關聯查詢：
1.Employee表:
id;lastName;email; gender;d_id(外鍵關聯Department的ID)
2.Department表：
id;deptName;
3。首先，為了關聯，Employee（javaBean）如下：
private Integer id;
private String lastName;
private String email;
private String gender;
private Department dept;
Department（javaBean）如下：
private Integer id;
private String deptName;
4.關聯三種方式：
場景：查詢部員工所在部門
4.1:新建resultMap (resultMap的id是select標簽的resultMap名字)
非Employee表的列通過<result column="數據庫列名" property="javaBean中該類的名稱.類屬性"/>
<resultMap type="com.mybatis.bean.Employee" id="myDifEmployee">
<id column="id" property="id"/>
<result column="gender" property="gender"/>
<result column="last_name" property="lastName"/>
<result column="d_id" property="dept.id"/>
<result column="dept_name" property="dept.deptName"/>
</resultMap>
<select id="getEmpAndDept" resultMap="myDifEmployee">
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 as e,tbl_department as d where e.d_id = d.id and e.id=#{id}
</select>
4.2:
<!-- association級聯 <association property="javaBean中該類的名稱" javaType="類類型地址">-->
<resultMap type="com.mybatis.bean.Employee" id="myDifEmployee2">
<id column="id" property="id"/>
<result column="gender" property="gender"/>
<result column="last_name" property="lastName"/>
<association property="dept" javaType="com.mybatis.bean.Department">
<id column="did" property="id"/>
<result column="dept_name" property="deptName"/>
</association>
</resultMap>
<select id="getEmpAndDept" resultMap="myDifEmployee2">
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 as e,tbl_department as d where e.d_id = d.id and e.id=#{id}
</select>
4.3：分步查詢
<!-- association分步
select:表明當前屬性是調用select指定方法查出的結果 是XXXMapper.xml中namespace。方法名
column；指定將哪一列的值傳給這個方法
-->
<resultMap type="com.mybatis.bean.Employee" id="myEmpByIdStep">
<id column="id" property="id"/>
<result column="gender" property="gender"/>
<result column="last_name" property="lastName"/>
<result column="email" property="email"/>
<association property="dept" select="com.mybatis.dao.DepartmentMapper.getDeptById" column="d_id">
</association>
</resultMap>
<select id="getEmpByIdStep" resultMap="myEmpByIdStep">
select * from tbl_employee where id = #{id}
</select>
5.延遲加載
延遲加載的以上是如果需要的僅僅是tbl_employee 表的值，那么sql語句就不查詢tbl_department 。
如果需要tbl_department ，sql語句才加載。
延遲加載的解決方式是在配置文件中的<settings>下加如下兩個配置：
<settings>
<!-- 延遲加載的全局開關。當開啟時，所有關聯對象都會延遲加載。 -->
<setting name="lazyLoadingEnabled" value="true"/>
<!-- 當開啟時，任何方法的調用都會加載該對象的所有屬性。否則，每個屬性會按需加載 -->
<setting name="aggressiveLazyLoading" value="false"/>
</settings>
6.關聯查詢collection返回集合
場景：查詢部門下的所有員工
Department（javaBeab）:
private Integer id;
private String deptName;
private List<Employee> emps;
方法1：
<resultMap type="com.mybatis.bean.Department" id="myDept">
<id column="did" property="id"/>
<result column="dept_name" property="deptName"/>
<!--ofType: collection返回值類型 -->
<collection property="emps" ofType="com.mybatis.bean.Employee">
<id column="eId" property="id"/>
<result column="gender" property="gender"/>
<result column="last_name" property="lastName"/>
<result column="email" property="email"/>
</collection>
</resultMap>
<select id="getDeptByIdPlus" resultMap="myDept">
select d.id did, d.dept_name dept_name,e.id eId, e.last_name last_name, e.gender gender, e.email email,e.d_id d_id from tbl_department as d left join tbl_employee as e ON d.id= e.d_id WHERE d.id=#{id}
</select>
方法二：
<resultMap type="com.mybatis.bean.Department" id="myDeptStep">
<id column="id" property="id"/>
<result column="dept_name" property="deptName"/>
<collection property="emps" select="com.mybatis.dao.EmployeeMapper.getEmpByDeptID" column="{deptId=id}" fetchType="lazy">
</collection>
</resultMap>
<select id="getDeptByIdStep" resultMap="myDeptStep">
select id,dept_name deptName from tbl_department where id = #{id}
</select>
==========多列返回值，collection；單個返回值：association==========
collection 將多列的值傳遞給方法：將多列值封裝成map傳遞；column=“{key1=column1,key2=column2}"
fetchType="lazy" 開啟懶加載 fetchType="eager"禁用懶加載；優先級大於全局的懶加載；
resultMap中discriminator標簽可以進行鑒別：
場景：如果是女員工把部門信息查出來，如果是男生將last_name這一列賦值給email
<resultMap type="com.mybatis.bean.Employee" id="myEmpByDis">
<id column="id" property="id"/>
<result column="last_name" property="lastName"/>
<result column="email" property="email"/>
<result column="gender" property="gender"/>
<!-- discriminator column:指定判斷的列名，javaType：列對應的java類型 -->
<discriminator javaType="string" column="gender">
<case value="0" resultType="com.mybatis.bean.Employee">
<association property="dept" select="com.mybatis.dao.DepartmentMapper.getDeptById" column="d_id">
</association>
</case>
<case value="1" resultType="com.mybatis.bean.Employee">
<id column="id" property="id"/>
<result column="gender" property="gender"/>
<result column="last_name" property="lastName"/>
<result column="last_name" property="email"/>
</case>
</discriminator>
</resultMap>
<select id="getEmpByDiscriminator" resultMap="myEmpByDis">
select * from tbl_employee where id = #{id}
</select>