All Work the Same An ugly big fat application layer with a proprietary language and referential integrity enforced in this layer. Domain constraints except for datatype are virtually unused.
Referential constraints are also unused.
The basic fact is that these systems are not relational data models, just a complex hodge-podge of the legacy file based systems from which they evolved. This makes an unstable set of data that easily loses its integrity. Logical transactions, even indexes are convoluted or missing in some ERP systems. Batch job restart or commit points to resume long transactions are crude or missing also. Configuration management of the software is a complex maze of intricate rules.