All the tables that can be included in a lock object must be linked with foreign keys . There are a number of restrictions to the valid relationships:
- The foreign key relationships of the 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 of the foreign key table.
- The foreign key relationships defined between the base tables of the lock objects cannot have any field that is checked against more than one other field. A field cannot occur twice as a foreign key field in a relationship and cannot be checked against two different fields in two different foreign key relationships.
- You must keep one restriction in mind for multi-structured foreign keys . If a field is assigned to a field that is outside the check table, the table containing this field must be in a subtree that contains the check table of this foreign key relationship as a root.
If the key of the foreign key table is an extension of the key of the check table, it always satisfies the listed conditions above.
Conditions 2, 3 and 4 are meaningless if the particular foreign key field was excluded from the assignment to the key fields of the check table by marking it as generic or setting it to a constant . This is also true for multi-structured foreign keys if the foreign key field refers to a table that is not contained in the lock object.