Classes With at Least 5 Students - Practice Coding Problems

Classes With at Least 5 Students - Problem

Database Easy

You are given a table Courses that contains information about student enrollments in various classes.

Table: Courses

Column Name	Type
student	varchar
class	varchar

The combination of (student, class) is the primary key for this table. Each row indicates the name of a student and the class in which they are enrolled.

Task: Write a SQL solution to find all the classes that have at least five students. Return the result table in any order.

Table Schema

Courses

Column Name	Type	Description
`student` PK	varchar	Name of the student
`class` PK	varchar	Name of the class the student is enrolled in

Primary Key: (student, class)

Note: Each row represents a unique student-class enrollment pair

Input & Output

Example 1 — Basic Class Count

Input Table:

student	class
A	Math
B	English
C	Math
D	Biology
E	Math
F	Computer
G	Math
H	Math
I	Math

Output:

class
Math

💡 Note:

Math has 6 students (A, C, E, G, H, I) which is ≥ 5, so it's included. English has 1 student, Biology has 1 student, and Computer has 1 student, all less than 5.

Example 2 — Multiple Qualifying Classes

Input Table:

student	class
Alice	Math
Bob	Math
Charlie	Math
David	Math
Eve	Math
Frank	English
Grace	English
Henry	English
Iris	English
Jack	English

Output:

class
English
Math

💡 Note:

Both Math and English have exactly 5 students each, so both classes are returned in the result.

Example 3 — No Qualifying Classes

Input Table:

student	class
Student1	Physics
Student2	Chemistry
Student3	Physics
Student4	Biology

Output:

class

💡 Note:

Physics has 2 students, Chemistry has 1 student, and Biology has 1 student. No class has at least 5 students, so the result is empty.

Constraints

1 ≤ number of students ≤ 1000
1 ≤ number of classes ≤ 1000
student and class are non-empty strings

Visualization

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Understanding the Visualization

Input

Student-class enrollment pairs

GROUP BY

Group by class and count students

HAVING

Filter classes with count >= 5

Key Takeaway

🎯 Key Insight: Use GROUP BY to aggregate data and HAVING to filter aggregated results

Asked in

a Amazon 23 m Microsoft 18 G Google 15

Use GROUP BY to group students by class, then HAVING COUNT(*) >= 5 to filter classes with at least 5 students. The key insight is using HAVING instead of WHERE since we need to filter after grouping.

Table Schema

Courses

Column Name	Type	Description
`student` PK	varchar	Name of the student
`class` PK	varchar	Name of the class the student is enrolled in

Primary Key: (student, class)

Note: Each row represents a unique student-class enrollment pair

Common Approaches

Approach	Time	Space	Notes
✓ GROUP BY with HAVING	O(n)	O(k)	Group students by class, count them, and filter classes with at least 5 students

GROUP BY with HAVING — Algorithm Steps

Step 1: GROUP BY class to group all rows by class name
Step 2: COUNT(*) to count students in each group
Step 3: HAVING COUNT(*) >= 5 to filter groups with at least 5 students

Visualization

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Step-by-Step Walkthrough

Group

GROUP BY class

Count

COUNT(*) students per class

Filter

HAVING COUNT(*) >= 5

Code -

solution.c — C

Time & Space Complexity

Time Complexity

⏱️

O(n)

Single pass to group and count rows

✓ Linear Growth

Space Complexity

O(k)

k is the number of unique classes for grouping

✓ Linear Space

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Smart Actions

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Table Schema

Input & Output

Constraints

Visualization

Related Problems

Table Schema

Common Approaches

GROUP BY with HAVING — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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