How to configure unique constraint that has an exception?

Question

Consider a table that has a unique constraint for fields a,b and c. For example something like

create table my_tbl (
  a number,
  b number,
  c varchar2(10),
  ...
  constraint constr_name_u unique(a,b,c)
);

Now I want to modify the constraint such that rows where b < a does not need to be unique. That means that I want to allow non-uniqueness only for rows where b < a. All other rows still have to be unique.

How can I create such a 'relaxed' unique constraint?

Answer 1

You can't code that logic in a constraint.

You can create a unique function-based index, however, which probably lets you accomplish the same task

CREATE UNIQUE INDEX idx_my_table
    ON my_table ( CASE WHEN a <= b THEN a ELSE NULL END,
                  CASE WHEN a <= b THEN b ELSE NULL END,
                  CASE WHEN a <= b THEN c ELSE NULL END );

This takes advantage of the fact that Oracle doesn't store completely NULL values in the index structure. This allows you to have as many rows where b < a as you'd like and none of them would be stored in the index structure.