SQL query with two tables, count and more info - sql-server

Question

I'm just learning this stuff and I'm having trouble with this one. I have two tables, STUDENTS and ADVISORS. The students are assigned advisors within the students table using a foreign key attached to the primary key of the advisors table.
The task here is this: Provide a list of all advisors and the number of active students assigned to each. Filter out any advisors with more than 1 student.
The current script is listed below:
select
Students.AdvisorID, count(Students.AdvisorID) as 'TotalStudents'
from
Students
left outer join
Advisors on Students.AdvisorID = Advisors.AdvisorID
where
Students.IsActive = 1
Group by
Students.AdvisorID
Having
count(Students.AdvisorID) < 2
This will output a proper list showing only the advisorID and total students.
I need to also display the
Advisors.FirstName + ' ' + Advisors.LastName as 'AdvisorName'
Any help would be greatly appreciated.
EDIT
students table
advisors table

Answer 1

I think your original attempt is on the right track, but you need to join again to the Advisors table to pull in the first and last name for each adviser. The reason for this is that after doing the aggregation all that remains is an ID for each adviser and a student count.
SELECT t1.AdvisorID,
t2.TotalStudents,
t1.FirstName + ' ' + t1.LastName AS AdvisorName
FROM Advisors t1
INNER JOIN
(
SELECT a.AdvisorID, COUNT(*) AS TotalStudents
FROM Advisors a
LEFT JOIN Students s
ON a.AdvisorID = s.AdvisorID
GROUP BY a.AdvisorID
HAVING COUNT(*) < 2
) t2
ON t1.AdvisorID = t2.AdvisorID
Other notes:
I chose to LEFT JOIN advisers to students, not the other way around, since you want a statistic for each adviser. Doing the join as you first had it could filter out advisers who do not match to any student. This is not the behavior you want, since an adviser who does not match to any student should have a student count of zero.

Answer 2

Here's a little sample data to work with
USE tempdb
GO
IF OBJECT_ID('tempdb.dbo.Advisors') IS NOT NULL DROP TABLE dbo.Advisors;
IF OBJECT_ID('tempdb.dbo.Students') IS NOT NULL DROP TABLE dbo.Students;
CREATE TABLE dbo.Advisors (AdvisorID int primary key, AdvisorName varchar(100));
CREATE TABLE dbo.Students
(
studentID int identity primary key,
AdvisorID int foreign key references dbo.Advisors(AdvisorID)
);
INSERT dbo.Advisors VALUES (1, 'Mr. White'),(2,'Walter Jr.'),(3,'Mr. Pinkman');
INSERT dbo.Students (AdvisorID)
SELECT TOP (20) abs(checksum(newid())%3)+1 FROM sys.all_columns;
No Left Join needed, I think this will give you what you are looking for.
SELECT a.AdvisorID, total_students = COUNT(*)
FROM dbo.Advisors a
INNER JOIN dbo.Students s ON a.AdvisorID = s.AdvisorID
GROUP BY a.AdvisorID
HAVING COUNT(*) < 2;