• Relational databases:
  • Online transaction processing (OLTP)
  • Online analytical processing (OLAP)
• NoSQL databases:
  • Document database
  • Key-value database
  • Wide-column database
  • Graph database

Online transaction processing
We have the good old relational database for OLTP, which typically follows these normalization rules:

Third normal form: A relation is in the third normal form if the second normal form is satisfied and if the column that is transitively dependent on the primary key should be eliminated and moved to another table, along with the determinant. So, if attribute A is the primary key, A is dependent on B, and B is dependent on C, then A to C form transitive dependency. So, the dependency between B and C should be moved to a different table.

First normal form: Each column in the table has atomic value, contains no duplicates, and contains a primary key, which is one or more ordered columns that uniquely identify a row.

Second normal form: A relation is in the second normal form if the first normal form is satisfied and a separate table is used to store all the values that apply to multiple rows and linked using foreign keys. A foreign key is one or more ordered columns that relate to a primary key in another table.

Online analytical processing
OLAP is typically used for data mart and data warehouse applications. This type requires Structured Query Language (SQL) to define, manipulate, query, and manage. They usually facilitate the following:

• Aggregations and roll-ups
• Drill down
• Pivots
• Slicing and dicing

For example, imagine a scenario where your business or data analyst needs to retrieve reporting summaries such as total employees joined in the last month and total cost to the company incurred. In this case, you will only query three columns. So, for all the rows selected, instead of retrieving all the columns, it makes sense to only retrieve three. This is a common pattern in analytical applications, and they use a column-oriented storage mechanism.

NoSQL database

There are NoSQL databases for semi-structured data – that is, data less structured than fully structured tabular data. There are a few types of NoSQL databases. There is no formal model or normalization requirement for this type.

Sometimes, it’s misleading when we hear that NoSQL options are schema-less. NoSQL options may not have a schema in the same pattern as relational databases, but they do have a structure to represent data. Four broad types of NoSQL databases are loosely based on a specific way of storing data. Let’s look at the logic for a data model in each case.

Document database

A document database stores data in document format, similar to a JSON document. Each document stores pairs of fields and values, with a wide variety of data types and data structures used as values. Document databases allow you to keep adding attributes as needed without making changes to the database schema each time.

For example, consider the scenario where you cannot fix all the attributes at the design stage and have to add more attributes as the process evolves, such as in the case of a retail store selling electronic equipment where a laptop has memory, a processor, a charger, and other configuration attributes and a washing machine has another set of attributes, such as weight, power, length, width, and so on. To search efficiently by these attributes, document databases allow for indexes. If you want to search an equipment by length, then you can create an index on length.

Key-value database

The key-value model consists of a key and a value, and it is the simplest type of database in terms of understanding and usage. The data model has two parts: data and a string associated with the data. Data can be accessed via direct request (send the key and get the data) rather than using any kind of query language. Key-value databases are simple if you have to search only on your key. However, if your value-data structure is complex, for example, a JSON object is stored as a value, and if you want to be able to search on items within the JSON structure, then a key-value database is not the best option. In that case, a document database would be a better option.

The following table is an example of a key-value database:

Key	Value
Key1	Value1
Key2	Value2
Key3	Value1
Key4	Value3

Wide-column database

Wide-column databases follow a table form structure that is both flexible and scalable. Each row has a key and one or more associated columns, called column families. Each row’s key-column family is allowed to have as many numbers of columns and the columns can have as many kinds of data as possible. Data is accessed using a query language. This type of column structure allows for faster summary (aggregation) queries. Wide-column databases are designed in such a way that they respond to specific queries. Instead of using indexes to allow efficient lookup of rows, wide-column databases store data in such a way that rows with similar keys (row keys are like primary keys in relational databases) are close together.

For example, consider log data from an application. The following table represents data that is organized to answer queries looked up by event ID and then time:

Event ID	Time	Description
1	5431023867	Event successful
2	5431023869	Error looking up…
3	5431023868	Finished with error

Graph database

Graph databases consist of nodes and edges. Nodes are connected by edges. Data is stored in the nodes and information about how the nodes are related is stored in the edges. Node and edge data is typically retrieved using query languages, Graph Query Language (GQL), and sometimes SQL as well. There are two types of query methods we can use to retrieve data from graph databases:

• Cypher Query Language, which specifies SQL-like statements that describe patterns in a graph
• Traversal Language, which specifies how to navigate from one node to another in the graph

People and connections are a great example of a use case for graph databases. A person could be designed as the node and the relationship of that person with other persons could be the links, also known as edges.

In the following diagram, persons 1, 2, 3, 4, 5, and 6 are all people and the link between them denotes friends: