Question:  I need to understand how to add a constraint statement to create a check constraint for a table.  How do you alter a table to add a check constraint?

Check Constraints to validate values

Check constraints validate that values in a given column meet a specific criteria. For example, you could create a check constraint on a varchar2 column so it only can contain the values T or F as in this example:

Create
   table my_status
   ( status_id     NUMBER      PRIMARY KEY,
     person_id     NUMBER      NOT NULL,
     active_record VARCHAR2(1) NOT NULL
         CHECK (UPPER(active_record)='t?
         or
         UPPER(active_record)=?F?),
  person_ssn   VARCHAR2(20) CONSTRAINT un_person_ssn UNIQUE
);

In this example we created a table called MY_STATUS using the create table command. Notice the constraint keyword, this indicates that we are getting ready to define a constraint. Recall that earlier in this chapter we discussed in-line and out of line constraints. This particular example is known as an in-line constraint because the constraint is being defined in the same line as the column being defined.

If you need to add a check constraint to a table after the fact, simply use the alter table command. Here is an example:

ALTER TABLE my_status ADD (CONSTRAINT ck_stats_01
  CHECK (UPPER(active_record)='t? or
         UPPER(active_record)=?F?) );

Boolean Expressions can be used with check constraints.

Get the Complete Oracle SQL Tuning Information  The landmark book "Advanced Oracle SQL Tuning  The Definitive Reference"  is filled with valuable information on Oracle SQL Tuning. This book includes scripts and tools to hypercharge Oracle 11g performance and you can buy it for 30% off directly from the publisher.