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?
The core SQL is identical among all of these - the basic syntax for inserting, deleting, modifying and retrieving data to/from tables, creating/modifying objects and other such tasks is identical. Where they differ is the various extensions (for example, where one may use LIMIT, another may use TOP) and functions (particularly those converting from one data type to another, string manipulations and date/time computations.) Additionally (for those DBMS that have one), the scripting language for each (T-SQL for SQL Server, PL/SQL for Oracle, etc.) will vary significantly.