Double values in a table - sql

Question

Is it possible to define a ID column to be unique but every value have to be occur twice?
For example:
table TRANSLATION:
id | name_id | translation
____________|_________|____________
1 | 1 | apple
____________|_________|____________
2 | 1 | apfel
____________|_________|____________
3 | 2 | pear
____________|_________|____________
4 | 2 | birne
I want name_id values to always occur twice, not once and not three times. name_id is a FK from table with my objects that needs to be translated.

Answer 1

No, this is impossible to enforce, though you can attempt it using triggers this is normally a pretty messy solution.
I'd change your table structure to be something like the following:
ID
NAME_ID
LANGUAGE_ID
TRANSLATION
You could then create a unique index on NAME_ID and LANGUAGE_ID. Theoretically, you'd also have a table LANGUAGES, and the LANGUAGE_ID column would have a foreign key back into LANGUAGES.ID - you could then restrict the number of times each NAME_ID appears by not having the data in LANGUAGES.
Ultimately this means that your schema would look something like this:
create table languages (
id number
, description varchar2(4000)
, constraint pk_languages primary key (id)
);
insert into languages values (1, 'English');
insert into languages values (2, 'German');
create table names (
id number
, description varchar(4000)
, constraint pk_names primary key (id)
);
insert into names values (1, 'apple');
insert into names values (2, 'pear');
create table translations (
id number
, name_id number
, language_id number
, translation varchar2(4000)
, constraint pk_translations primary key (id)
, constraint fk_translations_names foreign key (name_id) references names (id)
, constraint fk_translations_langs foreign key (language_id) references languages (id)
, constraint uk_translations unique (name_id, language_id)
);
insert into translations values (1, 1, 1, 'apple');
insert into translations values (2, 1, 2, 'apfel');
insert into translations values (3, 2, 1, 'pear');
insert into translations values (4, 2, 2, 'birne');
and you should be unable to break the constraints:
SQL> insert into translations values (5, 1, 3, 'pomme');
insert into translations values (5, 1, 3, 'pomme')
*
ERROR at line 1:
ORA-02291: integrity constraint (FK_TRANSLATIONS_LANGS) violated - parent
key not found
SQL> insert into translations values (5, 1, 2, 'pomme');
insert into translations values (5, 1, 2, 'pomme')
*
ERROR at line 1:
ORA-00001: unique constraint (UK_TRANSLATIONS) violated
See this SQL Fiddle

Answer 2

Do you mean a maximum of twice? or do you mean they have to occur twice (i.e., once only is not ok)
If the former, Once only IS ok) then you could Add a bit field and make the Primary Key composite on the actual id and the bit field.
If the latter (They have to occur twice), then put two id fields in the same row and make then each a single field unique key.