association分步查詢

場景一

查詢Employee的同時查詢員工對應的部門
Employee===Department
一個員工有與之對應的部門信息

Employee表:

Department表：

public interface DepartmentMapper {public Department getDeptById(Integer id);} public interface EmployeeMapperPlus {public Employee getEmpByIdStep(Integer id);} <!-- public Department getDeptById(Integer id); --><select id="getDeptById" resultType="com.atguigu.mybatis.bean.Department">select id,dept_name departmentName from tb1_dept where id = #{id}</select> <resultMap id="MyEmpStep" type="com.atguigu.mybatis.bean.Employee"><!-- 1.先按照員工id查詢員工信息--><!-- 2.根據查詢員工信息中的d_id值去部門表查出部門信息--><!-- 3.部門設置到員工中--><id column="id" property="id"></id><result column="last_name" property="lastName"></result><result column="email" property="email"></result><result column="gender" property="gender"></result><!-- association定義關聯對象的封裝規則select:表明當前屬性是調用select指定的方法查出的結果column:指定將哪一列的值傳給這個方法流程:使用select指定的方法(傳入column指定的這列參數的值)查出對象，并封裝給property指定的屬性--><association property="dept" select="com.atguigu.mybatis.dao.DepartmentMapper.getDeptById"column="d_id"></association></resultMap><!-- public Employee getEmpByIdStep(Integer id);--><select id = "getEmpByIdStep" resultMap="MyEmpStep">select * from tb1_employee where id = #{id}</select> @Testpublic void test03() throws IOException {SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();SqlSession sqlSession = sqlSessionFactory.openSession();try{EmployeeMapperPlus mapper = sqlSession.getMapper(EmployeeMapperPlus.class);Employee emp = mapper.getEmpByIdStep(1);System.out.println(emp);System.out.println(emp.getDept());}finally {sqlSession.close();}}

延遲加載

Employee ===> Dept
我們每次查詢Employee對象的時候，都將一起查詢出來
分段查詢的基礎上，使用延遲加載
可以使部門信息在我們使用的時候再去查詢

lazyLoadingEnabled & aggressiveLazyLoading

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

場景二

查詢部門的時候將部門對應的所有員工信息也查詢出來

public class Department {private Integer id;private String departmentName;private List<Employee> emps;} public interface DepartmentMapper {public Department getDeptByIdPlus(Integer id);} <resultMap id="MyDept" type="com.atguigu.mybatis.bean.Department"><id column="did" property="id"></id><result column="dept_name" property="departmentName"></result><!--collection定義關聯集合類型的屬性的封裝規則ofType:指定集合里面元素的類型--><collection property="emps" ofType="com.atguigu.mybatis.bean.Employee" ><!--定義這個集合中元素的封裝規則--><id column="eid" property="id"></id><result column="last_name" property="lastName"></result><result column="email" property="email"></result><result column="gender" property="gender"></result></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 genderFROM tb1_dept d LEFT JOIN tb1_employee e ON d.id = e.d_id WHERE d.id = #{id}</select> @Testpublic void test04() throws IOException {SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();SqlSession sqlSession = sqlSessionFactory.openSession();try{DepartmentMapper mapper = sqlSession.getMapper(DepartmentMapper.class);Department deptByIdPlus = mapper.getDeptByIdPlus(1);System.out.println(deptByIdPlus);System.out.println(deptByIdPlus.getEmps());}finally {sqlSession.close();}}

擴展：多列的值傳遞過去

將多列的值封裝map傳遞
column="{key1=column1,key2=column2}"
fetchType=“lazy”：表示使用延遲加載；
lazy：延遲
eager：立即

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