java Hibernate Left Outer Join 问题：加入时需要路径
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Hibernate Left Outer Join problem: path expected on join
提问by Nivas
I have two tables, something like:
Article and ArticleRelevance
They have a one to one relationship, and ArticleRelevance is optional, meaning a Article may (or may not) have a ArticleRelevance.
我有两个表，比如：
Article 和 ArticleRelevance
它们是一对一的关系，ArticleRelevance 是可选的，这意味着文章可能（也可能没有）有一个 ArticleRelevance。
I want to select all articles along with the ArticleRelevance details.
我想选择所有文章以及 ArticleRelevance 详细信息。
With traditional SQL, I will do a outer join on the tables, like
使用传统的 SQL，我将对表进行外部联接，例如
SELECT *
FROM ARTICLE A LEFT OUTER JOIN ARTICLE_RELEVANCE AR ON A.ARTICLE_ID = AR.ARTICLE_ID
I tried a similar query in HQL and got a Exception with message "Path Expected for Join"
我在 HQL 中尝试了一个类似的查询并得到了一个带有消息的异常 "Path Expected for Join"
I am unable to understand this message. What does this mean, and how do I solve it?
我无法理解此消息。这是什么意思，我该如何解决？
(As you probably have already guessed) I am an absolute hibernate newbie.
（您可能已经猜到了）我是一个绝对的休眠新手。
I haveto use HQL. Actually I have an HQL already, joining two or three tables, and I have to add this condition.
我必须使用 HQL。其实我已经有一个HQL，加入了两三个表，我必须添加这个条件。
StringBuffer hql = new StringBuffer();
hql.append(" select new service.orders.Order (order.orderNo, article.articleNo, article.articleName)");
hql.append(" from (Order order join Article article with order.articleNo = article.articleNo) left outer join ArticleRelevance articleRelevance with article.articleNo = articleRelevance.articleNo");
hql.append(" and order.orderNo = "+orderNumber);
In thisforum, someone says this is "missing functionality". True?
在这个论坛中，有人说这是“缺少功能”。真的？
回答by Pascal Thivent
I tried a similar query in HQL and got a Exception with message "Path Expected for Join"
我在 HQL 中尝试了一个类似的查询，并得到了一个消息“Path Expected for Join”的异常
With HQL, a [ LEFT [OUTER] | INNER ] JOINis done along an association path (constructed with an identification variable followed by a dot and an association field). So something like this in your case:
使用 HQL，a[ LEFT [OUTER] | INNER ] JOIN沿着关联路径完成（由标识变量、后跟点和关联字段构成）。所以在你的情况下是这样的：
from Article a left join a.articleRelevance
The keyword OUTER is optional.
关键字 OUTER 是可选的。
And your bigger query becomes:
你更大的查询变成：
select new service.orders.Order(order.orderNo, article.articleNo, article.articleName)
from Order order
join order.article article
left outer join article.articleRelevance
where order.orderNo = :orderNo
Note the use of a named parameter:orderNo(don't concatenate parameters). Use it like this:
注意命名参数的使用:orderNo（不要连接参数）。像这样使用它：
String queryString = "...";
Query q = session.createQuery(queryString);
q.setParameter("orderNo", orderNumber);
List<Order> results = q.list();
References
参考
- Hibernate Core Reference Guide
- Hibernate 核心参考指南
