按JPA和PostgreSQL 9.0分組

Question

我正嘗試在JPA查詢中使用該組。假設我有一個類Teacher和一個類Student。 A Teacher可以有更多的Student和Student只能有一個Teacher（一對多）。

以下JPA查詢：

Query q = this.em.createQuery( "SELECT teacher, COUNT(student)" + 
           " FROM StudentJpa student" + 
           " JOIN student.teacher teacher" + 
           " GROUP BY teacher" + 
           " ORDER BY COUNT(student) DESC"); 

生成以下SQL查詢：

select 
     teacherjpa1_.teacher_id as col_0_0_, 
     count(studentjpa0_.id) as col_1_0_, 
     teacherjpa1_.teacher_id as teacher1_0_, 
     teacherjpa1_.name as name0_ 
    from 
     student studentjpa0_ 
    inner join 
     teacher teacherjpa1_ 
      on studentjpa0_.teacher_id=teacherjpa1_.teacher_id 
    group by 
     teacherjpa1_.teacher_id 
    order by 
     count(studentjpa0_.id) DESC 

PostgreSQL的9.0，我得到以下錯誤：

org.postgresql.util.PSQLException: ERROR: column "teacherjpa1_.name" must appear in the GROUP BY clause or be used in an aggregate function

同樣的錯誤沒有按」 t出現在PostgreSQL 9.1中。

任何人都可以解釋我爲什麼嗎？ JPA似乎以錯誤的方式生成組：它應該包含所有Teacher屬性，而不僅僅是id。

這是我的JPA /休眠/ DB的配置，如果必要的話：

<beans xmlns="http://www.springframework.org/schema/beans" 
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
     xmlns:context="http://www.springframework.org/schema/context" 
     xmlns:tx="http://www.springframework.org/schema/tx" 
     xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-3.0.xsd"> 
    <context:property-placeholder location="/WEB-INF/jdbc.properties" /> 

    <bean id="dataSource" class="org.springframework.jdbc.datasource.TransactionAwareDataSourceProxy"> 
     <constructor-arg> 
      <bean class="org.springframework.jdbc.datasource.DriverManagerDataSource"> 
       <property name="driverClassName" value="org.postgresql.Driver" /> 
       <property name="url" value="${db.url}" /> 
       <property name="username" value="${db.username}" /> 
       <property name="password" value="${db.password}" /> 
      </bean> 
     </constructor-arg> 
    </bean> 

    <bean id="jpaAdapter" class="org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter"> 
     <property name="databasePlatform" value="org.hibernate.dialect.PostgreSQLDialect" /> 
     <property name="showSql" value="${db.showSql}" /> 
     <property name="generateDdl" value="${db.generateDdl}" /> 
    </bean> 

    <!-- enabling annotation driven configuration /--> 
    <context:annotation-config /> 
    <context:component-scan base-package="my.package" /> 

    <!-- Instructs the container to look for beans with @Transactional and decorate them --> 
    <tx:annotation-driven transaction-manager="transactionManager" proxy-target-class="true" /> 

    <!-- FactoryBean that creates the EntityManagerFactory --> 
    <bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean"> 
     <property name="jpaVendorAdapter" ref="jpaAdapter" /> 
     <property name="jpaProperties"> 
      <props> 
       <prop key="hibernate.format_sql">true</prop> 
       <prop key="hibernate.hbm2ddl.auto">update</prop> 
      </props> 
     </property> 
     <property name="dataSource" ref="dataSource" /> 
    </bean> 

    <!-- A transaction manager for working with JPA EntityManagerFactories --> 
    <bean id="transactionManager" class="org.springframework.orm.jpa.JpaTransactionManager"> 
     <property name="entityManagerFactory" ref="entityManagerFactory" /> 
    </bean> 
</beans> 

謝謝！

更新 - 解決方案是指定GROUP BY teacher.id, teacher.name而不是GROUP BY teacher，但這並不方便。有更好的解決方案嗎？

Answer 1

該查詢在PostgreSQL版本9.1中變得有效。看起來你在本地使用的是PostgreSQL 9.1版本，而Heroku使用的是更早的版本。

見9.1版本說明：

http://www.postgresql.org/docs/9.1/interactive/release-9-1.html

在該頁面上，根據查詢部分，它說：

Allow non-GROUP BY columns in the query target list when the primary key is specified in the GROUP BY clause (Peter Eisentraut)

The SQL standard allows this behavior, and because of the primary key, the result is unambiguous.

爲了得到這個在早期版本的PostgreSQL添加工作所有表達式從選擇列表中不使用聚合函數到GROUP BY子句。