有时需要按周统计信息，或者将没有数据的日期补齐返给前端形成统计图：条形图、饼图等

一、java方式

获取当前日期上一个自然周内数据，注意默认的是星期六到星期一，如果想让星期一是第一天~星期日，就要获取本周的星期日，上周的星期一，再通过sql传入计算的开始时间截止时间：

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;

public class DateUtil {

    public static final String DATE_PATTERN = "yyyy-MM-dd";
    public static final String DATE_TIME_PATTERN = "yyyy-MM-dd HH:mm:ss";

    /**
     * 获取星期的第一天，最后一天
     * n为推迟的周数，1本周，-1向前推迟一周，2下周，依次类推
     * m为星期几
     */
    public static String getWeek(int n, int m) {
        Calendar cal = Calendar.getInstance();
        cal.add(Calendar.DATE, n * 7);
        cal.set(Calendar.DAY_OF_WEEK, m);
        return new SimpleDateFormat(DateUtil.DATE_PATTERN).format(cal.getTime());
    }

    /**
     * 获取两个日期（含时分秒）相差的天数，包含今天
     *
     * @param startDate
     * @param endDate
     * @return
     * @throws ParseException
     */
    public static int dateBetweenIncludeToday(Date startDate, Date endDate) throws ParseException {
        return dateBetween(startDate, endDate) + 1;
    }

    public static int dateBetween(Date startDate, Date endDate) throws ParseException {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        startDate = sdf.parse(sdf.format(startDate));
        endDate = sdf.parse(sdf.format(endDate));
        Calendar cal = Calendar.getInstance();
        cal.setTime(startDate);
        long time1 = cal.getTimeInMillis();
        cal.setTime(endDate);
        long time2 = cal.getTimeInMillis();
        long between_days = (time2 - time1) / (1000 * 3600 * 24);
        return Integer.parseInt(String.valueOf(between_days));
    }

    /**
     * 将秒值转换为时分秒工具类
     *
     * @param second
     * @return
     */
    public static String secToTime(int second) {
        int hour = second / 3600;
        int minite = second % 3600 / 60;
        int sec = second % 60;
        String timeStr = "";
        if (hour > 0) {
            timeStr += hour + "小时";
        }
        if (minite > 0) {
            timeStr += minite + "分";
        }
        if (sec > 0) {
            timeStr += sec + "秒";
        }
        return timeStr;
    }
     public static void main(String[] args) {
        String startTime = DateUtil.getWeek(-1, Calendar.MONDAY);
        String endTime= DateUtil.getWeek(0, Calendar.SUNDAY);
    }
}

二、数据库方式

统计上一周的每天用户注册数量，没有数量的补齐日期，mysql为例：

  SELECT
            t1.timeDay as date,ifnull(t2.num,0) num
        FROM (
            SELECT
                date_format(lastWeek.`timeDay`,'%m-%d') as 'timeDay'
            FROM (
                select DATE_SUB(NOW(),interval 13 day) as 'timeDay'
                UNION ALL
                select DATE_SUB(NOW(),interval 12 day) as 'timeDay'
                UNION ALL
                select DATE_SUB(NOW(),interval 11 day) as 'timeDay'
                UNION ALL
                select DATE_SUB(NOW(),interval 10 day) as 'timeDay'
                UNION ALL
                select DATE_SUB(NOW(),interval 9 day) as 'timeDay'
                UNION ALL
                select DATE_SUB(NOW(),interval 8 day) as 'timeDay'
                UNION ALL
                select DATE_SUB(NOW(),interval 7 day) as 'timeDay'
                UNION ALL
                select DATE_SUB(NOW(),interval 6 day) as 'timeDay'
                UNION ALL
                select DATE_SUB(NOW(),interval 5 day) as 'timeDay'
                UNION ALL
                select DATE_SUB(NOW(),interval 4 day) as 'timeDay'
                UNION ALL
                select DATE_SUB(NOW(),interval 3 day) as 'timeDay'
                UNION ALL
                select DATE_SUB(NOW(),interval 2 day) as 'timeDay'
                UNION ALL
                select DATE_SUB(NOW(),interval 1 day) as 'timeDay'
            ) as lastWeek
            WHERE YEARWEEK(lastWeek.`timeDay`,1) = YEARWEEK(NOW(),1)-1
        ) as t1
        LEFT JOIN (
            SELECT
                count( DISTINCT user_id ) num,
                DATE_FORMAT( a.create_time, "%m-%d" )  date
            FROM
               user_info a
             
            GROUP BY
                DATE_FORMAT (a.create_time,"%Y-%m-%d")
        ) as t2 ON t1.timeDay = t2.date
        ORDER BY t1.timeDay