Question

这是我正在使用的数据库

CREATE TABLE Users(

    uid int PRIMARY KEY,

    name text,

    phone text

);
CREATE TABLE Messages(

    recipient int REFERENCES Users(uid),

    sender int REFERENCES Users(uid),

    time timestamp NOT NULL,

    message text NOT NULL,

    PRIMARY KEY (recipient, sender, time)

);

我需要回答的查询是 “考虑已收到给用户“ SomeUser”的消息的所有用户。还有多少其他用户已向所有 “ SomeUser”发送？”

我已经尝试过了，但是似乎不起作用，给出了count = 0（我想是因为我要我的数据库告诉我接收者（一个值）是否等于ALL创建的向量。）

SELECT count(*)
FROM Users AS U , Messages AS M
WHERE U.uid = M.sender AND U.name != 'SomeUser'  AND M.recipient =ALL
   (#id of users that received a message from “SomeUser”
    SELECT DISTINCT M.recipient 
    FROM Users AS U , Messages AS M
    WHERE U.name='SomeUser' AND U.uid=M.sender 
    );

如果此数据库中有这些值

INSERT INTO Users VALUES (0,'Maria','1123456123');

INSERT INTO Users VALUES (1,'John','1123456123');

INSERT INTO Users VALUES (2,'Freddy','1123456123');

INSERT INTO Users VALUES (3,'Joanna','1123456123');

INSERT INTO Users VALUES (4,'Jim','1123456123');

INSERT INTO Users VALUES (5,'Luis','1123456123');

INSERT INTO Messages VALUES (0,1,'2012-12-15 12:57:59.99999','Hello there');

INSERT INTO Messages VALUES (0,2,'2012-12-15 21:59:59.9999','Hello there');

INSERT INTO Messages VALUES (0,3,'2001-12-15 20:57:50.999','Hello theret');

INSERT INTO Messages VALUES (1,4,'2012-12-15 11:59:59.999','Hello there');

INSERT INTO Messages VALUES (2,3,'2012-12-15 21:52:50.9999','Hello there');

INSERT INTO Messages VALUES (2,4,'2012-12-15 11:59:59.99999','Hello there');

INSERT INTO Messages VALUES (3,4,'2012-12-15 20:57:50.999999','Hello there');

INSERT INTO Messages VALUES (4,1,'2012-12-15 23:59:59.99999','Hello there');

INSERT INTO Messages VALUES (4,2,'2012-12-15 20:57:59.9999','Hello there');

INSERT INTO Messages VALUES (2,3,'2012-12-15 21:51:59.999','Hello there');

INSERT INTO Messages VALUES (1,4,'2012-12-15 20:51:59.9999','Hello there');

INSERT INTO Messages VALUES (3,2,'2012-12-15 22:59:50.99999','Hello there');

INSERT INTO Messages VALUES (3,2,'2012-12-15 15:59:59.9999','Hello there');

INSERT INTO Messages VALUES (3,4,'2012-12-15 23:22:59.9999','Hello there');

INSERT INTO Messages VALUES (3,0,'2012-12-15 23:01:59.9999','Hello there');

INSERT INTO Messages VALUES (1,5,'2012-12-15 23:01:59.9999','Hello there');

INSERT INTO Messages VALUES (2,5,'2012-12-15 23:01:59.9999','Hello there');

INSERT INTO Messages VALUES (3,1,'2001-12-15 23:59:59.99999','Hello there');

INSERT INTO Messages VALUES (5,1,'1995-12-15 23:59:59.99999','Hello there');

INSERT INTO Messages VALUES (3,5,'2002-12-15 23:01:59.9999','Hello there');

考虑SomeUser ='Luis'，此查询需要返回 1（因为发送到uid 1,2和3的唯一一个是4）如果SomeUser ='Maria'，答案必须为4

提前感谢您的时间。

Answer 1

分组，加入并最终计数：

select count(*) counter from (
  select m.sender
  from Messages m inner join Users u
  on u.uid = m.sender
  where 
    u.name <> 'Maria' and
    m.recipient in (
      select distinct m.recipient 
      from Messages m inner join Users u
      on u.uid = m.sender
      where u.name = 'Maria')
  group by m.sender
  having 
    count(distinct m.recipient) = 
    (select count(distinct m.recipient) 
     from Messages m inner join Users u
     on u.uid = m.sender
     where u.name = 'Maria')
) t

请参见demo。

如何查找已向特定用户发送消息给相同用户的用户数量。的SQL

1 个答案: