JPA連接查詢的問題

Question

我與JPA查詢實驗，但保持在下面的查詢得到一個錯誤

SELECT p 
FROM usertype_permissions up 
JOIN permissions p ON up.PERMISSION_ID = p.ID 
WHERE up.USERTYPE_ID = 1 

錯誤：

Caused by: java.lang.IllegalArgumentException: An exception occurred while 
creating a query in EntityManager: 

Exception Description: Syntax error parsing the query 
[SELECT p 
FROM usertype_permissions up 
JOIN permissions p ON up.PERMISSION_ID = p.ID 
WHERE up.USERTYPE_ID = 1]. 

Answer 1

在JPQL你必須使用實體名稱（默認情況下它是Java類的名稱），對我來說它看起來很奇怪，usertype_permissions是一個實體名稱/ Java類...

它看起來像你使用的表名和加入後的單詞必須引用例如類的屬性l.books

例子：

@Entity 
public class Lib implements Serializable { 
     @OneToMany 
     private Set<Book> books; 
} 

@Entity 
public class Book implements Serializable { 
     @OneToOne 
     private Lib lib; 
} 

JPQL查詢應該是這樣的：

SELECT l FROM Lib JOIN l.books books