select * from Users
select *from Accounts
select * from Clients
exec getUserDetails 'username1','password1'

alter procedure getUserDetails
@username varchar(10), @password varchar(10)
as
BEGIN
    if not exists (select 'True' from Users where UserName=@username and Password=@password)
    begin
        Raiserror('User does not exist!',0,0)
    end
   else
   begin
select *
from Users U
left join Clients C on U.ClientId=C.Id
left join Accounts A on A.ClientId=C.Id
where U.UserName=@username and u.Password=@password
end

END
GO