Question

我需要选择10个唯一的位置，每个位置应该来自不同的用户。因此，user_id和location_id中的值应该（单独地）是唯一的。尝试了很多方法，但不知何故它没有用完。

不关心uniques的查询看起来像：

SELECT
    b.user_id, 
    l.location_id,
    ROUND(distance(transform(PointFromText('POINT(4.0 52.0)', 4326), 26986),transform("spatialPoint", 26986))) AS distance
 FROM  locations AS l
    JOIN b_user_location AS b USING(location_id)
 ORDER BY ROUND(distance(transform(PointFromText('POINT(4.0 52.0)', 4326) ASC
 LIMIT 10 OFFSET 0

似乎我需要这样的东西：

SELECT
   DISTINCT ON(user_id)
   DISTINCT ON(location_id)
   b.user_id,
   .....

但这是不允许的。

有人能指出我正确的方向吗？

[编辑] 链接到SqlFiddle http://sqlfiddle.com/#!12/7c090/2/0

SQL查询：

CREATE TABLE users (user_id SERIAL, username VARCHAR(255), CONSTRAINT users_pkey PRIMARY KEY (user_id));
CREATE TABLE locations(location_id SERIAL, "spatialPoint" INT,name VARCHAR(255),CONSTRAINT locations_pkey PRIMARY KEY (location_id));
CREATE TABLE b_user_location(user_id INT, location_id INT, CONSTRAINT user_id_fkey FOREIGN KEY(user_id) REFERENCES users(user_id),CONSTRAINT location_id_fkey FOREIGN KEY(location_id) REFERENCES locations(location_id));

INSERT INTO users(username)VALUES('user_1'),('user_2'),('user_3');
INSERT INTO locations("spatialPoint", name)VALUES(1,'location_1'), (2, 'location_2'), (3,'location_3');

INSERT INTO b_user_location(user_id, location_id)VALUES(1,1),(1,2),(1,3),(2,1),(2,2),(2,3),(3,1),(3,2),(3,3);

SELECT
  b.user_id, 
  l.location_id,
  l."spatialPoint" AS distance
FROM  locations AS l
JOIN b_user_location AS b USING(location_id)
ORDER BY l."spatialPoint" ASC
LIMIT 10 OFFSET 0

[/编辑]

Answer 1

尝试类似：

SELECT DISTINCT ON(location_id) * 
FROM (
SELECT DISTINCT ON(b.user_id)
    b.user_id, 
    l.location_id,
    ROUND(distance(transform(PointFromText('POINT(4.0 52.0)', 4326), 26986),transform("spatialPoint", 26986))) AS distance
 FROM  locations AS l
    JOIN b_user_location AS b USING(location_id)
 ORDER BY ROUND(distance(transform(PointFromText('POINT(4.0 52.0)', 4326) ASC
)
 LIMIT 10 OFFSET 0

它会为您提供包含唯一user_id和唯一location_id的10行，但这些行可能没有最低distance。

Postgresql：仅当2个字段是独立的时才选择字段

1 个答案: