What are the Differences Between Database Engines? An Overview for Beginners

If SQL is a standard language, why do databases like MySQL, SQL Server, Oracle, and PostgreSQL all exist? Do their differences matter? This article explains what sets them apart and why it’s useful to know – even if you’re not a database expert.

My husband is a software tester. Occasionally, he uses SQL to pull data from a database – like checking if a user with access to three open projects sees the correct list, or modifying data for testing purposes. He’s not a beginner, but he’s not an expert either.

Recently, he asked, “Why are there so many databases – MySQL, SQL Server, Oracle, PostgreSQL? Isn’t SQL just one language?” It’s a fair question. Many people assume SQL works the same everywhere, but databases have key differences that impact how they store, retrieve, and manage data.

In this article, I’ll explain these differences from a non-database person’s perspective. You’ll learn what sets these databases apart, why it’s important to know which ones you’ve worked with, and why employers ask about it. If you're just getting started with SQL, check out the SQL Basics course to build a solid foundation. And if you're preparing for a job interview, you might find SQL interview questions for software testers helpful.

What is a DBMS?

MySQL, SQL Server, Oracle, and PostgreSQL are all Database Management Systems (DBMS). Think of a DBMS as an operating system for databases – it helps store, organize, and retrieve data.

Just as you can browse the web, write documents, or send emails on both Windows and Mac, you can run SQL queries – such as SELECT, INSERT, or UPDATE – across different DBMSs in a similar way.

However, managing a database – just like managing an operating system – varies. Security settings, performance tuning, and administration tools differ from one DBMS to another.

Now, let’s look at what sets these database engines apart.

Differences in SQL Dialects

All major databases use SQL, but each has its own variation. The core commands – SELECT, FROM, JOIN, WHERE, GROUP BY – work the same everywhere, but the details can vary.

For example, ORDER BY behaves differently across databases. In PostgreSQL, MySQL, and SQL Server, NULL values appear first in ascending order, while Oracle places them last. Most databases sort text case-sensitively, meaning "apple" comes before "APPLE," but SQL Server sorts case-insensitively by default.

Another difference is in joins. MySQL does not support FULL OUTER JOIN, which is available in PostgreSQL, SQL Server, and Oracle.

Even selecting a limited number of rows varies. SQL Server uses the TOP keyword, while PostgreSQL and MySQL use LIMIT. If you want to find the top five highest-priced products, you would write different queries depending on the database.

These differences become important when writing anything beyond simple queries. It’s always best to check the syntax for the database you’re working with.

Differences in SQL Functions

While common functions like COUNT and AVG work the same in all databases, less frequently used functions can vary.

For example, concatenating text differs between databases. The SQL standard uses || to join text values:

SELECT 'Hello' || ' ' || 'World';

But in SQL Server and MySQL, || doesn’t work. Instead, you use the CONCAT function:

SELECT CONCAT('Hello', ' ', 'World');

String length functions also differ. MySQL and PostgreSQL use LENGTH, while SQL Server uses LEN.

The biggest differences appear in date and time functions, where each database has its own approach. Knowing these differences is important when writing queries across multiple systems.

Different Tools

Every database has its own tools for managing and querying data. SQL Server uses SQL Server Management Studio (SSMS), MySQL comes with the mysql command-line client, Oracle provides sqlplus and Oracle SQL Developer, and PostgreSQL offers psql or pgAdmin.

There are also universal tools like DBeaver that connect to multiple databases. However, you still need to know which database you’re working with in order to use the right features and syntax.

If you only work with one database, you’ll get comfortable with its tools. But if you switch to another database at a new job, expect a learning curve to find your way around the new interface. It’s not hard, but it’s something to be aware of.

Different Data Types and how They Affect Querying

Databases store numbers, text, and dates differently, which can affect how queries work. Integer types vary in range – MySQL and PostgreSQL have TINYINT (1 byte) for very small numbers, while SQL Server’s smallest option is SMALLINT (2 bytes). Oracle uses a different approach with its NUMBER type, which can store both integers and decimals, offering flexibility but requiring precise definition.

Text storage also differs. MySQL and PostgreSQL use VARCHAR for variable-length text, while SQL Server’s NVARCHAR is optimized for Unicode. Oracle’s VARCHAR2 behaves slightly differently in indexing and storage.

Date and time handling varies the most. PostgreSQL and MySQL offer TIMESTAMP with or without time zones. SQL Server distinguishes between DATETIME, DATETIME2, and SMALLDATETIME, while Oracle’s DATE type includes both date and time, unlike systems where DATE represents only a calendar day. These differences can affect how filters and comparisons work, sometimes requiring extra formatting.

For query writers, understanding these differences helps prevent errors. A DATE filter may return unexpected results if the database treats dates differently, and sorting text can depend on whether the system is case-sensitive or defaults to Unicode. Even though SQL syntax is similar across databases, how data is stored and processed can change how queries need to be written.

Different Internals

Relational databases work differently behind the scenes, which affects how they store, manage, and protect data. These differences are more important for administrators and developers than for those writing queries.

One key area is how data is stored. Every database organizes information in tables, but the way it saves data to disk and retrieves it can vary. Some databases focus on speed, while others prioritize data consistency or security.

Backup and recovery methods also differ. Every database allows you to save a copy of your data, but the tools and processes for doing so vary. In some databases, backups are automatic, while in others, they require manual setup. Restoring lost data can also work differently.

User management and permissions determine who can access or modify data. Some databases offer detailed control, letting administrators set rules on a table or even a single column, while others have simpler access settings.

Storage limits define how much data a database can handle. Some databases are optimized for small applications, while others are built to store massive amounts of data for large companies.

Concurrency control ensures multiple users can work with the database at the same time without causing conflicts. Some databases handle this by allowing different users to see different versions of the data, while others use locks to prevent conflicts.

Scaling and replication help databases handle more users or larger datasets. Some databases can split the workload across multiple servers, while others copy data to different locations to improve performance and reliability. Load balancing helps distribute requests efficiently so the system doesn’t slow down under heavy use.

These differences don’t change how SQL queries work, but they affect how a database is managed, secured, and scaled to meet different needs.

Summary

While all relational databases use SQL, they differ in how they store data, handle queries, manage users, and scale. For data analysts and software testers, these differences rarely matter. But for data engineers, DBAs, and developers, choosing the right database engine can impact performance, security, and administration. Even if you don’t manage databases, knowing which ones you’ve worked with can be useful – especially in interviews.

Want to master SQL across different database engines? The All Forever SQL Package at LearnSQL.com gives you lifetime access to SQL courses covering MySQL, PostgreSQL, SQL Server, and more. It’s the perfect way to build your SQL skills and confidently work with any database.