MyBatis 动态SQL

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="com.nowcoder.dao.FeedDAO">
    <sql id="table">feed</sql>
    <sql id="selectFields">id, user_id,created_date,data,type
    </sql>
    <select id="selectUserFeeds" resultType="com.nowcoder.model.Feed">
        SELECT
        <include refid="selectFields"/>
        FROM
        <include refid="table"/>
        where id &lt; #{maxId}
        <if test="userIds.size() != 0">
            AND user_id in 
            <foreach collection="userIds" index="index" item="item"
                     open="(" separator="," close=")">
                #{item}
            </foreach>
        </if>
        ORDER BY id DESC
        LIMIT #{count}
    </select>
</mapper>