Users With API Errors

A medium SQL interview practice problem on DataDriven. Write the query and run it against a real database, with instant feedback.

Domain
SQL
Difficulty
medium
Seniority
L3

Problem

How many unique users have triggered at least one API error? An API error is any call that returned a status code of 400 or higher.

Count unique users who have triggered an API error response

Tables

users

Columns: user_id INTEGER, username TEXT, email TEXT, signup_date TEXT, account_status TEXT, age_bucket TEXT

users: 10 of 200 rows
user_idusernameemailsignup_dateaccount_statusage_bucket
100alicealice@example.com2025-02-02inactive25-34
197aaron42aaron42@example.com2026-03-03suspended35-44
294 ameliaamelia@example.com2024-04-04pending_verification45-54
391arjun arjun@example.com2025-05-05active55-64
488ava99ava99@example.com2026-06-06inactive65+
585andrew_kandrew_k@example.com2024-07-07suspendedNULL
682anikaanika@example.com2025-08-08 16:34:00pending_verification18-24
779aidenaiden@example.com2026-09-09 14:53:00active25-34
876aria_baria_b@example.com2024-10-10inactive35-44
973brianbrian@example.com2025-11-11suspended45-54

api_calls

Columns: call_id INTEGER, endpoint TEXT, method TEXT, status INTEGER, latency REAL, user_id INTEGER, call_time TEXT, err_msg TEXT

api_calls: 10 of 670 rows
call_idendpointmethodstatuslatencyuser_idcall_timeerr_msg
8079/api/v1/ordersPOST2004.21002026-02-02 01:07:00NULL
8158/api/v2/productsPUT2007.91972026-03-03 02:14:00NULL
8237/api/v1/searchDELETE20111.62942026-04-04 03:21:00NULL
8316/api/v1/auth/loginPATCH20415.33912026-05-05 04:28:00NULL
8395/api/v1/auth/logoutget30119NULL2026-06-06 05:35:00NULL
8474/api/v2/analyticspost40022.75852026-07-07 06:42:00Not found
8553/api/v1/paymentsGET40126.46822026-08-08 07:49:00Unauthorized
8632/api/v1/users/POST40330.17792026-09-09 08:56:00Rate limit exceeded
8711/api/v2/products/PUT40433.88762026-01-10 09:03:00Internal server error
8790/api/v1/usersDELETE50037.5NULL2026-02-11 10:10:00Bad gateway

Expected output

First 1 of 1 rows
users_with_errors
67

Worked solution and explanation

Why this problem exists in real interviews

This tests filtering with a numeric comparison and counting distinct entities. Interviewers check whether you correctly define an error condition and produce a single aggregate count.

Break down the requirements

Step 1: Define error condition

WHERE status >= 400 identifies API calls that returned error codes (4xx and 5xx).

Step 2: Count unique users with errors

COUNT(DISTINCT user_id) returns the number of unique users who triggered at least one error.

The solution

Distinct count with numeric threshold filter
SELECT COUNT(DISTINCT user_id) AS users_with_errors
FROM api_calls
WHERE status >= 400

Common follow-up questions