DataFlex does not really know about nulls. It cannot (easily) distinguish between an empty string and a NULL value for example.

In SQL world NULLs are very common and have impact on the way SQL works. Declaring columns as nullable has for example impact on performance. SQL engines treat nullable columns different when creating an execution plan. The DataFlex SQL drivers rely on indexes and sorting to get good performance. Allowing NULLS in columns has negative impact on performance.

You can by the way configure if you want to create columns as nullable by specifying (DEFAULT_NULLABLE_* ) keywords in the driver configuration file (Mssqldrv.Int, Odbc_drv.int,DB2_drv.int). But this is considered advanced usage and not recommended unless you have very good reasons to do so.