderLineItem與他的父親Order全部初始化。

select * from OrderLineItem o left join Order p on o.OrderId=p.OrderId  where o.OrderLineItem_Id=?

false:表示不使用外連接抓取關聯的內容，當load(OrderLineItem.class,"id")時，Hibernate生成兩條SQL語句，一條查詢OrderLineItem表，另一條查詢Order表。這樣的好處是可以設置延遲加載，此處要將Order類設置為lazy=true。

select * from OrderLineItem o where o.OrderLineItem_Id=?
select * from Order p where p.OrderId=?

auto:具體是ture還是false看hibernate.cfg.xml中的配置

注意：如果使用HQL查詢OrderLineItem，如 from OrderLineItem o where o.id='id'，總是不使用外部抓取，及outer-join失效。

2、outer-join(集合)

由于集合可以設置lazy="true"，所以lazy與outer-join不能同時為true，當lazy="true"時，outer-join將一直是false，如果lazy="false"，則outer-join用法與1同

3、HQL語句會將POJO配置文件中的關聯一并查詢，即使在HQL語句中沒有明確join。

4、In HQL, the "fetch join" clause can be used for per-query specific outer join fetching. One important thing many people miss there, is that HQL queries will ignore the outer-join attribute you specified in your mapping. This makes it possible to configure the default loading behaviour of session.load() and session.get() and of objects loaded by navigating relationship. So if you specify

and then do

MyObject obj = session.createQuery("from MyObject").uniqueResult();

obj.getMySet().iterator().next();

you will still have an additional query and no outer-join. So you must explicily request the outer-join fetching:

MyObject obj = session.createQuery(

"from MyObject mo left join fetch mo.mySet").uniqueResult();

obj.getMySet().iterator().next();

Another important thing to know is that you can only fetch one collection reference in a query. That means you can just use one fetch join. You can however fetch "one" references in addition, as this sample from the Hibernate Docs demonstrates:

from eg.Cat as cat

inner join fetch cat.mate

left join fetch cat.kittens

We have once considered lifting this limitation, but then decided against it, because using more than one fetch-join would be a bad idea generally: The generated ResultSet becomes huge and is a major performance loss.

So alltogether the "fetch join" clause is an important instrument Hibernate users should learn how to leverage, as it allows tuning the fetch behaviour of a certain use case.

5、join fetch 與 join 的區別

如果HQL使用了連接，但是沒有使用fetch關鍵字，則生成的SQL語句雖然有連接，但是并沒有取連接表的數據，還是需要單獨的sql取數據，也就是 select a,b,d...中沒有連接表的字段

6、如果集合被聲明為lazy=true，在HQL中如果顯式的使用 join fetch 則延遲加載失效。

7、在one-to-many的one端顯式設置fecth="join",則無論如何都采取預先抓取（生成一個SQl），延遲加載失效(生成兩個SQL)

8、many-to-one的延遲加載是在配置文件的class標簽設置lazy="true",one-to-many和many-to- many的延遲加載是在set標簽中設置lazy="true"。而one-to-one不只要在calss標簽設置lazy="true",而且要在 one-to-one標簽中設置constrained="true".