显示服务器名称、实例 ID 和活动会话数的视图（如果结束时间戳为空，则会话处于活动状态）

Question

CREATE TABLE instances(
    ser_name VARCHAR(20) NOT NULL,
    id INTEGER NOT NULL ,
    ser_ip VARCHAR(16) NOT NULL,
    status VARCHAR(10) NOT NULL,
    creation_ts TIMESTAMP,
    CONSTRAINT instance_id PRIMARY KEY(id)
);

CREATE TABLE characters(
    nickname VARCHAR(15) NOT NULL,
    type VARCHAR(10) NOT NULL,
    c_level INTEGER NOT NULL,
    game_data VARCHAR(40) NOT NULL,
    start_ts TIMESTAMP ,
    end_ts TIMESTAMP NULL ,
    player_ip VARCHAR(16) NOT NULL,
    instance_id INTEGER NOT NULL,
    player_username VARCHAR(15),
    CONSTRAINT chara_nick PRIMARY KEY(nickname)
); 
ALTER TABLE
     instances ADD CONSTRAINT ins_ser_name FOREIGN KEY(ser_name) REFERENCES servers(name);
ALTER TABLE
    instances ADD CONSTRAINT ins_ser_ip FOREIGN KEY(ser_ip) REFERENCES servers(ip);
ALTER TABLE
    characters ADD CONSTRAINT chara_inst_id FOREIGN KEY(instance_id) REFERENCES instances(id);
ALTER TABLE
    characters ADD CONSTRAINT chara_player_username FOREIGN KEY(player_username) REFERENCES players(username);

insert into instances values
('serverA','1','138.201.233.18','active','2020-10-20'),
('serverB','2','138.201.233.19','active','2020-10-20'),
('serverE','3','138.201.233.14','active','2020-10-20');

insert into characters values
('characterA','typeA','1','Game data of characterA','2020-07-18 02:12:12','2020-07-18 02:32:30','192.188.11.1','1','nabin123'),
('characterB','typeB','3','Game data of characterB','2020-07-19 02:10:12',null,'192.180.12.1','2','rabin123'),
('characterC','typeC','1','Game data of characterC','2020-07-18 02:12:12',null,'192.189.10.1','3','sabin123'),
('characterD','typeA','1','Game data of characterD','2020-07-18 02:12:12','2020-07-18 02:32:30','192.178.11.1','2','nabin123'),
('characterE','typeB','3','Game data of characterE','2020-07-19 02:10:12',null,'192.190.12.1','1','rabin123'),
('characterF','typeC','1','Game data of characterF','2020-07-18 02:12:12',null,'192.188.10.1','3','sabin123'),
('characterG','typeD','1','Game data of characterG','2020-07-18 02:12:12',null,'192.188.13.1','1','nabin123'),
('characterH','typeD','3','Game data of characterH','2020-07-19 02:10:12',null,'192.180.17.1','2','bipin123'),
('characterI','typeD','1','Game data of characterI','2020-07-18 02:12:12','2020-07-18 02:32:30','192.189.18.1','3','dhiraj123'),
('characterJ','typeD','3','Game data of characterJ','2020-07-18 02:12:12',null,'192.178.19.1','2','prabin123'),
('characterK','typeB','4','Game data of characterK','2020-07-19 02:10:12','2020-07-19 02:11:30','192.190.20.1','1','rabin123'),
('characterL','typeC','2','Game data of characterL','2020-07-18 02:12:12',null,'192.192.11.1','3','sabin123'),
('characterM','typeC','3','Game data of characterM','2020-07-18 02:12:12',null,'192.192.11.1','2','sabin123');

这里我需要一个显示服务器名称、实例 ID 和活动会话数的视图（如果结束时间戳为空，则会话处于活动状态）。是我的代码有误还是其他？我开始学习所以希望得到积极的最佳答案。

我的观点

create view active_sessions as
select i.ser_name, i.id, count(end_ts) as active
from instances i, characters c
where i.id=c.instance_id and c.end_ts = null
group by i.ser_name, i.id;

Answer 1

这不是你想要的：

where i.id = c.instance_id and c.end_ts = null

没有什么等于 null。您需要 is null 来对照 null.

检查一个值

此外，count(end_ts) 将始终产生 0，因为我们已经知道 end_ts 是 null，而 count() 不考虑。

最后，我强烈建议使用标准连接（使用 on 关键字），而不是隐式连接（在 from 子句中使用逗号）：这种古老的语法来自几十年ago 不应在新代码中使用。我认为 left join 更接近您想要的（它还会考虑根本没有字符的实例）。

所以：

create view active_sessions as
select i.ser_name, i.id, count(c.nickname) as active
from instances i
left join characters c on i.id = c.instance_id and c.end_ts is null
group by i.ser_name, i.id;

显示服务器名称、实例 ID 和活动会话数的视图（如果结束时间戳为空，则会话处于活动状态）

A view that shows the name of the server, the id of the instance and the number of active sessions (a session is active if the end timestamp is null)

postgresql

join

count

where-clause

sql-view