Support for IF EXISTS/IF NOT EXISTS DDL constructs
IF EXISTS and IF NOT EXISTS are already part of many DBMSs DDL vocabulary. They have many use cases which are highly useful in some situations.
For instance, creating a table if it does not already exists or dropping a column if it exists.
It would be nice to have those directives with standard SQLAlchemy DDL methods. I guess they could be implemented using the native support for DBMSs that support them or with some introspection for those that do not.