TBH I cannot remember the last time someone thought of writing SQL statements that were so SQL compliant that they were cross-RDBMS compatible. Most places I have worked develop a RDBMS specific data access layer but some do so whilst ensuring that it takes business entities or data transfer objects (i.e. database neutral types) so that the data access layer/tier can be replaced whilst the remaining parts of the application can remain. The same is true for the UI e.g. swapping ASP.NET MVC front end for a Java Server Pages implementation.