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A relational database is nothing more than a data repository that stores information in structured tables with rows and columns. Because data is stored in a structured way, it can be retrieved using a query language that understands the table structure.
ACID (Atomicity, Consistency, Isolation, Durability) compliance ensuring database transactions are processed reliably
Referential integrity ensures that relationships between tables remain consistent
Fine-grained locking or synchronization mechanisms for managing simultaneous access of the same data by multiple users
Support for Unicode for multilingual capability
Ability to run database seamlessly on multiple platforms
Ability to recover data in the event of a failure
A table is referred to as a relation in the sense that each row contains data that is related to a single entity, while each column represents different keys or concepts.
When each row in a table is different, it is possible to use one or more columns to identify a particular row. There is always a “primary key” which serves as the unique identifier for each entity or record. For example, in the case of a table of company employees, the primary key might be employee number.
Data is retrieved from the table using a specialized language called Structured Query Language or SQL. SQL allows users to query the data using SELECT statements and WHERE clauses. A group of SQL statements can be called as a mini-program or executable which is called a stored procedure.
SQL is a widely understood language with a deep bench of practitioners in the marketplace. For this reason, tools using SQL are being developed to query non-relational big data stores like Hadoop, which use less well known, and harder to use, interfaces to retrieve data.
There are several robust free relational databases on the market like MySQL and PostgreSQL. Traditional highly-robust and fully-featured enterprise relational databases like Oracle start at about $45,000 for a single license for two processing cores. However, significant discounts are usually available.
MySQL is a popular open-source relational and embedded database, now owned by Oracle.
Microsoft Access is a database management system from Microsoft that combines the relational Microsoft Jet Database Engine with a graphical user interface and software-development tools.
Oracle Database 12c is a relational database software solution offered by Oracle.
Microsoft SQL Server is a relational database.
Oracle Database, or Oracle RDBMS, is an object-relational database management system.
SQL Developer is a relational database software solution offered by Oracle.
Amazon Relational Database Service (Amazon RDS) is a database-as-a-service (DBaaS) from Amazon Web Services.
DB2 is a relational database software solution offered by IBM.
Sequel Pro is a relational database software solution offered by Sequel Pro.
Amazon Aurora is a MySQL compatible relational database system from Amazon Web Services.
SAP® SQL Anywhere® solutions aim to deliver enterprise-level data management and synchronization capabilities. The solutions are designed to run in remote and mobile environments on a wide range of small-footprint devices and in real-world conditions.
Teradata is a relational database and data warehouse.
Informix is an embedded relational database offering from IBM.
kdb+ is a column-oriented relational database from kx headquartered in Palo Alto, California.
SAP HANA, Express Edition is a scaled down version of the HANA product that can run on laptops and other resource-constrained hosts, such as cloud-hosted virtual machines. The product is free to use for in-memory databases up to 32GB of RAM.
SAP IQ (or SAP Sybase IQ) is a relational database.
Presto is an open source SQL query engine supported by Teradata designed to run queries on data stored in Hadoop or in traditional databases. Teradata's development of Presto followed the acquisition of Hadapt and Revelytix.
Tibero is a relational database software solution offered by TmaxSoft.
Postgres Plus Advanced Server is an advanced deployment of the PostgreSQL relational database with greater features and Oracle compatibility, from EnterpriseDB headquartered in Bedford, Massachusetts.
MemSQL is a NoSQL database offering from the company of the same name in San Francisco.
MonetDB is an open source column-oriented relational database management system issued and supported by the Dutch MonetDB development team.
TimesTen from Oracle is a relational database.
Valentina is a relational database software solution offered by Paradigma Software.
NuoDB is a relational database from the Cambridge, Massachusetts based company of the same name.
