在使用

Question

CREATE  TABLE test(
id integer,
content text,
number integer
)

INSERT  INTO test(id,content,number) VALUES(1,'a'::text, 5);
INSERT  INTO test(id,content,number) VALUES(2,'b'::text, 2);
INSERT  INTO test(id,content,number) VALUES(3,'c'::text, 2);
INSERT  INTO test(id,content,number) VALUES(4,'d'::text, 3);
INSERT  INTO test(id,content,number) VALUES(5,'e'::text, 1);
INSERT  INTO test(id,content,number) VALUES(6,'f'::text, 3);
INSERT  INTO test(id,content,number) VALUES(7,'g'::text, 3);
INSERT  INTO test(id,content,number) VALUES(8,'h'::text, 2);
INSERT  INTO test(id,content,number) VALUES(9,'i'::text, 4);

我想要的是，将数字列和用id列排序的结果分组为desc，就像这样;

| id | number
----------------
| 9  |    4
| 8  |    2
| 7  |    3
| 5  |    1

此处所有具有多种外观的数字（如2,3和1）都会被分组，只能看一次，并且还会以id列desc进行排序。

我已尝试过此查询，但它对我不起作用;

SELECT DISTINCT ON (number) number, id FROM test  ORDER  BY number,id DESC LIMIT 4

Answer 1

使用派生表：

SELECT id, number
FROM (
    SELECT DISTINCT ON (number) number, id 
    FROM test
    ORDER BY number, id DESC
    ) s
ORDER BY id DESC
LIMIT 4;

 id | number 
----+--------
  9 |      4
  8 |      2
  7 |      3
  5 |      1
(4 rows)

Answer 2

你也可以：

select max(id) as id, number   
from test
group by number
order by id desc
limit 4