Best approach to maintain a single set of DDLs and DMLs for multiple database type
The best way to support multiple database types is to optimize the sql statements for each on of them. Otherwise you will one day end up with huge performance problems on one or the other database types.

Additionally to that it's unfair for your customers to tell them "I support Oracle, DB2, MSSQL, etc.) and not optimizing the SQL for each one of them. Why? Because your customer pays a lot for the database and your application does not use the advantages and features the database provides.