postgresql 如何顯示每天最頻繁的值

Question

我遇到了一個查詢問題，該查詢應該返回每個日期出現最多的值

+------------+------------------+
|    Date    |      value       |
+------------+------------------+
| 2020-01-01 | Programmer       |
| 2020-01-02 | Technician       |
| 2020-01-03 | Business Analyst |
+------------+------------------+

到目前為止我已經完成了

select count(headline) as asd, publication_date, employer -> 'name' as dsa from jobhunter
group by publication_date,dsa
ORDER BY publication_date  DESC

但它顯示 2020-12-31 19:06:00 而不僅僅是 YYYY-MM-DD 關於如何解決這個問題的任何想法？ 在此處輸入圖像描述

Answer 1

測試數據：

create table tbl (
    id serial primary key,
    row_datetime TIMESTAMP,
    row_val VARCHAR(60)
);

insert into tbl (row_datetime, row_val) values ('2021-01-01 00:00:00', 'a');
insert into tbl (row_datetime, row_val) values ('2021-01-01 01:00:00', 'a');
insert into tbl (row_datetime, row_val) values ('2021-01-01 02:00:00', 'b');
insert into tbl (row_datetime, row_val) values ('2021-01-02 00:00:00', 'a');
insert into tbl (row_datetime, row_val) values ('2021-01-02 01:00:00', 'b');
insert into tbl (row_datetime, row_val) values ('2021-01-02 02:00:00', 'b');

示例查詢：

SELECT dt, val, cnt
FROM (
   SELECT dt, val, cnt, ROW_NUMBER() OVER (PARTITION BY dt ORDER BY cnt DESC) AS row_num
   FROM (
      SELECT dt, val, COUNT(val) AS cnt
      FROM (
         SELECT DATE(row_datetime) AS dt, row_val AS val FROM tbl
      ) AS T1 GROUP BY dt, val
   ) AS T2
) AS T3
WHERE row_num=1
ORDER BY dt ASC

您還可以自定義查詢以優化性能、獲取更多字段等。