使用jpa进行两表联查时总会有字段名相同，所以需要用别名进行区分；
例子：
department表同时包含子级id和父级id：
查询语句为：
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select d.id,d.name,d.description,d.parent_id,temp.name
as parent_name
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from department d,department temp
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where d.parent_id=?
1
and
if(d.parent_id=
-1,d.id=temp.id,d.parent_id=temp.id)
此时希望同时获取部门信息、部门的父级id和父级部门名称，但不希望在department表在新建一列parent_name，所以我们可以使用@Transient注解表示该属性并非是一个要映射到数据库表中的字段；
但使用@Transient注解需要注意：
1、直接将@Transient注解直接加在parentName属性上，数据库不会创建parent_name字段，上述查询也无法获取值；
2、将@Transient注解加在parentName属性的get方法上，数据库会创建parent_name字段，上述查询可以获取值；
3、将@Transient注解加在parentName属性的set方法上，数据库不会创建parent_name字段，上述查询可以获取值；
关键代码如下：
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package cn.test.bean;
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import java.util.List;
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import javax.persistence.Entity;
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import javax.persistence.GeneratedValue;
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import javax.persistence.GenerationType;
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import javax.persistence.Id;
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import javax.persistence.Table;
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import javax.persistence.Transient;
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/**
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* 部门实体类
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*
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*/
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@Entity
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@Table(name=
"department")
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public
class Department {
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@Id
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@GeneratedValue(strategy=GenerationType.IDENTITY)
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private Long id;
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private String name;
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private Long parentId;
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private String description;
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-
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private String parentName;
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@Transient
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private List<Department> childrens;
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public Long getId() {
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return id;
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}
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public void setId(Long id) {
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this.id = id;
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}
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public String getName() {
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return name;
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}
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public void setName(String name) {
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this.name = name;
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}
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public Long getParentId() {
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return parentId;
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}
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public void setParentId(Long parentId) {
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this.parentId = parentId;
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}
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public String getDescription() {
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return description;
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}
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public void setDescription(String description) {
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this.description = description;
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}
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public String getParentName() {
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return parentName;
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}
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@Transient
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public void setParentName(String parentName) {
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this.parentName = parentName;
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}
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public List<Department> getChildrens() {
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return childrens;
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}
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public void setChildrens(List<Department> childrens) {
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this.childrens = childrens;
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}
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}
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package cn.test.dao;
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import java.util.List;
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import org.springframework.data.jpa.repository.JpaRepository;
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import org.springframework.data.jpa.repository.Query;
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import cn.test.bean.Department;
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public
interface DepartmentRepository extends JpaRepository<Department, Long>{
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@Query(value=
"select d.id,d.name,d.description,d.parent_id,temp.name as parent_name " +
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"from department d,department temp " +
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"where d.parent_id=?1 and if(d.parent_id=-1,d.id=temp.id,d.parent_id=temp.id)",nativeQuery=
true)
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public List<Department> testAs(Long parentId);
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}
原文地址：https://blog.csdn.net/woshihm/article/details/93721300
