解析：

<select id="ID标识" resultMap="resultMapID标识" >
	SQL语句
</select>

<resultMap id="resultMapID标识" type="返参类型">
	<collection property="反参中映射的字段" ofType="collection内select返参类型" javaType="java.util.List" column="select中的字段名(collection内select的入参)" select="另一个select的ID标识"/>
</resultMap>

<select id="另一个select的ID标识" resultType="另一个select返参类型" >
	SQL语句
</select>

示例：

<select id="queryUser" resultMap="userMap">
	SELECT user_id from user
</select>

<resultMap id="userMap" type="com.test.User">
	<result column="user_id" property="userId"/>
	<collection property="urls" ofType="java.util.List" javaType="java.util.List" column="user_id " select="userUrls"/>
</resultMap>

<select id="userUrls" resultType="java.lang.String">
    select url from user_url where user_id = #{userId};
</select>

//实体类

@Data
public class User {
    private String userId;
    private List<String> urls;
}