| Database Type | Examples | Pros | Cons | Use cases |
| Relational Databases | Oracle, MySQL, SQL, DB2 etc. | Organize data into tables with columns, each with a specified name and datatype.Rows are identified with a unique attribute, or grouping of attributes, called a primary key (typically a single column, called a field)Relationships between tables are defined through foreign keys which reference primary keysStrict schema (data model) enforcementData accessed via Structured Query Language (SQL)ACID compliance (Atomicity, Consistency, Isolation, and Durability)Extra features like Triggers & Stored Procedures | SQL databases cannot handle unstructured or semi-structured data, their tables don’t necessarily map to objects, they require complicated ETL (Extract, Transform, Load) and maintenance, have row locking, and pricing for some products (Oracle, SAP) can be expensive. Note: While some RDBMS systems can now handle JSON, they are not purpose built to do so. | ACID compliance, Datawarehouse, OLAP, OLTP, structured data analysis. |
| Key-Value Store | MongoDB, Redis, DynamoDB, Cosmos DB | Simple, basic, & Schema-less (not as canonical as an RDMS) Provides basic functionality for retrieving arbitrary data via a specific keyValues can be anything: single values, arrays, objects, files, etc.Database does not evaluate the data it is storingData structure can be referred to as a dictionary or hash table | Key value stores provide fast, low-complexity access to data, are flexible, and can scale quickly and cheaply.They have extremely limited functionality, cannot handle complex structures or query or search by anything other than key, do not scale well as data models grow, and they require more programming overhead for complex implementations. | Embedded systems, URL shorteners, configuration data, application variables and flags for web applications, state information, and data represented by a dictionary or hash. |
| Document Stores | MongoDB, DynamoDB, Couchbase, and Firebase are similar to key-value stores, but the value is a document. | Document formats are JSON, BSON, or XML documentsSchema-less, no data structure enforcement (documents can be different)Data accessed and modified via NoSQL (or proprietary language)Well-suited for unstructured and semi-structured dataSeen as easier for developmentFlexibility and scalability, Schema-less, fast writes, ideal for semi-structured and unstructured data, and developers do not need to know data structure ahead of time / it can change overtime without downtime. | Not ACID compliant (DynamoDB IS ACID compliant but this is an exception), limited to querying within a document, relationships/cross references are not enforced, slow searching, cannot join documents/collections in a single query, lack of database enforcement requires developer discipline and vigilance for application-level enforcement, and they typically result in data duplication. | Unstructured or semi-structured data, content management, rapid prototyping, and collecting of high traffic data. |
| Graph Database | Neo4j, OrientDB, and TitanDB, are ideal for when relationships or connections are top priority. | Based on mathematical graph theoryRepresent data as a network of related nodes, edges, and propertiesDatabase stores data items within nodes and relationships in edges that connect nodesNodes are connected by relationships and grouped according to labelsFacilitate data visualizations and graph analyticsEach node contains free-form datahave advanced features for relationship querying, traversing, and tracking, are optimized for querying related data, and they avoid row locking | Difficult to understand and use, high overhead for simple use cases, lack of standardization, poor performance of aggregate queries, and devs typically need to learn a custom query language. | Great for analysis of heterogeneous data points, fraud prevention, advanced enterprise operations, social networking, payment systems, and GeoSpatial routing/visualization. |
| Time Series Database | InfluxDB, Kdb+, and Prometheus | Focused on datasets that change over timeHeavily write orientedDesigned to handle constant streams of dataTypically append-only (no modification after ingestion)Rollup/aggregation/down sampling features to lower archive data footprintTime series databases are designed for dealing with linear data over time, can handle high ingestion rates, have built-in features specifically for dealing with time-based data, a schema optimized for time-series arrays, and batch delete features | Time series databases only deal with time-series data, do not support full SQL, their read speed suffers compared to writes, they have no transaction capability and are append-only (not optimized for updates). | Managing infrastructure, IoT sensor collection, and log monitoring and alerting. |
| Search Engines | Elasticsearch, Splunk and Apache Solr | Built for non-relational, document-based dataArranged and optimized for storage and rapid retrieval of dataIndexes data across a variety of sources including file systems, intranets, document -Management systems, e-mail, and databasesfocused on optimized searching, highly scalable and Schema-less, and they have advanced search options like full text search, suggestions, and complex search operations. | They are expensive, have low durability and poor security, have no transaction support, are not efficient for writing and retrieving data outside of searching, and are difficult to manage. | Search engines are great when search results are top priority, logging, product catalogues, and blogs |