Error: DynamoDB ConditionExpression not working

ConditionExpression is evaluated against the existing item before DynamoDB applies a write. A false condition produces ConditionalCheckFailedException and leaves the item unchanged. A malformed expression produces a validation error. Distinguish those outcomes before changing the logic.

• Use ConditionExpression with PutItem, UpdateItem, DeleteItem, or supported transaction actions. Query and Scan use FilterExpression for non-key filtering.
• Bind every #name and :value placeholder, use the client’s expected value format, and keep attribute names case-sensitive. String "1" and Number 1 are different.
• Remember that attribute_exists checks presence, including NULL or empty values. Use attribute_type or size when those distinctions matter.
• For an upsert guard, test the primary key at the full requested item key. Conditions do not search other items for uniqueness.
• Use parentheses to make mixed AND and OR logic explicit. Functions and SQL predicates are not interchangeable.

When a condition fails, inspect the current item or request ReturnValuesOnConditionCheckFailure=ALL_OLD where supported. Recompute a stale version or reject the business operation; removing the condition can introduce lost updates. A condition testing the new value being assigned will still see the old stored value.

See conditional-write patterns and AWS documentation.