Which of the following is an element of a NoSQL database?

Primary keys are fundamental for uniquely identifying records or documents within a database, and even NoSQL databases, which are often schemaless, still rely on a unique identifier for each item. For instance, MongoDB uses an '_id' field as a primary key to ensure uniqueness for each document in a collection.

Constraints, like foreign key or unique constraints, are more characteristic of relational databases to enforce data integrity across tables, which NoSQL generally relaxes.

NoSQL databases are typically schema-less or have a flexible schema, meaning their structure is not rigidly defined upfront like in relational databases.

Tables are a core structural element of relational databases, whereas NoSQL databases often use collections, documents, graphs, or key-value pairs instead.

Delimiters are characters used to separate data fields, which is a general data formatting concept, not a structural element specific to NoSQL databases.