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Accepted
Shir K
Asked: 2020-05-09 01:21:03 +0800 CST

使用 'case when count()=0' & 'where' 时结果不正确

  • 772

如果有人能帮我解决这个问题，我将不胜感激。case when count()=0使用&时我似乎无法得到正确的结果where

我使用 SQL Server，使用类似于“Facebook”的系统数据库。

我正在尝试编写一个显示电子邮件地址、标记状态和过去一个月发布的帖子数量的查询。标记状态应如下： 如果标记 1-2 个帖子 - 显示“few” 如果标记 3-5 个帖子 - 显示“medium” 如果标记 5+ 个帖子 - 显示“many” 如果根本不标记 - 显示“none” .

下面是我写的查询。我写的查询的问题是，由于 WHERE 中的条件，我永远不会收到“无”。

如何更改查询以显示正确的结果？

select U.Mail, count(P.ID) as PostCount, 
  case 
    when count(P.ID) = 0 then 'none'
        when count(P.ID) <= 2 then 'few'
        when count(P.ID) <= 5 then 'medium'
    else 'many'
  end PostCountCategory
from Users U 
    left join Tagging T on U.Mail = T.Mail
    left join Post P on T.IDPost = P.ID
where datediff(day,P.DatePosted,getdate()) <= 30 --Because of this condition I would never get 'none'
group by U.Mail, U.Gender

数据举例：

所需的输入应该是：Kelly-'none'、Lilly-'none'、Nelly-'few'、Owen-'none'。

create table Users
(
Mail nvarchar (20) primary key check(Mail like '_%@_%._%' and (Mail like '%[0-9]%' Or Mail like '%[a-z]%'Or Mail like '%[A-Z]%')),
Password nvarchar (8) check (Password like '%[0-9]%' and Password like '%[az]%' and len(password) <= 8) not null,
FirstName nvarchar (20) not null,
LastName nvarchar (20) not null,
BirthDate date check (datediff(year,BirthDate,getdate())>=18) not null,
JoinDate date check (JoinDate<=getdate()) not null,
Gender nchar(1) check(Gender = 'F' or Gender = 'M' or Gender = 'O'),
NickName nvarchar(20),
Photo nvarchar(20),
Phone bigint check (Phone like '%[0-9]%' and len(Phone) <= 10) not null
)

INSERT INTO Users
VALUES
('[email protected]','k1000000','Kelly','Ka','1992-05-15','2016-09-04','F','Kelly','Kelly.jpg','546296100'),
('[email protected]','l1101111','Lilly','La','1999-04-03','2012-04-04','F','Lilly','Lilly.jpg','542448300'),
('[email protected]','n130131','Nelly','Na','1994-03-07','2020-04-13','F','Nelly','NellyNa.jpg','541234567');
('[email protected]','o140141','Owen','Oa','1992-02-02','2020-05-13','M','Owen','OwenOa.jpg','541234567');


create table Post
(
ID int identity(1,1) primary key,
Photo nvarchar(20),
Text nvarchar(200),
Location nvarchar(50),
Video int,
DatePosted date check (datediff(month,DatePosted,getdate())<=3) not null,
UserMail nvarchar (20) references Users(Mail) on delete cascade on update
cascade not null
)

INSERT INTO Post
VALUES
('','my name is nellu','','','2020-05-08','[email protected]'),
('','hii','','','2020-02-19','[email protected]');


create table Tagging
(
Mail nvarchar (20) references Users(Mail) not null,
IDPost int references Post(ID) not null,    
TagMail nvarchar(20) references Users (Mail) not null,
primary key (Mail,IDPost);
)

INSERT INTO Tagging
VALUES
('[email protected]','1','[email protected]'),
('[email protected]','1','[email protected]');
sql-server relational-theory

2 个回答

  1. Best Answer

    只需将 where 条件移动到连接部分：

    select U.Mail, count(P.ID) as PostCount, 
  case 
    when count(P.ID) = 0 then 'none'
        when count(P.ID) <= 2 then 'few'
        when count(P.ID) <= 5 then 'medium'
    else 'many'
  end PostCountCategory
from Users U 
    left join Tagging T on U.Mail = T.Mail
    left join Post P on T.IDPost = P.ID AND datediff(day,P.DatePosted,getdate()) <= 30
group by U.Mail, U.Gender

    您的原始条件也不允许 SQL Server 使用索引查找。最好这样重写：

    P.DatePosted >= dateadd(day, -30, cast(getdate() as date))
    • 6


  2. 首先，将 Post 表与 Tagging 表结合起来，在哪里进行计数，如下所示：

     SELECT T.Mail,COUNT(P.ID) as myCount
 FROM Post as P
    INNER JOIN Tagging as T 
        ON T.IDPost = P.ID
 WHERE datediff(day,P.DatePosted,getdate()) <= 30
 GROUP BY T.Mail

    最后，将其与用户表重新加入（不再需要 Group By，如果邮寄的话；它是唯一的）

    select U.Mail, p.myCount as PostCount, 
  case 
    when p.myCount = 0 then 'none'
    when p.myCount <= 2 then 'few'
    when p.myCount <= 5 then 'medium'
    when p.myCount > 5 then 'many'
  else 'N/A'
  end PostCountCategory
from Users U 
    --left join Tagging T on U.Mail = T.Mail
    left join 
    (
     SELECT T.Mail,COUNT(P.ID) as myCount
     FROM Post as P
        INNER JOIN Tagging as T 
            ON T.IDPost = P.ID
     WHERE datediff(day,P.DatePosted,getdate()) <= 30
     GROUP BY T.Mail
     )as p 
        ON P.Mail = U.Mail
--where datediff(day,P.DatePosted,getdate()) <= 30 --Because of this condition I would never get 'none'
--group by U.Mail, U.Gender

    dbfiddle在这里

    • 4

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