Postgre/Oracle users are used to use Sequences to generate values of Primary Key of tables. mySQL/Access users are used to use auto increment flag on a PK field for the same task. Generally speaking Sequences are more powerful and flexible, but more verbose.
We have add Sequences into Valentina 5.0 in 99% same way as they work in PostgreSQL, so now Valentina developer can choose between auto-increment and sequences if they develop true Relational model.
You can read details here:
Reminder: If developer uses modern ObjectPtr and Binary links of Valentina DB, then he can avoid PK-FK headache at all.