You can define a value for a table column or user-defined datatype that is automatically inserted if a user does not explicitly enter a value. For example, you can create a default that has the value “???” or the value “fill in later.” You can also define rules for that table column or datatype to restrict the types of values users can enter for it.
In a relational database management system, every data element must contain some value, even if that value is null. As discussed in Chapter 7, “Creating Databases and Tables,” some columns do not accept the null value. For those columns, some other value must be entered, either a value explicitly entered by the user or a default entered by Adaptive Server.
Rules enforce the integrity of data in ways not covered by a column’s datatype. A rule can be connected to a specific column, to several specific columns or to a specified, user-defined datatype.
Every time a user enters a value, Adaptive Server checks it against the most recent rule that has been bound to the specified column. Data entered prior to the creation and binding of a rule is not checked.
As an alternative to using defaults and rules, you can use the default clause and the check integrity constraint of the create table statement to accomplish some of the same tasks. However, these items are specific to each table and cannot be bound to columns of other tables or to user-defined datatypes. For more information about integrity constraints, see Chapter 7, “Creating Databases and Tables.”
