如何按属性分组并按日期排序

Question

我有两个表：

医护人员

CREATE TABLE "medic" (
   "id" BIGINT NOT NULL, 
   "name" CHARACTER VARYING(255) NOT NULL,
   PRIMARY KEY ("id")

);

CREATE TABLE IF NOT EXISTS "comment" (
   "id" BIGINT NOT NULL,
   "medic_id" BIGINT NOT NULL,
   "comment" CHARACTER VARYING(1024) NOT NULL,
   "created_at" TIMESTAMP WITHOUT TIME ZONE NOT NULL DEFAULT now(),
   CONSTRAINT pk_comment PRIMARY KEY (id),
   CONSTRAINT fk_comment_medic FOREIGN KEY (medic_id)
   REFERENCES medic(id) ON UPDATE NO ACTION ON DELETE NO ACTION
);

现在我想得到 medic_id, name, comments_count 和所有 ordered by created_at

这是我目前尝试过的方法：

SELECT m.id, m.name, COUNT(c.id)
FROM COMMENT AS c
JOIN medic AS m ON m.id = c.medic_id
GROUP BY m.id, m.name, c.created_at
ORDER BY c.created_at DESC

但是显然这行不通，因为按日期分组是没有意义的，尽管我想按日期排序时必须这样做。

另一种方法是使用 window 函数。特别是rank() over (partition by m.id order by c.created_at desc)。但在这种情况下，我失去了对所有记录的排序。

这是一些 SQLFiddle。

我正在使用 Postgres 9.3

Answer 1

我猜你想在最近的评论日期之前订购：

SELECT m.id, m.name, COUNT(c.id)
FROM COMMENT c JOIN
     medic m
     ON m.id = c.medic_id
GROUP BY m.id, m.name
ORDER BY MAX(c.created_at) DESC;

如何按属性分组并按日期排序

How to group by an attribute and order by date

postgresql

group-by

sql-order-by