Help with GROUP BY clause

need your 开发者_运维知识库help with GROUP BY clause.

I have two tables: Managers and Clients (I simplified it for a question).

Managers:
managerId int PK,
ManagerName nvarchar(50)

and

Clients:
ClientId int,
ClientName nvarchar(50),
ManagerId int, WhenAdded datetime

I store managers list in Managers table. Each client from Clients table has a corresponding manager.

I want a sql query which returns following table:

ManagerName nvarchar(50),
NumberOfClients int

My problem is: if manager doesn't have a client(s). Sql query doesn't include ManagerName in resulting table.

I tried this query:

SELECT M.ManagerName, COUNT(*) 
FROM
Clients AS C left join Managers AS M 
ON
C.ManagerId = M.ManagerId
GROUP BY 
M.ManagerName

Second question

Same as a first question, but if I want to add a where clause to the WhenAdded field in Clients). Query must return ManagerName and NumberOfClients even if NumberOfClients=0.

You can use this:

SELECT ManagerName, (SELECT Count(ClientID) FROM Clients C WHERE
    C.ManagerID=M.ManagerID) As NumberOfClients FROM Managers M;

or this:

SELECT ManagerName, Count(C.ClientID) As NumberOfClients FROM Managers M
    LEFT JOIN Clients C ON M.ManagerID=C.ManagerID GROUP BY M.ManagerID,
    M.ManagerName;

The problem with your query is that you're using Clients table as basic table.

For the second question you can use this:

SELECT ManagerName, (SELECT Count(ClientID) FROM Clients C WHERE
    WhenAdded<DATEADD(day, 1, GETDATE()) AND M.ManagerID=C.ManagerID) As
    NumberOfClients FROM Managers M;

This should work. Not very optimized though!

select m.ManagerName , count(c.ManagerId) from Managers m, Clients c where 
m.ManagerId = c.ManagerId group by c.ManagerId