Query with subquery-count and groupby - sql

Question

Below is the ERD
I want to count number of gender ('Male' and 'Female') for each month irrespective of year.
What I have tried so far is that I can count number of males and females for each month separately like below
Query
Select u.Gender, datename(month, p.EntryDate) month, COUNT(p.User_Id) count
from [HospitalManagement].[dbo].[Patients] p,[HospitalManagement].[dbo].[Users] u
where u.Id = p.User_Id
group by datename(month, p.EntryDate), u.Gender
Result
I want it like below
Expected Result
Month | MaleCount | FemaleCount
June | 0 | 2
November | 1 | 1
To achieve above I try following query
Query
Select datename(month, p.EntryDate) month,
(select count(u.gender) from [HospitalManagement].[dbo].[Users] u
where u.Id = p.User_Id and u.Gender = 'Female'
group by u.Gender) female,
(select count(u.gender) from [HospitalManagement].[dbo].[Users] u
where u.Id = p.User_Id and u.Gender = 'Male'
group by u.Gender) male
from [HospitalManagement].[dbo].[Patients] p
group by datename(month, p.EntryDate)
Error
Column 'HospitalManagement.dbo.Patients.User_Id' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.
Below are the create statements of tables (I am using MSSql)
-- Creating table 'Users'
CREATE TABLE [dbo].[Users] (
[Id] bigint IDENTITY(1,1) NOT NULL,
[Email] nvarchar(max) NULL,
[Password] nvarchar(max) NULL,
[UserName] nvarchar(max) NULL,
[Age] bigint NULL,
[Gender] nvarchar(max) NULL,
[NRIC] nvarchar(max) NULL,
[Comments] nvarchar(max) NULL,
[Address] nvarchar(max) NULL,
[ContactNo] nvarchar(max) NULL,
[FullName] nvarchar(max) NULL
);
GO
-- Creating table 'Patients'
CREATE TABLE [dbo].[Patients] (
[Id] bigint IDENTITY(1,1) NOT NULL,
[Disease] nvarchar(max) NULL,
[Occupation] nvarchar(max) NULL,
[EntryDate] datetime NULL,
[EntryTime] time NULL,
[User_Id] bigint NOT NULL
);
GO

Answer 1

As the error says, you are directly using User_Id column in select clause which is not present in the GROUP BY.
You can change the correlated subqueries to faster LEFT JOINs (assuming the User_Id is unique in the Patients table).
select
datename(month, p.EntryDate) mon,
count(case when u.Gender = 'Female' then 1 end) female_cnt,
count(case when u.Gender = 'Male' then 1 end) male_cnt
from [Patients] p left join [Users] u
on p.User_Id = u.Id
group by datename(month, p.EntryDate);
EDIT:
you can use a lookup CTE to generate all months and then do LEFT JOIN with it like this:
;WITH months(mn, mon) AS
(
SELECT 1, DATENAME(MONTH,DATEADD(month,0,GETDATE())) mon
UNION ALL
SELECT mn+1, DATENAME(MONTH,DATEADD(MONTH,mn,GETDATE()))
FROM months
WHERE mn < 12
)
select
m.mon mon,
count(case when u.Gender = 'Female' then 1 end) female_cnt,
count(case when u.Gender = 'Male' then 1 end) male_cnt
from months m left join [Patients] p
on m.mon = datename(month, p.EntryDate)
left join [Users] u
on p.User_Id = u.Id
group by m.mon;

Answer 2

You can try the following query:
Select
datename(month, p.EntryDate) month,
COUNT(IF(u.Gender = 'Male', 1, NULL) AS MaleCount,
COUNT(IF(u.Gender = 'Female', 1, NULL) AS FemaleCount
from
[HospitalManagement].[dbo].[Patients] p,[HospitalManagement].[dbo].[Users] u
where
u.Id = p.User_Id
group by
datename(month, p.EntryDate)

Answer 3

This is how we do in MySQL or Oracle. Have not tried in MSSQL though. But as these are standard SQL function (and is available in MSSQL), this should work in MSSQL as well.
Select datename(month, p.EntryDate) month , SUM(CASE WHEN u.Gender = 'Male' THEN 1 ELSE 0) MaleCount, SUM(CASE WHEN u.Gender = 'Female' THEN 1 ELSE 0) FemaleCount,
from [HospitalManagement].[dbo].[Patients] p,[HospitalManagement].[dbo].[Users] u
where u.Id = p.User_Id
group by datename(month, p.EntryDate)
Typically I am adding 1 in case of the specific gender to get the result.
Note: If there are any syntax errors please let me know with the error, I can correct it.