All major databases share core SQL, but each adds its own types, functions and syntax. Knowing the differences saves hours when you switch engines.

The major engines

Engine	Typical use	Notable traits
MySQL / MariaDB	Web apps (LAMP), SaaS back ends	Open source, InnoDB engine, very popular, replication-friendly
PostgreSQL	General purpose, analytics, GIS	Most standards-compliant open-source engine; rich types (`JSONB`, arrays), extensions
SQL Server	Microsoft / enterprise stacks	T-SQL dialect, strong tooling, `TOP`, `IDENTITY`
Oracle	Large enterprises, banking	PL/SQL, `ROWNUM`, `DUAL`, powerful but commercial
SQLite	Mobile apps, browsers, embedded, tests	A library, not a server — the whole DB is one file; dynamic typing

Dialect differences you will meet

Task	MySQL	PostgreSQL	SQL Server	Oracle	SQLite
First N rows	`LIMIT n`	`LIMIT n` / `FETCH FIRST n ROWS ONLY`	`TOP (n)` / `OFFSET … FETCH`	`FETCH FIRST n ROWS ONLY` (12c+)	`LIMIT n`
Auto-increment id	`AUTO_INCREMENT`	`GENERATED … AS IDENTITY` / `SERIAL`	`IDENTITY(1,1)`	`GENERATED AS IDENTITY`	`INTEGER PRIMARY KEY`
String concat	`CONCAT(a, b)`	`a \|\| b`	`a + b` / `CONCAT`	`a \|\| b`	`a \|\| b`
Current timestamp	`NOW()`	`NOW()`	`GETDATE()`	`SYSDATE`	`CURRENT_TIMESTAMP`
NULL replacement	`IFNULL` / `COALESCE`	`COALESCE`	`ISNULL` / `COALESCE`	`NVL` / `COALESCE`	`IFNULL` / `COALESCE`
Boolean type	`TINYINT(1)`	`BOOLEAN`	`BIT`	`NUMBER(1)` (`BOOLEAN` in 23ai)	integer `0`/`1`
Identifier quoting	`name`	`"name"`	`[name]`	`"name"`	`"name"` (also accepts the others)

Top 3 highest-paid employees:

SQLite / MySQL / PostgreSQL

SELECT name, salary FROM employees ORDER BY salary DESC LIMIT 3;

Output

name  | salary
------+-------
Asha  | 150000
Ravi  | 120000
Priya | 110000

SQL Server

SELECT TOP (3) name, salary FROM employees ORDER BY salary DESC;

Oracle 12c+ / PostgreSQL / SQL standard

SELECT name, salary FROM employees ORDER BY salary DESC
FETCH FIRST 3 ROWS ONLY;

Which SQL database should you choose for a new project?

Is SQLite a “real” database?