Column 'Course.Course_Name' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause - sql

Question

I need to link two tables columns, please help me. This my code:
SELECT Student.Stu_Course_ID, Course.Course_Name, COUNT(Student.Stu_ID) AS NoOfStudent FROM Student
INNER JOIN Course
ON Student.Stu_Course_ID=Course.Course_ID
GROUP BY Stu_Course_ID;
This is my course table:
__________________________________________
|Course_ID | Course_Name |
|1 | B.Eng in Software Engineering |
|2 | M.Eng in Software Engineering |
|3 | BSC in Business IT |
I got number of students from student table
_____________________________
|Stu_Course_ID | NoOfStudents |
|1 | 30 |
|2 | 12 |
|3 | 20 |
This is what i want
____________________________________________________________
|Stu_Course_ID | Course_Name | NoOfStudents|
|1 | B.Eng in Software Engineering | 30 |
|2 | M.Eng in Software Engineering | 12 |
|3 | BSC in Business IT | 20 |

Answer 1

You need to add Course.Course_Name to your group by clause:
SELECT Student.Stu_Course_ID,
Course.Course_Name,
COUNT(Student.Stu_ID) AS NoOfStudent
FROM Student
INNER JOIN Course
ON Student.Stu_Course_ID=Course.Course_ID
GROUP BY Student.Stu_Course_ID, Course.Course_Name;
Imagine the following simple table (T):
ID | Column1 | Column2 |
----|---------+----------|
1 | A | X |
2 | A | Y |
Your query is similary to this:
SELECT ID, Column1, COUNT(*) AS Count
FROM T
GROUP BY Column1;
So, you know you have 2 records for A in column1, so you expect a count of 2, however, you are also selecting ID, there are two different values for ID where Column1 = A, so the following result:
ID | Column1 | Count |
----|---------+----------|
1 | A | 2 |
Is no more or less correct than
ID | Column1 | Count |
----|---------+----------|
2 | A | 2 |
This is why ID cannot be contained in the select list, unless it included in the group by clause, or as part of an aggregate function.
For what it's worth, if Course_ID is the primary key in the table Course then following query is legal according to the SQL Standard, and will work in Postgresql, and I suspect at some point Microsoft will build this functionality into SQL Server too:
SELECT Course.Course_ID,
Course.Course_Name,
COUNT(Student.Stu_ID) AS NoOfStudent
FROM Student
INNER JOIN Course
ON Student.Stu_Course_ID=Course.Course_ID
GROUP BY Course.Course_ID;
The reason for this is that since Course.Course_ID is the primary key of Course there can be no duplicates of this in the table, therefore there can only be one value for Course_name for each Course_ID

Answer 2

give columns names after group by statements which you want to retreive so you have to also give Course.Course_Name as well...