What's the difference between MySql, T-Sql , Oracle , MS Access when it comes to SQL commands?

Why is there so many different ways of writing SQL commands?

Aren't they all doing the same thing? So why does there have to be a difference in the way the commands are written?

I know how to write them in regular SQL.

But what does it mean when someone say , write it for MySql or Oracle Sql or Transact SQL or for MS Access?What's the difference between MySql, T-Sql , Oracle , MS Access when it comes to SQL commands?
sql follows a standard syntax, however, different vendors apply more reserved word or more functions in their 'versions' or extensions. what that someone is telling you is to use standard sql and not extensions' syntax.



For example: PL/SQL (used in oracle db) allows you to use "+=" to indicate a left outer join, but T-SQL (microsoft sqlserver) doesn't recognize it.