Best Free Plans for No-code Database Platforms
What are the basic components of any app? Data and interfaces. This is what these spreadsheet-style app platforms aim to combine into one no-code solution. You create tables with all the required columns you need and fill them up through imports, forms or direct input. Then choose the view style (table, card, calendar, map, etc.) that fits the data.
Features to consider: The platforms differ mainly in two areas: Integerations with external tools/data and ability to share your apps with external users or public visitors.

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Airtable is popular category-leader. It's very flexible with lots of field types and table views. With the interface designer you can create an app-like experience. Automations are great but limited on the free plan. Extension are only for paid users.
Free plan
Unlimited Bases (Sheets)
1,000 Records / Base
1 GB Files / Base
100 Automation Runs / Month
5 Users

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Stackby looks a lot like Airtable (with less polish) and offer the same basic table features. Great: You can link columns to external APIs and directly pull live data in. For workflow automations it relies on tools like Zapier.
Free plan
Unlimited Users & Guests
20 Stacks (Sheets)
1,500 Rows / Stack
2 GB Files / Stack
1 Dashboars / Stack
2 API Connectors Total

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Baserow wants to be the open source equivalent of Airtable. The feature set not as extensive. Webhooks are supported, but no built-in automations. The application builder works like a simple webiste editor, where you can freely place tables, forms, buttons and even iframe. Great potential.
Free plan
3,000 Rows Total
Unlimited Users
Ulimited Databases
2 GB Storage

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This is the other big Airtable open source alternative. The basic column types and table views are available. Good filter options. There are webhooks to integrate with external tools and full API access is included in the free tier. The best feature likely won't be available in the free plan: Connecting your own Postgres/MySQL database.
Free plan
10,000 Rows Total
1,000 Rows / Table
Unlimite Tables
5 Users
1 GB Storage
Technical Preview (all plans are temporarily free, but details might change)

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SmartSuite clearly has an enterprise focus. Lots of features for bigger teams like permission system, commenting etc. The automation feature is mostly meant for workflows inside SmartSuite, but there are some intergrations. API yes, but no webhooks yet.
Free plan
5 Solutions (Sheets)
1000 Records / Solution
100 MB Storage / Solution
3 Users, 1 Guest
100 Workflow Runs / Month