MyBatis學習總結（九）---基於XML多表聯合查詢（一對一、一對多、多對多）
阿新 • • 發佈：2018-12-02
1、一對一的關聯
使用association，association元素用於處理“has-one”（一對一）這種型別關係。
作用：針對pojo物件屬性的對映，它的兩個主要引數此時對應的值： javaType對應pojo類名, property對應pojo的屬性名， 。
示例：
Employee.java
public class Employee implements Serializable{ private int empId; private String empName; private Date empBirthDay; private String empSex; private Department dept; //Getters and Setters //consructor }
Department.java
public class Department {
private int deptId;
private String deptName;
private List<Employee> emps;
//...
}方式一：巢狀結果：使用巢狀結果對映來處理重複的聯合結果的子集，封裝聯表查詢的資料(去除重複的資料)。
根據條件查詢員工，每一個員工對應一個部門
employeeMapper.xml
!-- 一對一關聯，方式一：巢狀結果：使用巢狀結果對映來處理重複的聯合結果的子集 封裝聯表查詢的資料(去除重複的資料) --> <select id="selectEmpsByYears" parameterType="hashmap" resultMap="emp" > SELECT * FROM tb_emp WHERE empBirthDay between #{startTime} and #{endTime} ; </select> <resultMap type="com.mybatisstudy.model.Employee" id="emp"> <id property="empId" column="empId" /> <result property="empName" column="empName"/> <result property="empBirthDay" column="empBirthDay"/> <result property="empSex" column="empSex"/> <association property="dept" javaType="com.mybatisstudy.model.Department"> <result property="deptId" column="deptId"/> </association> </resultMap>
方式二：巢狀查詢：通過執行另外一個SQL對映語句來返回預期的複雜型別
employeeMapper.xml
<select id="selectAllEmp" resultMap="allEmp"> SELECT * FROM tb_emp; </select> <resultMap type="com.mybatisstudy.model.Employee" id="allEmp"> <id property="empId" column="empId" /> <result property="empName" column="empName"/> <result property="empBirthDay" column="empBirthDay"/> <result property="empSex" column="empSex"/> <association property="dept" javaType="com.mybatisstudy.model.Department" column="deptId" select="com.mybatisstudy.dao.IDepartmentDao.selectDeptById"> </association> </resultMap>
departmentMapper.xml
<select id="selectDeptById" resultMap="baseCardResultMap">
SELECT * FROM tb_dept d WHERE d.deptId=#{deptId};
</select>
<resultMap type="com.mybatisstudy.model.Department" id="baseCardResultMap">
<id property="deptId" column="deptId" />
<result property="deptName" column="deptName"/>
</resultMap>2 、一對多關聯
使用collection ,此時其ofType對應pojo的類名，javaType對應ArrayList。
方式一： 方式一: 巢狀結果: 使用巢狀結果對映來處理重複的聯合結果的子集
departmentMapper.xml
<!-- 一對多 ， 方式一: 巢狀結果: 使用巢狀結果對映來處理重複的聯合結果的子集 -->
<select id="getDeptsAndEmps" resultMap="deptsAndEmps" >
SELECT empId,empName,empBirthDay,empSex,tb_dept.deptId,tb_dept.deptName FROM `tb_emp`,tb_dept WHERE tb_emp.deptId = tb_dept.deptId;
</select>
<resultMap id="deptsAndEmps" type="com.mybatisstudy.model.Department">
<id property="deptId" column="deptId" />
<result property="deptName" column="deptName"/>
<collection property="emps" ofType="com.mybatisstudy.model.Employee">
<result property="empId" column="empId" />
<result property="empName" column="empName" />
<result property="empBirthDay" column="empBirthDay" />
<result property="empSex" column="empSex" />
</collection>
</resultMap>方式二：巢狀查詢：通過執行另外一個SQL對映語句來返回預期的複雜型別
departmentMapper.xml
<!-- 一對多， 方式二：巢狀查詢：通過執行另外一個SQL對映語句來返回預期的複雜型別 -->
<resultMap id="allDeptandEmp" type="com.mybatisstudy.model.Department" >
<id property="deptId" column="deptId" />
<result property="deptName" column="deptName"/>
<collection property="emps"
column="deptId"
ofType="com.mybatisstudy.model.Employee"
select="com.mybatisstudy.dao.IEmployeeDao.selectEmpsBydeptId">
</collection>
</resultMap>
<select id="selectAllDeptAndEmp" resultMap="allDeptandEmp">
SELECT * FROM tb_dept;
</select> employeeMapper.xml
<!-- 協助一對多，方式二 -->
<resultMap id="baseStudentResultMap" type="com.mybatisstudy.model.Employee">
<id property="empId" column="empId" />
<result property="empName" column="empName"/>
<result property="empBirthDay" column="empBirthDay"/>
<result property="empSex" column="empSex"/>
</resultMap>
<select id="selectEmpsBydeptId" resultMap="baseStudentResultMap">
SELECT * FROM tb_emp WHERE deptId=#{deptId};
</select> 2 、多對多關聯
如商品表和訂單表之間就是一種多對多的關聯。
此處主要是通過巢狀查詢的方式去處理。（如在findGoodsById的resultMap中巢狀一個根據商品的Id去查詢訂單）。
Goods.java
public class Goods implements Serializable {
private Integer goodsId;
private String name;
private Double price;
private String remark;
private List<Order> orders;
//getters and setters
//constructor
}
goodsMapper.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "mybatis-3-mapper.dtd" >
<mapper namespace="com.gec.mapper.goodsMapper">
<resultMap type="article" id="basegoodsResultMap">
<id column="id" property="goodsId"/>
<result column="name" property="name"/>
<result column="price" property="price"/>
<result column="remark" property="remark"/>
</resultMap>
<resultMap type="article" id="findArtcleByIdResultMap" extends="basegoodsResultMap">
<collection property="orders" javaType="ArrayList"
ofType="com.gec.domain.Goods" column="id"
select="com.gec.mapper.OrderMapper.findOrderBygoodsId"
>
</collection>
</resultMap>
<!-- 根據訂單id查詢商品 -->
<select id="findGoodsByOrderId" resultMap="basegoodsResultMap">
select * from tb_goods where id
in (select goods_id from tb_item where order_id=#{id})
</select>
<select id="findGoodsById" resultMap="findGoodsByIdResultMap">
select * from tb_goods where id=#{id}
</select>
</mapper>
Order.java
public class Order {
private Integer orderid;
private String code;
private Double total;
private List<goods> goods;
//...
}
orderMapper.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "mybatis-3-mapper.dtd" >
<mapper namespace="com.gec.mapper.OrderMapper">
<resultMap type="order" id="baseOrderResultMap">
<id column="orderId" property="orderid"/>
<result column="code" property="code"/>
<result column="total" property="total"/>
</resultMap>
<resultMap type="order" id="queryOrderByUserIdRsultMap" extends="baseOrderResultMap">
<collection property="goods" javaType="ArrayList"
ofType="Goods" column="orderId"
select="com.gec.mapper.ArticleMapper.findArtcleByOrderId">
</collection>
</resultMap>
<!-- 根據商品id查詢訂單 -->
<select id="findOrderBygoodsId" resultMap="baseOrderResultMap">
select * from tb_order where id
in (select order_id from tb_item where goods_id=#{id})
</select>
</mapper>