1，dao：

List <HashMap <String, String>> selectSubjectNum (@Param ("startTime") String beginTime , @Param ("endTime") String endTime);

2，mapper：

    <select id = "selectSubjectNum" resultType = "java.util.HashMap">
        SELECT `subject` AS 'key', count(subject) AS 'value'
        FROM tb_contract
        where DATE_FORMAT(`create_time`, '%Y-%m-%d') BETWEEN '2022-04-19' AND '2022-04-20'
        group by `subject`
    </select>

3，* 实体类：

注意：多个map时，必须将map封装到list中，一个map对应数据库查询出来的一行； 因为从数据库出来的结果是这样的： 【（”key”,”zhangsan”）,(“value”,”20”)】 【 (“key”,”0”)，(“value”，”4”) 】 必须要将键和值分开；

/**
     * 学科统计
     *
     * @return
     */
    @Override
    public List <SubjectInfoVo> subjectStatistics (String beginTime , String endTime) {
        //查询有几个学科的合同
        //new map
        List <SubjectInfoVo> list = new ArrayList();
        Map <String, String> map = new HashMap();
        List <HashMap <String, String>> mapList = reportMpper.selectSubjectNum(beginTime , endTime);
        if (mapList.size() != 0) {
            //遍历装map的list
            for (HashMap <String, String> hashMap : mapList) {
                String key = null;
                String value = null;
                //遍历单个map,取出键和值
                Set <Map.Entry <String, String>> entries = hashMap.entrySet();
                for (Map.Entry <String, String> entry : entries) {
                    if ("key".equals(entry.getKey())) {
                        key = String.valueOf(entry.getValue());
                    } else if ("value".equals(entry.getKey())) {
                        value = String.valueOf(entry.getValue());
                    }
                    //再封装会map中
                    map.put(key , value);
                }
            }
        }
        System.out.println("map = " + map);
        //再次遍历map
        Set <Map.Entry <String, String>> entries = map.entrySet();
        for (Map.Entry <String, String> entry : entries) {
            SubjectInfoVo subjectInfoVo = new SubjectInfoVo();
            String key = entry.getKey();
            if (key != null){
                String subjectName = reportMpper.selectSubjectName(key);
                subjectInfoVo.setSubject(subjectName);
                subjectInfoVo.setNum(Integer.parseInt(entry.getValue()));
                //将结果封装回list
                list.add(subjectInfoVo);
            }
        }
        return list;
    }