Dataford
Interview QuestionsInterview GuidesExperiencesMock InterviewsPricing
Get started
Dataford
Popular roles
Software EngineerData AnalystData ScientistData EngineerBusiness AnalystAI EngineerMachine Learning EngineerProduct Manager
Browse
Browse All RolesEvery role hub, from analyst to MLBrowse All CompaniesCompany-specific interview loopsAll Interview GuidesThe full guide library
Top questions by role
Software EngineerData AnalystData ScientistData EngineerBusiness AnalystAI EngineerMachine Learning EngineerProduct Manager
Top questions by skill
SQLPythonStatisticsMachine LearningA/B TestingSystem DesignGenerative AIProduct SenseMetricsBehavioral
Browse all questions →Try a mock interview
Experiences
Practice
Mock InterviewsTimed interview simulations with feedbackSuccess PathYour 6-week structured planModulesCurated lessons by topicWebinarsTalks from ex-Big Tech data leadsPlaygroundA free-form scratch editor
Learn
BlogInterview strategy and career adviceTech Job Market ReportHiring trends across data and AI rolesFor UniversitiesDataford for career centersAbout DatafordWho we are and how we build
Pricing
Build my plan
Optimize SQL Queries
00:00
5 left

Optimize SQL Queries

MediumSQL · PostgreSQL

Problem

How do you optimize SQL queries?

Demonstrate your approach by writing an efficient PostgreSQL query for the provided customer, account, and order data. Return active customers whose completed orders during 2025 exceed the stated threshold, and briefly explain performance considerations.

Output

  1. One row per qualifying customer with customer_id, customer_name, segment, completed_order_total, and latest_completed_order_date.
  2. Include only active customers with an account and more than 1,000 in completed 2025 orders.
  3. Order by total descending, then customer ID ascending.

Schema

customers
ColumnTypeDescription
customer_idPKINTUnique customer identifier
customer_nameVARCHAR(100)Customer display name
account_idINTLinked account identifier
statusVARCHAR(20)Customer lifecycle status
accounts
ColumnTypeDescription
account_idPKINTUnique account identifier
account_nameVARCHAR(100)Account name
segmentVARCHAR(30)Account market segment
orders
ColumnTypeDescription
order_idPKINTUnique order identifier
customer_idINTCustomer who placed the order
order_dateDATEOrder creation date
statusVARCHAR(20)Order processing status
total_amountNUMERIC(12,2)Order total amount
Tablescustomersaccountsorders
Interviewer

Your question is Optimize SQL Queries. Start with the requirements and the three tables in the Question tab.

Run and submit as often as you like. When you're ready, talk me through your approach or go straight to the code.

You need to log in / sign up to run or submit.
CodePostgreSQL
Sign up free to run your codeLog inLn 1
Run your query to see results here.