SQL queries contain complex joins, subqueries, and some aggregates

In the following section we will see an example of an advanced query:

Where we get the,
Department's names with all the students for each department
Students name separated with comma and
Showing the department having at least three students in it

SELECT
d.DepartmentName,
COUNT(s.StudentId) StudentsCount,
GROUP_CONCAT(StudentName) AS Students
FROM Departments AS d
INNER JOIN Students AS s ON s.DepartmentId = d.DepartmentId
GROUP BY d.DepartmentName
HAVING COUNT(s.StudentId) >= 3;

We added a JOIN clause to get the DepartmentName from the Departments table. After that we added a GROUP BY clause with two aggregate functions:
"COUNT" to count the students for each department group.
GROUP_CONCAT to concatenate students for each group with comma separated in one string.
After the GROUP BY, we used the HAVING clause to filter the departments and select only those departments that have at least 3 students.

The result will be as following:

sqlite> SELECT
...>   d.DepartmentName,
...>   COUNT(s.StudentId) StudentsCount,
...>   GROUP_CONCAT(StudentName) AS Students
...> FROM Departments AS d
...> INNER JOIN Students AS s ON s.DepartmentId = d.DepartmentId
...> GROUP BY d.DepartmentName
...> HAVING COUNT(s.StudentId) >= 3;
DepartmentName  StudentsCount  Students
--------------  -------------  -----------------
IT              3              Michael,John,Jack
Physics         3              Sara,Sally,Nancy