How to give a unique constraint to a combination of a column and a fixed value in Oracle?

Question

I have a table with with 3 columns: A (number), B (number) and C (boolean).

I would need to create a rule that would prevent records from being created with columns A and B and with C equal to true. For example.

This would be allowed:

A  B  C
1  2  true
1  2  false
1  2  false

But this, no:

A  B  C
1  2  true
1  2  true
1  2  false

Answer 1

Slight variation on MarmiteBomber's approach, to avoid concatenating the values (which could cause accidental clashes with non-integer values):

create table t (a number, b number, c varchar2(5),
  constraint t_chk check (c in ('true', 'false'))
);

create unique index t_unq
on t (case when c = 'true' then a end, case when c = 'true' then b end);

insert into t(a,b,c) values (1,2,'true');

1 row inserted.

insert into t(a,b,c) values (1,2,'false');

1 row inserted.

insert into t(a,b,c) values (1,2,'false');

1 row inserted.

insert into t(a,b,c) values (1,2,'true');

ORA-00001: unique constraint (MY_SCHEMA.T_UNQ) violated

select * from t;

         A          B C    
---------- ---------- -----
         1          2 true 
         1          2 false
         1          2 false

---

Quick example of why non-integers (if they can exist) might be a problem:

create unique index uq_true on test(case when c = 'true' then a||'.'||b end);

insert into test(a,b,c) values (1.1, 2,'true');

1 row inserted.

insert into test(a,b,c) values (1, 1.2,'true');

ORA-00001: unique constraint (MY_SCHEMA.UQ_TRUE) violated

select * from test;

         A          B C    
---------- ---------- -----
       1.1          2 true 

... because for both '1.1' ||'.'|| '2' and '1' ||'.'|| '1.2' resolve to the same string, '1.1.2'.

This can also be a problem when combining string values rather than numbers. In either case you can avoid it by using a delimiter which cannot exist in either value; harder to do with strings, but with numbers any punctuation other than a period (or comma to be safe) would probably do - unless someone has a weird setting for nls_numeric_characters...