我是JPA的新手,当我尝试使用MAX()函数查询数据库时遇到问题.
我的功能代码如下.谁能帮我？谢谢.
我的功能代码如下.谁能帮我？谢谢.
public int getMaxId(){
entityManager = this.entityManagerFactory.createEntityManager();
Query query = entityManager.createquery("SELECT * FROM user WHERE id = (SELECT MAX(u.id) FROM user u)");
User user = (User) query.getSingleResult();
int id = user.getId();
return id;
}
我正在使用JPA,TopLink和Apache Derby.我的方法应该返回表用户的最大id.
编辑：我从服务中调用该函数：
try {
int id = userDAO.getMaxId();
logger.info("Max id: " + id);
user.setId(id+1);
}
catch (Exception ex){
logger.error("Unable to get the max id.");
}
user.setId()的值始终为“0”.
编辑(2)：记录
Caused by: Exception [EclipseLink-8034] (Eclipse Persistence Services - 2.3.0.v20110604-r9504): org.eclipse.persistence.exceptions.JPQLException
Exception Description: Error compiling the query [SELECT u FROM user u WHERE u.id = (SELECT MAX(uu.id) FROM user uu)]. UnkNown entity type [user].
at org.eclipse.persistence.exceptions.JPQLException.entityTypeNotFound(JPQLException.java:483)
at org.eclipse.persistence.internal.jpa.parsing.ParseTreeContext.classForSchemaName(ParseTreeContext.java:138)
at org.eclipse.persistence.internal.jpa.parsing.SelectNode.getClassOfFirstvariable(SelectNode.java:327)
at org.eclipse.persistence.internal.jpa.parsing.SelectNode.getReferenceClass(SelectNode.java:316)
at org.eclipse.persistence.internal.jpa.parsing.ParseTree.getReferenceClass(ParseTree.java:436)
at org.eclipse.persistence.internal.jpa.parsing.ParseTree.adjustReferenceClassForQuery(ParseTree.java:75)
at org.eclipse.persistence.internal.jpa.parsing.JPQLParseTree.populateReadQueryInternal(JPQLParseTree.java:103)
at org.eclipse.persistence.internal.jpa.parsing.JPQLParseTree.populateQuery(JPQLParseTree.java:84)
at org.eclipse.persistence.internal.jpa.EJBQueryImpl.buildEJBQLDatabaseQuery(EJBQueryImpl.java:219)
at org.eclipse.persistence.internal.jpa.EJBQueryImpl.buildEJBQLDatabaseQuery(EJBQueryImpl.java:190)
at org.eclipse.persistence.internal.jpa.EJBQueryImpl.<init>(EJBQueryImpl.java:142)
at org.eclipse.persistence.internal.jpa.EJBQueryImpl.<init>(EJBQueryImpl.java:126)
at org.eclipse.persistence.internal.jpa.EntityManagerImpl.createquery(EntityManagerImpl.java:1475)
... 35 more
我的实体User声明如下：
@Entity
@Table(name = "user")
public class User {
@Id
private int id;
private String name;
private String lastName;
private String city;
private String password;
解决方法
您可以直接使用更简单的JPQL
return (Integer)entityManager.createquery("select max(u.id) from User u").getSingleResult();
或者使用TypedQuery
return entityManager.createquery("select max(u.id) from User u",Integer.class).getSingleResult();
编辑：
UnkNown entity type [user]
您应该使用User而不是user.