L10 Non Relational Databases

Question 1

Name features provided by object-relational database systems which are not available in (pure) relational database systems.

Answer 1

• user-defined types
• type inheritance
• table inheritance
• array and multiset types
• object identity
• reference types
• methods

Question 2

What is a NoSQL database?

Answer 2

• NoSQL stands for “Not only SQL”
• Used to refer to various approaches which are non-relational
• examples:
  
  • Key-value stores
  • XML databases
  • Document stores
  • Graph databases

Offer scalability and extensibility over

loosely-coupled clusters of commodity hardware.

As a result, they are often used in large-scale

web-based applications.

They typically do not offer the extensive transaction

management (ACID) capabilities of RDBMSs.

Question 3

Give three limitations of the relational model which have led to alternative data models being proposed.

Answer 3

1. all data is value-based - all relationships are expressed through common values
2. data must always be represented in ’flat’ (first normal form) relations
3. for some applications, performance is not fast enough

Question 4

Name four categories of NoSQL database systems

Answer 4

• kev-value store
• xml database
• document store
• graph database

Question 5

What is a key-value store?

Answer 5

• A key-value store enables the storage and retrieval of values by key.
• It is essentially a big hash table.
• The values are simply sequences of bytes.
• It is up to an application to interpret any logical structure within the values. See Redis

Question 6

What is a xml database?

Answer 6

• Data model is that of XML (elements and attributes).
• XML allows for flexible, nested structures (no schema is necessary).
• The query language is usually XQuery
• example eXist

Question 7

What is a document store?

Answer 7

• Like a key-value store, a document store enables the storage and retrieval of values by key.
• In this case, the values do have structure (each is a “document”).
• The syntax used for documents is usually JSON (JavaScript Object Notation).
• Example: MongoDB

Question 8

What is a graph database?

Answer 8

• Graph: A graph store represents information in a graph consisting of nodes and edges.
• Nodes and edges both typically have properties associated with them.
• Common queries in graphs involve findin connections (paths) between nodes.
• An example system is Neo4J (www.neo4j.com).
• Neo4J uses its own query language called Cypher.

Question 9

What features of relational database systems are often missing in NoSQL database systems?

Answer 9

They typically do not offer the extensive transaction management (ACID) capabilities of RDBMSs.