Question

有一行像：

formatter = {
    generateStub = true
}

城镇都有整数值，其中城镇3和城镇4的数量最多

我想为用户选择两个顶级城镇，因此最终结果应为：

user | town 1 | town 2 | town 3 | town 4 | town 5 | town 6|

Answer 1

这是正确规范化的模型：

create table users (
    user_id serial primary key,
    user_name varchar(100)
);

create table town (
    town_id serial primary key,
    town_int int
);

create table user_town (
    town_id int references town (town_id),
    user_id int references users (user_id),
    primary key (user_id, town_id)
);

insert into users (user_name) values ('John');
insert into town (town_int) values (1),(2),(3),(4),(5),(6);
insert into user_town (user_id, town_id) values (1,1),(1,2),(1,3),(1,4),(1,5),(1,6);

如何查询：

select user_id, user_name, town_id, town_int
from
    user_town
    inner join
    users using (user_id)
    inner join
    town using (town_id)
where user_id = 1
order by town_int desc
limit 2
;
 user_id | user_name | town_id | town_int 
---------+-----------+---------+----------
       1 | John      |       6 |        6
       1 | John      |       5 |        5

Answer 2

有时候，我们拥有的东西，例如设计糟糕的遗留数据库。然后

WITH t AS (
SELECT 100 as userid, 11 as town1, 23 as town2, 77 as town3, 14 as town4, 15 as town5, 16 as town6
UNION ALL
SELECT 101 as userid, 21 as town1, 235 as town2, 177 as town3, 24 as town4, 25 as town5, 26 as town6
) 
SELECT userid, max(r.town) as top1, min(r.town) as top2
FROM t
CROSS JOIN LATERAL(
    SELECT town1 as town FROM t t2 WHERE t2.userid=t.userid
    UNION ALL 
    SELECT town2 FROM t t2 WHERE  t2.userid=t.userid
    UNION ALL 
    SELECT town3 FROM t t2 WHERE  t2.userid=t.userid
    UNION ALL 
    SELECT town4 FROM t t2 WHERE  t2.userid=t.userid
    UNION ALL 
    SELECT town5 FROM t t2 WHERE  t2.userid=t.userid
    UNION ALL 
    SELECT town6 FROM t t2 WHERE  t2.userid=t.userid
    ORDER BY town DESC LIMIT 2) AS r
 GROUP BY userid;

Postgresql选择两个顶级城镇

2 个答案: