mybatis多表联查怎么写

MyBatis多表联查可以通过使用嵌套查询或者使用MyBatis提供的关联查询功能来实现。

1. 嵌套查询

在Mapper.xml文件中，可以使用select标签嵌套其他select标签来实现多表联查。例如：

<select id="getUserOrders" resultMap="userOrderMap">
  select * from user
  <where>
    <if test="userId != null">
      and user.id = #{userId}
    </if>
  </where>
  <foreach collection="orders" item="order" open="and (" close=")" separator="or">
    <if test="order.status != null">
      order.status = #{order.status}
    </if>
  </foreach>
  and exists (
    select 1 from order where order.user_id = user.id
  )
</select>

2. 关联查询

MyBatis提供了关联查询功能，可以通过定义resultMap来实现多表联查。例如：

<resultMap id="userOrderMap" type="User">
  <id property="id" column="user_id"/>
  <result property="username" column="username"/>
  <collection property="orders" ofType="Order">
    <id property="id" column="order_id"/>
    <result property="status" column="status"/>
  </collection>
</resultMap>

<select id="getUserOrders" resultMap="userOrderMap">
  select * from user
  left join order on user.id = order.user_id
  <where>
    <if test="userId != null">
      and user.id = #{userId}
    </if>
    <foreach collection="orders" item="order" open="and (" close=")" separator="or">
      <if test="order.status != null">
        order.status = #{order.status}
      </if>
    </foreach>
  </where>
</select>