Best Relational Databases include:
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Relational Databases Overview
Relational Database Software Overview
What is Relational Database Software?
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.
Relational Database Features & Capabilities
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
How it works
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.
The SQL Query Language
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.