Relational Model Versus Document Model

The relational model was a theoretical proposal, and many people at the time doubted whether it could be implemented efficiently. However, by the mid-1980s, relational database management systems (RDBMSes) and SQL had become the tools of choice for most people who needed to store and query data with some kind of regular structure.

The Birth of NoSQL

Although SQL stands against the test of time well and are used wildly, in the 2010s, NoSQL is the latest attempt to overthrow the relational model's dominance. The driving forces behind the adoption of NoSQL databases includes:

• A need for greater scalability than relational databases
• A widespread preference for free and open source software
• Specialized query operations that are not well supported by SQL
• Frustration with the restrictiveness of relational schemas, and a desire for a more dynamic and expressive data model

There are different advantages and drawbacks on choosing between SQL and NoSQL, such as complexity of application code, schema flexibility, data locality... etc. An application should wisely choose which model to use based on its own need and the context.

Query Languages for Data

SQL is a declarative query language, as it followed the structure of relational algebra. An imperative language tells the computer to perform certain operations in a certain order. In a declarative query language, however, you just specify the pattern of the data you want—what conditions the results must meet, and how you want the data to be transformed, then the query will fetch for you.

Graph-Like Data Models

The Property Graphs

In the property graph model, each vertex consist of:

• A unique identifier
• A set of outgoing edges
• A set of incoming eges
• A collection of properties (key-value pairs)

Each edge consist of:

• A unique identifier