Data Extensions 🗄

What are Data Extensions?

A Data Extension (DE) is a database table inside SFMC. It has rows (records) and columns (fields) — just like a spreadsheet or a database table. Data Extensions are the professional standard for managing subscriber data in SFMC and are used for virtually everything: send audiences, personalisation data, transactional records, and automation results.

Lists vs Data Extensions

Field types

When you create a DE, each column has a data type. Choosing the right type matters for storage, filtering, and SQL operations.

DE types

Creating a Data Extension

• 1
  Navigate to Email Studio → Subscribers → Data Extensions → Create

  Or use Audience Builder or Contact Builder for DEs that need to be part of the contact model.
• 2
  Name it clearly

  Use a consistent naming convention. Recommended format: [Type]_[Purpose]_[YYYYMM] or [Programme]_[Description]. Examples: Segment_ActiveUK_202606, Master_Subscribers, Temp_WelcomeJourneyEntry.
• 3
  Set data retention

  For segment/temp DEs, set a data retention period (e.g. 6 months). For master subscriber DEs, set to "None" or indefinite. Retention policies auto-delete data after the set period — be careful with master DEs.
• 4
  Add fields

  Add all the columns you need. For every DE that will be used for email sends, include at minimum: SubscriberKey (Text, Primary Key) and EmailAddress (EmailAddress type).
• 5
  Set the primary key

  Mark the field(s) that uniquely identify each row as the Primary Key. For subscriber DEs, this is almost always SubscriberKey. The primary key determines how upsert operations work — if a record with the same key exists, it's updated; otherwise it's inserted.

Importing data

You can get data into a DE several ways:

Manual import

In the DE record, click Import. Upload a CSV file and map columns to fields. Choose the import type:

• Add & Update — upsert: updates matching rows, inserts new ones (most common)
• Add Only — inserts new rows, skips existing
• Update Only — updates existing rows, skips new
• Overwrite — replaces ALL data in the DE with the import file

Import Activity (Automation Studio)

For scheduled or file-triggered imports. The file is placed on SFMC's SFTP server and the import activity picks it up. This is the standard approach for nightly data feeds from CRM or e-commerce systems.

REST API

Insert or upsert records via the Data Events API endpoint. Used for real-time updates from websites or apps (e.g. form submissions, purchase events).

AMPscript / SSJS

Use UpsertDE(), InsertDE(), etc. in AMPscript for writes from emails or Cloud Pages.

Sendable Data Extensions

A Sendable DE is a DE you can use as a send audience in Email Studio or Journey Builder. To make a DE sendable:

1. Check the "This data extension can be used for sending" option when creating it
2. Map the EmailAddress field to the send (tells SFMC which column contains the email address)
3. Map the Subscriber Key relationship (tells SFMC which column is the unique identifier — typically SubscriberKey)

Data model design

A well-designed SFMC data model makes everything easier: faster queries, cleaner journeys, simpler personalisation. Here are the core principles.

Recommended DE structure

Design principles

• One source of truth — keep master subscriber data in one DE, not scattered across dozens
• Separate concerns — demographic data, transactional data, and preferences in different DEs, joined via Subscriber Key
• Never modify a running send audience — segment DEs should be purpose-built for a specific send or journey, not the master DE
• Temp DEs for automation outputs — SQL query results should write to dedicated segment DEs, not into master data
• Set retention on temporary DEs — avoids accumulating stale data that inflates storage costs

Contact Builder

Contact Builder is the data model manager for SFMC. It lets you define relationships between your DEs and the central Contact record — creating a unified profile for each subscriber.

Why it matters

• Journey Builder can query linked DEs when making decision split choices
• Enables unified contact profiles visible to support and sales teams (if using MC Connect)
• Required for Einstein features that analyse cross-DE data
• Allows cross-DE suppression based on contact attributes

Setting up relationships

In Contact Builder, go to Data Designer. Link your DEs to the Contact record via a shared key (usually Subscriber Key). Define whether the relationship is one-to-one (one DE row per contact) or one-to-many (multiple DE rows per contact — e.g. orders, events). Once linked, Journey Builder and other tools can traverse these relationships.