MyBatis结果集映射

1、select返回List

mybatis会把结果集自动的封装成List

    <!-- public List<Employee> getEmpsByLastNameLike(String lastName); -->
    <!--resultType：如果返回的是一个集合，要写集合中元素的类型  -->
    <select id="getEmpsByLastNameLike" resultType="com.bw.mybatis.bean.Employee">
        select * from tbl_employee where last_name like #{lastName}
    </select>

2、select返回Map

多条记录封装一个map：Map:键是这条记录的主键，值是记录封装后的javaBean

//@MapKey:告诉mybatis封装这个map的时候使用哪个属性作为map的key
@MapKey("id")
public Map<String, Employee> getEmpByLastNameLikeReturnMap(String lastName);
<select id="getEmpByLastNameLikeReturnMap" resultType="com.bw.mybatis.bean.Employee">
     select * from tbl_employee where last_name like #{lastName}
</select>
//返回一条记录的map；key就是列名，值就是对应的值
public Map<String, Object> getEmpByIdReturnMap(Integer id);
<select id="getEmpByIdReturnMap" resultType="map">
     select * from tbl_employee where id=#{id}
</select>

3、嵌套对象查询

public class Employee {    
    private Integer id;
    private String lastName;
    private String email;
    private String gender;
    private Department dept;
}
public class Department {
    private Integer id;
    private String departmentName;
}

查询查询Employee的同时查询员工对应的部门

1、级联属性封装结果集

<!--
  联合查询：级联属性封装结果集
-->
<resultMap type="com.bw.mybatis.bean.Employee" id="MyDifEmp">
    <id column="id" property="id"/>
    <result column="last_name" property="lastName"/>
    <result column="gender" property="gender"/>
    <result column="did" property="dept.id"/>
    <result column="dept_name" property="dept.departmentName"/>
</resultMap>
<!--  public Employee getEmpAndDept(Integer id);-->
<select id="getEmpAndDept" resultMap="MyDifEmp">
    SELECT e.id id,e.last_name last_name,e.gender gender,e.d_id d_id,
    d.id did,d.dept_name dept_name FROM tbl_employee e,tbl_dept d
    WHERE e.d_id=d.id AND e.id=#{id}
</select>

2、association指定联合的javaBean对象

    <resultMap type="com.bw.mybatis.bean.Employee" id="MyDifEmp2">
        <id column="id" property="id"/>
        <result column="last_name" property="lastName"/>
        <result column="gender" property="gender"/>
        <!--  association可以指定联合的javaBean对象
        property="dept"：指定哪个属性是联合的对象
        javaType:指定这个属性对象的类型[不能省略]
        -->
        <association property="dept" javaType="com.bw.mybatis.bean.Department">
            <id column="did" property="id"/>
            <result column="dept_name" property="departmentName"/>
        </association>
    </resultMap>

3、association分步查询

<resultMap type="com.bw.mybatis.bean.Employee" id="MyEmpByStep">
    <id column="id" property="id"/>
    <result column="last_name" property="lastName"/>
    <result column="email" property="email"/>
    <result column="gender" property="gender"/>
    <!-- association定义关联对象的封装规则
    select:表明当前属性是调用select指定的方法查出的结果
    column:指定将哪一列的值传给这个方法
    流程：使用select指定的方法（传入column指定的这列参数的值）查出对象，并封装给property指定的属性
    -->
    <association property="dept" 
                 select="com.bw.mybatis.dao.DepartmentMapper.getDeptById"
                 column="d_id">
    </association>
</resultMap>
<!--  public Employee getEmpByIdStep(Integer id);-->
<select id="getEmpByIdStep" resultMap="MyEmpByStep">
    select * from tbl_employee where id=#{id}
</select>
<!--public Department getDeptById(Integer id);  -->
<select id="getDeptById" resultType="com.bw.mybatis.bean.Department">
    select id,dept_name departmentName from tbl_dept where id=#{id}
</select>

可以设置以下两个属性实现延迟加载

在加载employee的时候先不查询department的信息，department会在第一次使用的时候查询出来。

<setting name="lazyLoadingEnabled" value="true"/>
<setting name="aggressiveLazyLoading" value="false"/>

4、Collection定义关联集合查询

public class Department {
    private Integer id;
    private String departmentName;
    private List<Employee> emps;
}

查询department的时候把employee也查询出来

    <resultMap type="com.bw.mybatis.bean.Department" id="MyDept">
        <id column="did" property="id"/>
        <result column="dept_name" property="departmentName"/>
        <!-- 
            collection定义关联集合类型的属性的封装规则 
            ofType:指定集合里面元素的类型
        -->
        <collection property="emps" ofType="com.bw.mybatis.bean.Employee">
            <!-- 定义这个集合中元素的封装规则 -->
            <id column="eid" property="id"/>
            <result column="last_name" property="lastName"/>
            <result column="email" property="email"/>
            <result column="gender" property="gender"/>
        </collection>
    </resultMap>
    <!-- public Department getDeptByIdPlus(Integer id); -->
    <select id="getDeptByIdPlus" resultMap="MyDept">
        SELECT d.id did,d.dept_name dept_name,
                e.id eid,e.last_name last_name,e.email email,e.gender gender
        FROM tbl_dept d
        LEFT JOIN tbl_employee e
        ON d.id=e.d_id
        WHERE d.id=#{id}
    </select>

5、Collection分步查询

    <!-- fetchType="lazy"：表示使用延迟加载；
                - lazy：延迟
                - eager：立即
    如果需要传递多个值，可以把多个值封装成一个map，column="{key1=column1,key2=column2}"
    下面的column可以写成column="{deptId=id}"
    -->
<resultMap type="com.bw.mybatis.bean.Department" id="MyDeptStep">
        <id column="id" property="id"/>
        <id column="dept_name" property="departmentName"/>
        <collection property="emps" 
            select="com.bw.mybatis.dao.EmployeeMapperPlus.getEmpsByDeptId"
            column="{id}" fetchType="lazy"></collection>
    </resultMap>
    <!-- public Department getDeptByIdStep(Integer id); -->
    <select id="getDeptByIdStep" resultMap="MyDeptStep">
        select id,dept_name from tbl_dept where id=#{id}
    </select>
    <!-- public List<Employee> getEmpsByDeptId(Integer deptId); -->
    <select id="getEmpsByDeptId" resultType="com.bw.mybatis.bean.Employee">
        select * from tbl_employee where d_id=#{deptId}
    </select>