The Expression Filter
The Expression Filter uses the Rules Manager to work with expressions.
Once you’ve completed an optimal logical database
design, youmu st go back and consider what indexes youshou ld add to improve the
anticipated performance of the database and whether youshou ld designate any
tables as part of a cluster or hash cluster.
Constraints
A constraint enforces certain aspects of data integrity within a database. When youadd a constraint to a particular column, Oracle automatically ensures that data violatingthat constraint is never accepted. (unique, not null)
Each table can have, at most, a single primary key constraint. The primary keymay consist of more than one column in a table. The primary key constraint forces each primary key to have a unique value. Itenforces both the unique constraint and the NOT NULL constraint. A primarykey constraint will create a unique index, if one doesn’t already exist for thespecified column(s).
The foreign key constraint is defined for a table (known as the child) that has arelationship with another table in the database (known as the parent). The value entered in a foreign key must be present in a unique or primary key of another specific table. For example, the column for a department ID in an employee table might be a foreign key for the department ID primary key in the department table. Normally, you cannot delete a row in a parent table if it causes a row in the childtable to violate a foreign key constraint. However, you can specify that a foreignkey constraint causes a cascade delete, which means that deleting a referenced row in the parent table automatically deletes all rows in the child table that reference the primary key value in the deleted row in the parent table.
All constraints can be either immediate or deferred. An immediate constraint is enforced as soon as a write operation affects a constrained column in the table. A deferred constraint is enforced when the SQL statement that caused the change in the constrained column completes.
You can temporarily suspend the enforcement of constraints for a particular table. When youenable the operation of the constraint, you can instruct Oracle tovalidate all the data for the constraint or simply start applying the constraint to thenew data. When you add a constraint to an existing table, youcan also specifywhether you want to check all the existing rows in the table. (126)