Find Customers With Positive Revenue this Year

Find Customers With Positive Revenue this Year - Problem

Database Easy

Given a table Customers that contains customer information including their customer ID, year, and revenue data, write a SQL solution to find all customers who had positive revenue in the year 2021.

Note: Revenue can be negative, so we only want customers where revenue > 0 for the year 2021.

Return the result table in any order.

Table Schema

Customers

Column Name	Type	Description
`customer_id` PK	int	Unique identifier for each customer
`year` PK	int	The year of the revenue record
`revenue`	int	Customer revenue for that year (can be negative)

Primary Key: (customer_id, year)

Note: The combination of customer_id and year forms the primary key

Input & Output

Example 1 — Mixed Revenue Records

Input Table:

customer_id	year	revenue
1	2021	1000
2	2021	-1000
3	2020	2000
1	2020	4000

Output:

customer_id
1

💡 Note:

Only customer 1 has positive revenue in 2021 (1000). Customer 2 has negative revenue in 2021 (-1000), and customer 3's positive revenue is from 2020, not 2021.

Example 2 — No Positive Revenue in 2021

Input Table:

customer_id	year	revenue
1	2021	-500
2	2021	0
3	2020	1000

Output:

customer_id

💡 Note:

No customers have positive revenue in 2021. Customer 1 has negative revenue, customer 2 has zero revenue (not positive), and customer 3's positive revenue is from 2020.

Example 3 — Multiple Positive Customers

Input Table:

customer_id	year	revenue
1	2021	500
2	2021	1500
3	2021	200

Output:

customer_id
1
2
3

💡 Note:

All three customers have positive revenue in 2021, so all customer IDs are returned in the result.

Constraints

1 ≤ customer_id ≤ 100000
2000 ≤ year ≤ 2025
-1000000 ≤ revenue ≤ 1000000

Visualization

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Understanding the Visualization

Input

Customer revenue records across years

Filter

WHERE year = 2021 AND revenue > 0

Output

Customer IDs with positive 2021 revenue

Key Takeaway

🎯 Key Insight: Use compound WHERE conditions to filter data by multiple criteria simultaneously

Asked in

A Amazon 15 G Google 12 M Microsoft 8

Use a simple WHERE clause with year = 2021 AND revenue > 0 to filter customers with positive revenue in the specified year.

Table Schema

Customers

Column Name	Type	Description
`customer_id` PK	int	Unique identifier for each customer
`year` PK	int	The year of the revenue record
`revenue`	int	Customer revenue for that year (can be negative)

Primary Key: (customer_id, year)

Note: The combination of customer_id and year forms the primary key

Common Approaches

Approach	Time	Space	Notes
✓ Basic Query with WHERE Filter	O(n)	O(1)	Use WHERE clause to filter by year and positive revenue

Basic Query with WHERE Filter — Algorithm Steps

Step 1: Select customer_id from Customers table
Step 2: Apply WHERE filter for year = 2021 AND revenue > 0

Visualization

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Step-by-Step Walkthrough

Scan

Check each row

Filter

Apply year = 2021 AND revenue > 0

Select

Return matching customer_id

Code -

solution.c — C

Time & Space Complexity

Time Complexity

⏱️

O(n)

Linear scan through table rows

✓ Linear Growth

Space Complexity

O(1)

No additional space needed for filtering

✓ Linear Space

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Smart Actions

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Table Schema

Input & Output

Constraints

Visualization

Related Problems

Table Schema

Common Approaches

Basic Query with WHERE Filter — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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