Hi Drew: Could you please comment on this text? Particularly if something is wrong or incomplete. Thanks!
Mariano On table attributes of not null, unique and key Base allows you to specify certain attributes for the columns in your tables. In this tutorial we will consider the following options: Key: This option tells Base that this column will hold the primary key of the table. Base will prepare then to see it associated with other tables. Unique: When you specify this option, Base will make sure that records in this column are not repeated. If, for example, you specify that the 'surname' column be unique, then the second time you try to enter 'Dumas' Base will reject it as an invalid entry. It makes a lot of sense to make sure that a column set to KEY is also set to UNIQUE. Not null: This option means that records can not be left with this attribute empty. Base will display an error message if you try to enter a record that leaves a NO NULL column empty. This forces whomever is using your database to at least have the information requested in the NOT NULL columns if they want to entrer the record. For example, if you set the 'surname' and 'date of birth' as NOT NULL, then a user can not input a new author if he doesn't have at least the surname and the date of birth. Again, it makes sense that Key columns are also set to NOT NULL. On the different deletion options and what they mean. Because being able to relate the object in one class to another object in another class is so important, and the strength of relational databases, special care must be placed on the subject or deleting records. Think about this: Let's say that there is an an author that wrote only one book and you discover that you no longer have that book in your collection. As you update your database and erase that title, what will happen to the 'author' information? Should it be deleted too? Should it be kept as an historical record or in case you find and buy that book again? Actually, both options are valid and you can chose the one that reflects the purpose of your database best. But your application will not know what to do unless you make explicit what your preference is. For this reason, when you are developing your application and defining relationships, Base will ask you how to handle the deletion of records and will offer you the following options. This is what they mean: No action: Base will not delete the record but... Delete Cascade: With this option, Base will delete the record you are requesting to delete and will also delete all other records that have the deleted record's key as a foreign key. This option is called cascade because it elicits the image of a deletion creating further deletions. Set Null: With this option, Base will delete the record you are requesting but will not delete the others related to it. Instead it will erase their foreign keys to reflect that they are no longer associated. Note that this requires that NUT NULL is not a condition of the foreign key column. Set Default: When deleting an object, the foreign key column of the associated tables will be populated with a default parameter that you previously specified. --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
