Two way optional?
The mother-child unary relationship must be optional in both directions. The optional nature of the parent-side of the relationship (i.e., the top circle) means that Willaim's and Abigail's mother does not have to be represented by a row in the table. Conversely, the optional nature of the child-side of the relationship (i.e., the bottom circle) means that Willaim, Thomas and Robert do not have to be anyone's mom. For that matter, Sally Anne and Susan don't have to be moms, either. They might be moms, but they aren't required to be moms.
| ID |
Name |
Born |
MotherID |
| 100 |
Willaim |
1848 |
-- |
| 101 |
Abigail |
1850 |
-- |
| 102 |
Thomas |
1869 |
101 |
| 103 |
Robert |
1873 |
101 |
| 104 |
Sally Anne |
1876 |
101 |
| 105 |
Susan |
1899 |
104 |
|
etc. |
|
|