mybatis多表查询

1.多对一

• 准备(两张表employee,department,两个domain,两个mapper.xml)
• 查询(嵌套结果[一条sql],嵌套查询[n+1条sql])

• 嵌套查询

• <select id="findAll" resultMap="employeeMap">
      select e.id eid,e.name ename,e.age,e.sex,d.id did,d.name dname
        from employee e join department d on d.id = e.dept_id
  </select>
  
  <resultMap id="employeeMap" type="employee">
      <!-- 手动完成基本映射 -->
      <id property="id" column="eid" />
      <result property="name" column="ename" />
      <result property="sex" column="sex" />
      <result property="age" column="age" />
      <!-- property:属性名,javaType:属性类型 -->
      <association property="dept" javaType="department">
          <id property="id" column="did" />
          <result property="name" column="dname" />
      </association>
  </resultMap>

• 嵌套查询

• 会产生n+1条sql
• 需要去找到对应的那条sql并且执行
• 保证MyBatis能找到这两个xml

• employeeMapper.xml

  <select id="findAll" resultMap="employeeMap">
      SELECT * FROM employee
  </select>
  <!--嵌套查询方案-->
  <resultMap id="employeeMap" type="employee">
      <id property="id" column="id" />
      <result property="name" column="name" />
      <result property="sex" column="sex" />
      <result property="age" column="age" />
  <!--
      查询相应的部门
          column:数据库对应的列 select:查询对应的sql的路径
   -->
      <association property="dept" javaType="department"
               column="dept_id" select="cn.itsource._04_many2one.DepartmentMapper.findById" />
  </resultMap>
  

  departmentMapper.xml

  <mapper namespace="cn.itsource._04_many2one.DepartmentMapper">
      <select id="findById" parameterType="long" resultType="department">
          select * from department where id = #{id}
      </select>
  </mapper>

• 一对多

• 5.1 准备domain

  • Employee

  public class Employee {
  
      private Long id;
      private String name;
      private Integer age;
      private Boolean sex;
       //... getter,setter与toString
  }
  

  • Department

  public class Department {
      private Long id;
      private String name;
  
      private List<Employee> employees = new ArrayList<>();
  

  5.2 级连保存

  • 准备两个Mapper
  • 保存部门后需要马上拿到它的id
  • 保存员工传的是Map{List<Employee>,deptId}

  departmentMapper.xml

  <!--保存后需要拿到id-->
  <insert id="save" parameterType="department"
          useGeneratedKeys="true" keyProperty="id" keyColumn="id">
      insert into department (name) values (#{name})
  </insert>
  

  employeeMapper.xml

  <insert id="batchSave" parameterType="map">
      insert into employee (name,age,sex,dept_id) values
      <foreach collection="list" item="emp" separator=",">
          (#{emp.name},#{emp.age},#{emp.sex},#{deptId})
      </foreach>
  </insert>
  

  5.3 嵌套结果

  • 查询的sql要关连多张表(一定要取别名,不然有些名称会产生冲突)
  • 当我们使用了collection 后默认的映射失败,需要自己手动完成映射

  <resultMap id="departmentMap" type="department">
      <id property="id" column="did" />
      <result property="name" column="dname" />
      <collection property="employees" ofType="employee">
          <id property="id" column="eid" />
          <result property="name" column="ename" />
          <result property="age" column="age" />
          <result property="sex" column="sex" />
      </collection>
  </resultMap>
  
  <select id="findAll" resultMap="departmentMap">
      select d.id did,d.name dname,e.id eid,e.name ename,e.age,e.sex
      from department d join employee e on e.dept_id = d.id
  </select>
  

  5.4 嵌套查询

  依赖需要找到对应的SQL

  • departmentMapper.xml

  <resultMap id="departmentMap" type="department">
      <id property="id" column="id" />
      <result property="name" column="name" />
      <collection property="employees" ofType="employee" column="id" select="cn.itsource._05_one2many.EmployeeMapper.findByDeptId">
      </collection>
  </resultMap>
  <select id="findAll" resultMap="departmentMap">
      select *  from department
  </select>
  

  • employeeMapper.xml

  <!--这个命名空间的名称就是咱们Mapper接口的全限定名-->
  <mapper namespace="cn.itsource._05_one2many.EmployeeMapper">
  
      //...
      
      <select id="findByDeptId" resultType="employee" parameterType="long">
          select * from employee where dept_id = #{deptId}
      </select>
  
  </mapper>