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Overlap Columns

Obsolete

Overlap columns were replaced by multi-segment relationships in DataFlex 11.0 and are now considered obsolete.

Overlap columns are native embedded database columns defined in terms of other physical columns, which must be contiguous. They do not take up space in the table, as they are "virtual" columns.

Overlap columns were designed to allow developers to create multi-segment (multi-column) relationships before full support for multi-segment relationships was added to DataFlex (introduced in revision 11.0). Overlap columns are now considered a legacy technique and are supported only as such. Developers are encouraged to use proper multi-segment relationships instead of overlap columns when creating new relationships and to convert existing overlap columns to proper multi-segment relationships whenever possible.

The DataFlex embedded database is the only database that natively supports overlap columns. Various Connectivity Kits allow embedded database tables that use overlap columns to be converted to other databases that do not support overlap columns, but this is also just for legacy support purposes.

Developers have used overlap columns for various purposes that differ from their designed and intended purpose and may appear to work under some circumstances, but were never officially supported. We encourage developers to migrate away from these techniques, which include:

  • Using overlap columns for data display: Instead, use the columns that make up the overlap column to display the data in question. Another alternative is to use a function that concatenates data from multiple database columns and displays it in a data entry object (DEO).

  • "Underlap" columns: These have a smaller byte range than a single overlapped column. Instead, use the columns that make up the overlap column to display the data in question. Another alternative is to use a function that concatenates data from multiple database columns and displays it in a data entry object (DEO).