Find all students with an `age` less than `21`.

Problem Statement

<p>Find all <code>students</code> with an <code>age</code> less than <code>21</code>.</p>

Examples

Input: students collection: +-----+--------+-----+ | _id | name | age | +-----+--------+-----+ | 1 | John | 20 | | 2 | Emma | 22 | | 3 | Liam | 19 | | 4 | Olivia | 23 | +-----+--------+-----+

Output: +-----+------+-----+ | _id | name | age | +-----+------+-----+ | 1 | John | 20 | | 3 | Liam | 19 | +-----+------+-----+

Explanation: The query retrieves all documents from the collection that satisfy the specified filter criteria.

Complexity

Time Complexity: -

Space Complexity: -

Hints

💡 Hint 1: Use comparison operators ($gt, $gte, $lt, $lte) to filter documents based on numeric or date ranges. 💡 Hint 2: Construct the filter condition: { fieldName: { $gt: value } } inside db.<collection>.find(...). 💡 Hint 3: Chain projection or sorting if requested, and invoke .toArray() to produce the result array.

Editorial & Approach

Problem Overview & Intuition

To solve "Less Than ($lt)", we query the MongoDB document store. The goal is to find all students with an `age` less than `21`. Using a targeted find query, the database engine filters and structures the BSON documents efficiently.

Step-by-Step Approach

Identify Target Collection: Access the collection through the db instance.
Construct Query / Pipeline: Build the query filter with appropriate comparison operators.
Resolve Cursor: Invoke .toArray() to transform the query cursor into the required array of documents.

Optimal Implementation (MongoDB)

function solve(db) {
  return db.students.find({ age: { $lt: 21 } }).toArray();
}

Complexity Analysis

Time Complexity O(N) collection scan (O(log N) if index is present on filtered fields).

Space Complexity O(K) where K is the number of returned documents in memory.

Key Considerations & Edge Cases

Empty Collections: If no documents match, the query cleanly returns an empty array [].
Missing / NULL Fields: Missing fields in documents are handled safely without throwing runtime exceptions.
Type Coercion: BSON types (ObjectId, Numbers, Strings) are compared strictly according to MongoDB specifications.

Less Than ($lt)

Easy

Find all students with an age less than 21.

Example Scenarios

1Example 1

Input:

students collection

_id	name	age
1	John	20
2	Emma	22
3	Liam	19
4	Olivia	23

Output:

_id	name	age
1	John	20
3	Liam	19

Explanation:

The query retrieves all documents from the collection that satisfy the specified filter criteria.

MongoDB Editor

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Query Results (JSON)

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