Conditions Required of Foreign Keys

Conditions Required of Foreign Keys

All the tables which can be included in a lock object must be connected with their foreign key. There are a number of restrictions to the valid relationships.

The foreign key relationships of the Basis tables of the lock object must form a tree. The tables are the nodes of the tree. The links of the tree mean ‘is the check table of’.

The foreign key fields must be key fields.

The foreign key relationships of the Basis tables of the lock objects may not have any field which is checked against more than one other field. A field therefore may not occur twice as foreign key field in a relationship and may not be checked against two different fields in two different foreign key relationships.

There is one restriction for multi-structured foreign keys ( Multi-Structured Foreign Keys). If a field is assigned to a field which is outside the check table, the table containing this field must be in a sub-tree which contains the check table of this foreign key relationship as a root.

These conditions are always satisfied if the key of the foreign key table is an extension of the key of the check table.

Conditions 2, 3 and 4 are meaningless if the relevant foreign key field was removed from the assignment of the key fields of the check table, by marking it as generic or setting it to be constant. This is also true for multi-structured foreign keys if the foreign key field refers to a table which is not contained in the lock object.