Causes DynamoDB to evaluate the value before attempting a conditional operation:
If Exists is true, DynamoDB will check to see if that attribute value
already exists in the table. If it is found, then the operation succeeds. If it is not found, the operation fails with a ConditionalCheckFailedException.
If Exists is false, DynamoDB assumes that the attribute value does not
exist in the table. If in fact the value does not exist, then the assumption is valid and the operation succeeds. If the value is found, despite the assumption that it does not exist, the operation fails with a ConditionalCheckFailedException.
The default setting for Exists is true. If you supply a Value all by itself,
DynamoDB assumes the attribute exists: You don't have to set Exists to true, because it is implied.
DynamoDB returns a ValidationException if:
Exists is true but there is no Value to check. (You expect a value to
exist, but don't specify what that value is.)
Exists is false but you also provide a Value. (You cannot expect an attribute
to have a value, while also expecting it not to exist.)