Question

Here is the most relevant part of my database schema:

create table TEST (
    ID integer not null,
    NAME text not null,
    constraint PK_TEST primary key (ID),
    constraint UNQ_TEST_NAME unique (NAME)
);

create table SESSION (
    ID integer not null,
    constraint PK_SESSION primary key (ID)
);

create table SESSION_TEST (
    SESSION_ID integer not null,
    TEST_ID integer not null,
    ORDINAL integer not null,
    constraint PK_SESSION_TEST primary key (SESSION_ID, TEST_ID),
    constraint FK_SESSION_TEST_SESSION_ID foreign key (SESSION_ID) references SESSION (ID) on delete cascade,
    constraint FK_SESSION_TEST_TEST_ID foreign key (TEST_ID) references TEST (ID) on delete cascade,
    constraint UNQ_SESSION_TEST_SESSION_ID_ORDINAL unique (SESSION_ID, ORDINAL)
);

There are SESSIONS that consist of multiple TESTS. TESTs in SESSIONS have ORDINALs (are ordered). SESSION_TEST is a link table for a many-to-many relationship: one test can be a part of multiple sessions, and one session consists of multiple tests (but one test can be in a session only once, which is the PK).

I am having problems writing an SQL statement that would return true (or 1 actually, as I am using SQLite) for a test with a given ID, if there is a session that has that test and only that test (in other words, the session consists of only one test, the one I am looking for).
For example:

TEST:
ID|NAME
1|aaa
2|bbb
3|ccc

SESSION:
ID
1
2
3
4

SESSION_TEST:
SESSION_ID|TEST_ID|ORDINAL
1|1|1
1|2|2
2|1|1
3|3|1

SESSION with ID = 1 has two TESTs, and SESSIONs 2 and 3 have one test each. I would need to have a select that would return 1/true for inputs 1 and 3, but 0 for 2 (as this TEST is only in SESSION 1, but it’s not the only one).

(Sorry about the title, I really didn’t know how to put it in a few sentences and make it clear!).

Answer 1

RETURN EXISTS SELECT SESSION_ID FROM 
SESSION_TEST
WHERE TEST_ID = @id
AND ORDINAL = 1
AND SESSION_ID NOT IN (SELECT SESSION_ID FROM SESSION_TEST WHERE ORDINAL > 1);

The RETURN EXISTS syntax might be TSQL only, but you get the picture. If the query returns rows, return true, if not return false.