Reducing Unnecessary Database Hits | Dataford Interview Questions

What are the primary strategies for reducing unnecessary database hits?

Demonstrate the set-based approach with one PostgreSQL query that retrieves active Depop listings, seller usernames, and whether user 1 has liked each listing. Include listings even when seller or like data is missing.

Output

One row per active listing.
Columns: listing_id, listing_title, seller_username, liked_by_user, liked_at.
Sort by newest created_at, then descending listing_id.

Schema

users

Column	Type	Description
idPK	INT	User identifier
username	VARCHAR(80)	Depop username
is_active	BOOLEAN	Whether the account is active

listings

Column	Type	Description
idPK	INT	Listing identifier
seller_id	INT	User who created the listing
title	VARCHAR(150)	Listing title
status	VARCHAR(20)	Listing lifecycle status
created_at	TIMESTAMP	Listing creation timestamp

listing_likes

Column	Type	Description
idPK	INT	Like identifier
listing_id	INT	Liked listing
user_id	INT	User who liked the listing
created_at	TIMESTAMP	Time the like was created

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Problem