mybatis中多层foreach用法

MyBatis中多层foreach用法可以实现多层嵌套的循环操作，可以方便地处理嵌套的集合数据。具体实现方法如下：

1. 外层foreach循环嵌套内层foreach循环

<select id="selectUserOrders" resultMap="orderResultMap">
  select * from orders o
  <where>
    <foreach collection="userIds" item="userId" separator="or">
      o.user_id = #{userId}
    </foreach>
  </where>
  and o.status = #{status}
  <foreach collection="orders" item="order" index="index" open=" and (" separator=" or " close=")">
    o.order_id = #{order.orderId}
  </foreach>
</select>

2. 外层foreach循环嵌套if条件语句

<select id="selectUserOrders" resultMap="orderResultMap">
  select * from orders o
  <where>
    <foreach collection="userIds" item="userId" separator="or">
      o.user_id = #{userId}
    </foreach>
    <if test="status != null">
      and o.status = #{status}
    </if>
  </where>
  <if test="orders != null and orders.size() > 0">
    and (
    <foreach collection="orders" item="order" index="index" separator=" or ">
      o.order_id = #{order.orderId}
    </foreach>
    )
  </if>
</select>

3. 外层foreach循环嵌套where条件语句

<select id="selectUserOrders" resultMap="orderResultMap">
  select * from orders o
  <where>
    <foreach collection="userIds" item="userId" separator="or">
      o.user_id = #{userId}
      <if test="orders != null and orders.size() > 0">
        and (
        <foreach collection="orders" item="order" index="index" separator=" or ">
          o.order_id = #{order.orderId}
        </foreach>
        )
      </if>
    </foreach>
    <if test="status != null">
      and o.status = #{status}
    </if>
  </where>
</select>