Question: I am trying to run my Django server via 'python manage.py runserver' on a github repository i cloned and always get this runtime error message:

> 
> class AbstractBaseUser(models.Model):
> 
> 
> RuntimeError: \_\_\_\_class\_\_\_\_ not set defining 'AbstractBaseUser' as <'class 'django.contrib.auth.base\_user.AbstractBaseUser'>. Was \_\_\_\_classcell\_\_\_\_ propagated to type.\_\_\_\_new\_\_\_\_?
> 
> 
> 

Details of installed packages in my virtual environment are:
Django 1.9 |
django-crispy-forms 1.6.0 |
django-markdown-deux 1.0.5 |
django-pagedown 0.1.1 |
markdown2 2.3.1 |
olefile 0.46 |
Pillow 7.1.1 |
pip 19.2.3 |
setuptools 41.2.0 |

I am using python version 3.8.1

please does anyone have any suggestions on how to solve this error. Thank you.

Answer: 
You cant string aggregation.

In MySQL, use `group_concat()`:

```
select
    studentId,
    studentName, 
    group_concat(courseName order by courseName separator ', ') courseNames
from mytable
group by studentId, studentName

```

In SQL Server 2017 or higher, use `string_agg()`:

```
select
    studentId,
    studentName, 
    string_agg(courseName, ', ') within group(order by courseName) courseNames
from mytable
group by studentId, studentName

```
For MySql try,

```
    SELECT StudentId, StudentName, dbo.GROUP_CONCAT(CourseName) 
          FROM TableName 
          GROUP BY StudentId, StudentName

```

For SQL Server try, 

```
select StudentId, StudentName, stuff((select '; ' + a.CourseName
          from TableName a
           Group By StudentId, StudentName
          for xml path ('')
         ), 1, 2, '')  as CourseNames

```