The notation represents Update and Delete operations enforced by referential integrity rules.
If a Primary Key (PK) is linked to a Foreign Key (FK) in another table, we need to ensure that referential integrity is maintained. Depending on what makes business sense, the update rule can be either:
- U:R (Update:Restrict) – Disallow updates of values in the PK
- U:C (Update:Cascade) – Allow updates, but cascade changes to all related FKs in other tables
Similarly, the delete rule can be either:
- D:R (Delete:Restrict) – Disallow deletion of records
- D:C (Delete:Cascade) – Allow deletion of records, but cascade deletions in all related tables that contain a FK linked to this table
Notations available in Visio are:
- R (No action) - Don't change the child table
- C (Cascade) - Replicate the change in the child table
- D (Set default) - Insert the default value you previously set for that column in the child table
- N (Set Null) - Set the child table to NULL
- (Blank) - Do not enforce referential integrity on the relationship
See the Preserve referential integrity in Database Model diagrams article for more details.