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.

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