Mybatis一对多查询问题怎么处理 - 问答

Mybatis提供了很好的支持来处理一对多查询问题。下面是一些处理一对多查询问题的方法：

使用嵌套查询：在主查询中进行嵌套查询，通过主查询的结果来获取关联的多个子对象。例如，可以使用<collection>标签来定义一对多的关联关系。

<resultMap id="userMap" type="User">
  <id column="id" property="id" />
  <result column="name" property="name" />
  <collection property="orders" ofType="Order">
    <id column="order_id" property="id" />
    <result column="order_name" property="name" />
  </collection>
</resultMap>

<select id="getUserWithOrders" resultMap="userMap">
  SELECT u.id, u.name, o.id as order_id, o.name as order_name
  FROM user u
  LEFT JOIN order o ON u.id = o.user_id
  WHERE u.id = #{id}
</select>

使用嵌套查询映射：如果一对多的关系比较复杂，可以考虑使用嵌套查询映射来处理。通过在主查询中使用<association>标签来定义关联对象，然后在关联对象的映射中使用<collection>标签来定义关联的多个子对象。

<resultMap id="userMap" type="User">
  <id column="id" property="id" />
  <result column="name" property="name" />
  <association property="department" javaType="Department">
    <id column="department_id" property="id" />
    <result column="department_name" property="name" />
    <collection property="employees" ofType="Employee">
      <id column="employee_id" property="id" />
      <result column="employee_name" property="name" />
    </collection>
  </association>
</resultMap>

<select id="getUserWithDepartmentAndEmployees" resultMap="userMap">
  SELECT u.id, u.name, d.id as department_id, d.name as department_name, e.id as employee_id, e.name as employee_name
  FROM user u
  LEFT JOIN department d ON u.department_id = d.id
  LEFT JOIN employee e ON d.id = e.department_id
  WHERE u.id = #{id}
</select>

使用嵌套查询的方式来处理：在查询主对象的同时，通过子查询来获取关联的多个子对象。这种方式适用于一对多关系比较简单的情况。

<select id="getUserWithOrders" resultType="User">
  SELECT u.id, u.name, (SELECT o.id, o.name FROM order o WHERE o.user_id = u.id) as orders
  FROM user u
  WHERE u.id = #{id}
</select>

这些方法可以根据具体的业务需求来选择使用，可以根据关联关系的复杂程度和查询性能来进行选择。

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