A client sends SFMC a daily file. Most days the file contains today's new signups plus updates to existing customers. Some days the file is a full snapshot of every active customer. The same Import Activity handles both cases very differently depending on which update type you pick.
Four options, four behaviors. Picking wrong corrupts the DE silently.
The four update types
Update typeWhat it doesUse whenAdd OnlyInsert new records, skip existing onesFile contains only new signups, guaranteed no duplicates with existing DEAdd and UpdateInsert new + update existing (matched by Primary Key)Daily delta files: mix of new records and updates to existing onesUpdate OnlyUpdate existing records, skip new onesField refresh on existing subscribers - you don't want new records addedOverwriteDelete every row in the DE, then insert from fileFull snapshot: DE should exactly match the file contents
How the match actually works
Update types that modify existing records (Add and Update, Update Only, Add Only when checking for duplicates) need the DE to have a Primary Key. SFMC uses the Primary Key to decide "is this incoming row a new record or an existing one to update?"
Without a Primary Key:
- Add Only creates duplicates on every import.
- Add and Update becomes effectively Add Only.
- Update Only does nothing.
Confirm the DE has a Primary Key before choosing anything other than Overwrite.
When each one is the right answer
Add Only
Daily signups file. You're confident the file only contains new people, not updates to existing customers. Used with forms + form-export jobs.
Without a Primary Key, this creates duplicates. With Primary Key, existing matches are silently skipped (not updated).
Add and Update
The workhorse. Delta files from CRM or e-commerce platforms: mostly existing customers with updates, plus occasional new ones. Catches both. This is the default choice when in doubt.
Update Only
Specific field refreshes. Example: client wants to push loyalty point balances without allowing the file to add new customers to the list. Use when the file is an enrichment feed, not a new-customer feed.
Overwrite
Snapshot files. The source system exports the full list of currently-active customers each night, and SFMC's DE should exactly match that list. Overwrite deletes the DE contents first and reloads from scratch.
Overwrite is destructive. If the source system exports an empty file by mistake, the DE ends up empty. Combine Overwrite with a Verification Activity that halts the automation if the file row count is outside expected bounds.
Mistakes we keep fixing
Add Only without Primary Key
Every daily import adds the same subscriber again. After a week the DE has each customer 7 times. Audience counts inflate. Sends target duplicates. Fix: add a Primary Key to the DE.
Overwrite on the wrong DE
The classic nightmare: an engineer thinks they're running Overwrite on a staging DE and points at the production DE instead. Records wiped. Restoring requires a backup or re-running the source export.
Mitigation: add confirmation checkpoints in runbook. Test Overwrite against staging first, then flip to production only after confirming the target.
Import Activity says "File Not Found"
Two common causes:
- Filename mismatch - the Import Activity is configured for file.csv but the actual file is File.csv (different capitalization). SFMC matches case-sensitively.
- Wrong folder - file was uploaded to SFTP root instead of /Import. Import Activity reads from /Import by default unless you reconfigure.
Both silent failures. Confirm both before assuming the automation is broken.
Column names in file don't match DE field names
Import fails or data lands in wrong fields. File has first_name, DE has FirstName. Use the Map Attributes step in the Import Wizard or Activity to map file columns to DE fields explicitly. Don't rely on auto-match.
Manual import without SFTP
No Enhanced SFTP means no Import Activity. For one-off imports, use the Import Subscriber Wizard in Email Studio - upload a local file through the web UI. Only works for one-offs, not scheduled automations.
Takeaway
Update type is decided before the Import Activity runs the first time, and it's surprisingly costly to change later. Get the Primary Key sorted, match update type to file semantics (delta vs snapshot), and pair Overwrite with Verification. The rest of the Import Activity is plumbing.
Setting up SFMC data imports? Our Salesforce team ships Import Activities, Primary Key strategies, and file validation on production engagements. Get in touch ->
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