DynamoDB — cannot specify projection when Select is COUNT

TL;DR — Your Query/Scan set Select: COUNT and a ProjectionExpression (or the legacy AttributesToGet). COUNT returns only the number of matching items, never the items themselves, so naming attributes to return is contradictory — DynamoDB rejects it. Pick one: Select: COUNT with no projection, or a projection with Select: SPECIFIC_ATTRIBUTES (the default when you supply a ProjectionExpression).

What it means

ValidationException: Cannot specify the AttributesToGet when choosing to get only the Count
ValidationException: Cannot use ProjectionExpression when Select is COUNT

Select controls what a read returns. COUNT asks DynamoDB for a tally (Count/ScannedCount) and no item data. A ProjectionExpression/AttributesToGet asks for specific attributes of returned items. Those two are mutually exclusive — you can't count items and also project their attributes in the same call.

Why it happens

• Both parameters set at once — Select: COUNT left in place while a ProjectionExpression was added (or vice versa).
• Legacy AttributesToGet combined with Select: COUNT.
• A query builder that always attaches a projection, even when the caller only wanted a count.
• Misunderstanding Select — expecting COUNT to also hand back the items it counted.

How to fix it

1. Just counting? Drop the projection. Set Select: COUNT and remove ProjectionExpression/AttributesToGet.
2. Need specific attributes? Drop Select: COUNT. Supply a ProjectionExpression; DynamoDB defaults Select to SPECIFIC_ATTRIBUTES, so you usually don't set Select at all.
3. Need both the count and the items? Make one call for the items (with your projection) and read its Count, or make a separate Select: COUNT call.
4. Prefer ProjectionExpression over AttributesToGet — the legacy parameter is discouraged and only works with Select: SPECIFIC_ATTRIBUTES.

Just want a quick count or a projected view while exploring? The DynoTable desktop app runs queries and shows the matched count without you hand-assembling conflicting Select parameters.

Related errors

• Cannot use both expression and non-expression parameters — mixing legacy and expression params.
• Query key condition not supported — a malformed key condition on the same request.
• Learn: Count, sum & aggregate · Projection expressions