An RDMS is a database that stores data in tables using rows and columns. The values in the tables are related to one other, and the tables may also be related to one another, hence the term relational. This relationship makes it possible to access data across multiple tables with a single query.

In this section, we will review the top four relational database management systems. The top four are Oracle, MySQL, SQL Server, and PostgreSQL.

According to the DB-Engines Ranking, here are the scores for the top RDMSes at the time of writing this book:

The preceding screenshot can be found at https://db-engines.com/en/ranking.

Oracle was first released in 1979. Oracle was the first commercially available SQL-based RDMS. It does have one free version, Oracle Database XE, which has some limitations compared to its licensed versions. Oracle runs best on Linux, but can be installed on Windows. Oracle is a great choice for organizations that need an RDMS and can handle very large databases and a variety of features.

The advantages of Oracle are that it offers a lot of functionality for system and database administrators, it's fast and stable, and it has lots of support and documentation.

The disadvantages of Oracle are that licensing is expensive and it may require significant database administrator resources to maintain it after installation.

MySQL is a free, open source SQL database that started in 1995. It also has proprietary licensing available, which includes support and maintenance. Sun Microsystems bought MySQL in 2008, which was then acquired by Oracle in 2010. MySQL is commonly used in conjunction with PHP web applications. MySQL is a great choice for organizations that need a good RDMS but have a tight budget.

The advantages of MySQL are that it's available for free, it offers a lot of functionality for system and database administrators, it's easy to use and implement, and it's fast and stable.

The disadvantages of MySQL are that while support is available, it's not free. Also, since it's under Oracle, not all features are free, including paid-for options, such as enterprise monitoring, backup, high availability, scalability, and security.

Initially released in 1989, SQL Server is available with a commercial license. It does have one free version, SQL Server Express, with a limitation of 10 GB per database, along with other resource limitations. SQL Server is usually installed on Windows, but can also be installed on Linux. SQL Server is a great choice for organizations that need a good RDMS, and use a lot of other Microsoft products.

The advantages of SQL Server are that it offers a lot of functionality, including replication, and high availability and partitioning works very well with other Microsoft products, such as .NET Framework and Visual Studio. It is also fast and stable.

The disadvantages of SQL Server are that licensing is expensive, especially for the Enterprise edition, and not all features are included in all editions, such as some high-availability options and partitioning.

The first release of PostgreSQL was in 1989. This indefinitely and doesn't enforce any limits at all. PostgreSQL is usually installed on Linux machines and can be used to store structured and unstructured data. PostgreSQL is a great choice for organizations that need a good RDMS, already use Linux, and don't want to spend a lot of money on licensing.

The advantages of PostgreSQL are that it offers a lot of functionality, such as high availability and partitioning, it's scalable and can handle terabytes of data, and it's fast and stable.

The disadvantages of PostgreSQL are that documentation may be harder to come by and configuration can be confusing. It also runs on Linux, and you need to know how to run commands from Command Prompt.

Even though there is an ANSI/ISO standard, there are different versions of SQL. Still, to be compliant, they all similarly support the major commands, so SELECT, WHERE, INSERT, UPDATE, and DELETE would all have a syntax that matches.

Each subsequent chapter of this book will also note differences in the language or functionality of SQL where there are differences between MySQL and SQL Server, PostgresSQL, and Oracle.