In the case of a relational database consisting of tables, columns, rows, keys and foreign keys
, the mapping can be done using the following principles :
The two explanations of foreign key
are quite similar, the oral account being a grammatically less structured statement.
will be retrieved later when the user starts to collect time study data and will be saved together with these data as a foreign key
in the database.
Referential integrity constraint: For each leg [L.sub.ij] (except [L.sub.11]), there should be a referential constraint from [Tau] ([E.sub.1]) to [Tau] ([E.sub.i]), with [l.sub.ij]_[k.sub.i] being the foreign key
for a foreign key
and 2) the designer selected a special value to represent null.
In the case of Sophie Acorn's record, the contact ID is 31, so the criteria string will be CID=31, where CID is the field name of the primary key field for the Contact table as well as the foreign key
field name in each of the related tables.
Key and foreign key
constraints are represented by attribute grammars, adding semantic rules to schema grammars, to carry key and foreign key
values (to verify their properties).
In the ATTRIBUTES section, the user must insert all of the fields of a record with the exception of the foreign key
fields, specifying for each attribute: name, type and value.
The Room segment becomes a relational table, but must have the HNAME field added as a foreign key
to establish the one-to-many relationship between it and the Hotel segment and to fully identify the room, since room number is unique only within each hotel.
Referential constraints enforce primary key-foreign key relationships so that a foreign key
cannot be entered that does not conform to its parent primary key.
(In related tables, CustomerID is a foreign key
defined as a Long Integer with no default value.)