Using a CHECK constraint to check for minimum age

Question

Is it possible to use a check constraint in a table to check whether a person meets a minimum age? For example, if a person is registering as a member, I want to make sure the person is at least 18 years old (contract age), so that a minor doesn't try to register as an adult.
From what I understand, I can't use SYSDATE in a check statement. Is there another way? What about using CURRENT_DATE and FLOOR to form a statement for this check?
Here's the DDL for the table (table is not fully decomposed to keep it simple; "dob" represents "Date of Birth" and "zipcode" is a FK to this table):
CREATE TABLE member     (
member_id     VARCHAR2(10)     NOT NULL,
fname          VARCHAR2(30)     NOT NULL,
lname          VARCHAR2(30)     NOT NULL,
address1     VARCHAR2(50),
address2     VARCHAR2(25),
zipcode     NUMBER(5)     NOT NULL,
phone          VARCHAR2(15)     NOT NULL,
dob          DATE          NOT NULL,
gender     CHAR(1)          NOT NULL,
CONSTRAINT ck_gender     gender IN ('M','F'),
CONSTRAINT pk_member     PRIMARY KEY (member_id),
CONSTRAINT fk_member_zipcode (zipcode) REFERENCES zipcode)
Thanks for your help.

Answer 1

Like SYSDATE, CURRENT_DATE is a non-deterministic function and so cannot be used in this context.
From the manual:
Restrictions on Check Constraints
You cannot specify a check constraint for a view. However, you can define the view using the WITH CHECK OPTION clause, which is equivalent to specifying a check constraint for the view.
The condition of a check constraint can refer to any column in the table, but it cannot refer to columns of other tables.
Conditions of check constraints cannot contain the following constructs:
Subqueries and scalar subquery expressions
Calls to the functions that are not deterministic (CURRENT_DATE, CURRENT_TIMESTAMP, DBTIMEZONE, LOCALTIMESTAMP, SESSIONTIMEZONE, SYSDATE, SYSTIMESTAMP, UID, USER, and USERENV)
Calls to user-defined functions
Dereferencing of REF columns (for example, using the DEREF function)
Nested table columns or attributes
The pseudocolumns CURRVAL, NEXTVAL, LEVEL, or ROWNUM
Date constants that are not fully specified
To achieve this sort of checking you will need to use a trigger