[TOC] ## Solution --- [Pandas](#pandas) [Database](#database) # pandas ### Approach: Group By #### Algorithm We need to count the number of each unique classes, so we can group the DataFrame `courses` by the `class` column using the function `groupby('class')`. Then, we use the function `size()` to count the number of occurrences for each unique class, which gives us the count of students in each class. We use `reset_index(name='count')` to name this column as `count`. The result DataFrame `df` consists of two columns: `class` and `count`. ```python df = courses.groupby('class').size().reset_index(name='count') ``` We will have the following DataFrame `df`:

class	count
Biology	1
Computer	1
English	1
Math	6

Next, we filter `df` to select only the rows where the `count` column is greater than or equal to 5, which helps us identify the classes with more than 5 students. ```python df = df[df['count'] >= 5] ```

class	count
Math	6

We need to return the required column `class`, thus the complete code is as follows: #### Implementation We will obtain the following DataFrame:

class
Math

--- # database ### Approach: Group By #### Algorithm First, we can count the student number in each class. And then select the ones have more than 5 students. To get the student number in each class. We can use `GROUP BY` and `COUNT`, which is very popular used to statistic bases on some character in a table. ```sql SELECT class, COUNT(student) FROM courses GROUP BY class ; ```

class	COUNT(student)
Biology	1
Computer	1
English	1
Math	6

To continue, we can filter the classes by taking the above query as a sub-query. #### Implementation ```sql SELECT class FROM (SELECT class, COUNT(student) AS num FROM courses GROUP BY class) AS temp_table WHERE num >= 5 ; ``` >Note: Make an alias of `COUNT(student)` ('num' in this case) so that you can use in the `WHERE` clause because it cannot be used directly over there.
### Approach 2: Using `GROUP BY` and `HAVING` condition #### Algorithm Using sub-query is one way to add some condition to a `GROUP BY` clause, however, using [`HAVING`](https://dev.mysql.com/doc/refman/5.7/en/group-by-handling.html) is another simpler and natural approach. So we can rewrite the above solution as below. #### Implementation ```sql SELECT class FROM courses GROUP BY class HAVING COUNT(student) >= 5 ; ```

Classes More Than 5 Students

Problem Description

Problem Description

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