n8n Data Tables

Data Tables are n8n's built-in tabular storage: real tables inside the n8n instance with columns, types, rows, and CRUD via the dataTable node and data-table MCP tools.

Use them for local persistent state: lookup tables, recent events, per-session inventories, counters, idempotency tracking, dedup state when there's row-level logic or external visibility (plain "have I seen this value?" dedup belongs in the Remove Duplicates node). Small-to-moderate volume (tens of thousands of rows fine, millions belong in a real DB).

Non-negotiables

1. System-managed columns + external IDs. Three columns auto-exist on every table: id (bigserial), createdAt, updatedAt. Don't declare them in create_data_table (errors or shadows the system column). Don't write them on insert. For domain identifiers from outside (arxivId, stripeCustomerId, requestId), add a separate column and key dedup/lookup on that.
2. Only primitives in columns, nested data uses string + _object postfix. No JSON/object/array column types exist. For nested data (arrays, parsed objects), use a string column with JSON.stringify(...) on write and JSON.parse(...) on read. Mark the column with _object (e.g., keyInsights_object). The postfix is the contract that tells readers to parse. See references/SCHEMA_DESIGN.md.

Strong defaults

• Don't add a Set node before a Data Table node to modify fields. The Data Table node's per-column expression slots are just as powerful as Set fields, so the Set node is doing zero work the Data Table node can't do itself. (Same Set-node antipattern called out in n8n-expressions.)
• Match n8n's column casing: camelCase. The auto-managed columns are camelCase (createdAt, updatedAt), so user columns read more cleanly when they match: arxivId, paperId, taxRate. Mixed casing in the same query (createdAt >= ... AND arxiv_id eq ...) reads as a typo. Keep the _object postfix on stringified-blob columns regardless (keyInsights_object), the underscore is a contract marker, not casing.

• Verify the columns parameter via get_workflow_details after create/update. The UI has a display quirk in manual mapping mode ("Currently no items exist" with no actual data loss). Checking the JSON confirms what's persisted.
• Relational design works when the shape calls for it. For genuine parent-child data (papers → summaries, customers → orders), reference parents by id, name columns explicitly (paperId, customerId), and enforce integrity in workflow logic. Don't force it on flat use cases (dedup, lookup, audit) where there's no relationship to model.
• Storage format is not interface format. Parse _object fields before returning them from a sub-workflow. Callers should never receive stringified shells they have to parse themselves. See references/SCHEMA_DESIGN.md "Storage format ≠ interface format".

The default columns

Every Data Table has these whether you declare them or not:

In practice:

• Don't declare them in create_data_table. Already there.
• Use them in queries without your own timestamp columns. "Created today": createdAt >= '<today ISO>'. "Updated since last sync": updatedAt >= $('Last Sync').item.json.timestamp.
• Don't use them as cross-system identifiers. Auto-id is internal, and resets on table recreate or instance migration. For domain identifiers, use your own column.

Relational design when the shape calls for it

Data Tables don't enforce foreign keys, but you can still model parent-child data across tables when the data genuinely has that shape. The catch: integrity is your responsibility, not n8n's.

• Reference parent rows by id. A child table holds the parent's id in a column.
• Document references in column names. paperId, customerId, eventId make the relationship obvious.
• Enforce integrity in workflow logic. Before inserting a child, look up the parent. Before deleting a parent, decide what happens to children (delete, orphan, archive). n8n won't cascade.
• Watch for stale references. Children pointing at deleted parents are silent bugs. Soft-delete, or run cleanup workflows.

For complex relational structure (3+ tables with joins, transactional writes), reach for an actual SQL DB.

Operations: which one for what

For the full operation surface (filter syntax, matchType, sort patterns), see references/OPERATIONS.md.

The "Currently no items exist" UI quirk

When the SDK saves manual-mode column mappings (mappingMode: 'defineBelow'), the n8n UI's "Values to insert" pane can render empty ("Currently no items exist") even though runtime persists data correctly. If the user reports the Insert node "looks broken" or "has no fields," tell them: it's a UI display issue, press the reload (refresh) button on the columns parameter, and it repopulates the schema and the mappings render. No data loss, safe to do anytime.

Common patterns

Dedup by external ID

Default to the Remove Duplicates node ("items seen in previous executions" mode) for plain "have I seen this value?" dedup. It's a one-node solution with an internal store, no schema to maintain. Data Tables only earn the slot when there's a reason for the dedup state to live in a real table:

• You'll query or inspect the dedup state. Dashboards, audit, "what have we processed in the last week?"
• Row-level logic on hits. Per-category TTL ("expire after 30 days for category A, 7 days for category B"), conditional re-process based on stored state, branching on a status column.
• Per-tenant or per-user namespacing that the Remove Duplicates history-store can't express.

When that bar is met:

[Source: { arxivId, ... }]
   ↓
[Data Table Get: filter arxivId eq $json.arxivId, limit 1]
   ↓
[IF: result has items?]
   ├── Yes → [Skip, or apply row-level logic from the stored row]
   └── No  → [Process] → [Data Table Insert: { arxivId, ...rest }]

For the full pattern surface (upsert, rowNotExists, Get+IF, idempotency keys), see references/DEDUP_PATTERNS.md.

Lookup tables

Stable reference data (country → tax rate, plan → feature flags). Edited via n8n UI, and workflows read at execution:

[Data Table Get: filter country eq $json.country, limit 1]
   ↓
[Use the looked-up row's taxRate, etc.]

Recent events / audit trail

Append-only insert, queried later:

[Workflow event] → [Data Table Insert: { userId, eventType, payloadSummary }]

createdAt makes "recent events in the last hour" trivial without your own timestamp.

Reference files

For expression discipline ($json vs $('Node Name').item.json, the Set-node antipattern), see n8n-expressions. For Merge convergence and same-shape branches, see n8n-node-configuration references/MERGE_NODE.md.

Anti-patterns

Verification before publishing

After creating or updating a workflow that uses Data Tables:

1. validate_workflow passes.
2. get_workflow_details and inspect each Data Table node's columns. Both value and (for manual map) schema populated.
3. test_workflow with pinned data. Insert response should include id, createdAt, updatedAt.
4. Inspect actual Data Table contents via UI or follow-up Get to confirm columns aren't silently NULL.

Step 4 especially the first time you wire a new Insert. Context-stripping intermediates + manual map + UI quirk silently produce NULL columns.