Incorrect result when using 'case when count()=0' & 'where'

Question

I would very appreciate if someone could help me out with this query. I can't seem to get the correct result when using case when count()=0 & where

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I work with SQL Server, with a DB of a system that is alike "Facebook".

I'm trying to write a query that shows the email address, tagging status & the number of posts posted in the past month. The tagging status should be as follows: If tagging 1-2 posts - display "few" If tagging 3-5 posts - display "medium" If tagging 5+ posts - display "many" If not tagging at all - display "none".

Below is the query I wrote. The problem with the query I wrote is that because of the condition in WHERE, I will never receive 'none'.

How can I change the query in order to display the correct result?

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

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Data for example:

The required input should be: 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
('Kelly@gmail.com','k1000000','Kelly','Ka','1992-05-15','2016-09-04','F','Kelly','Kelly.jpg','546296100'),
('Lilly@gmail.com','l1101111','Lilly','La','1999-04-03','2012-04-04','F','Lilly','Lilly.jpg','542448300'),
('Nelly@gmail.com','n130131','Nelly','Na','1994-03-07','2020-04-13','F','Nelly','NellyNa.jpg','541234567');
('Owen@gmail.com','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','Nelly@gmail.com'),
('','hii','','','2020-02-19','Lilly@gmail.com');


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
('Nelly@gmail.com','1','Kelly@gmail.com'),
('Nelly@gmail.com','1','Owen@gmail.com');

Answer 1

just move where conditions to the join part:

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

also your original condition doesn't alloow SQL Server to use index seek. It would be better to rewrite it this way:

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

Answer 2

First , combine Post table with Tagging table ,where will do the count, like this:

 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

and finally, rejoin it with the Users table (no longer need Group By, if mail it;s unique)

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 here