SELECT  * ,
 (SELECT . . .

SELECT seminar.* ,
 (SELECT test.guid from test where test.id IN (SELECT course_test.test_id FROM course_test WHERE course_test.course_id = 2)) AS test_guid 
FROM seminar WHERE seminar.id IN (SELECT course_seminar.seminar_id FROM course_seminar WHERE course_seminar.Course_id = 2)

Database:  localhost/3253:/var/db/fb25/tmp.fdb
SQL> show version;
ISQL Version: LI-V2.5.3.26744 Firebird 2.5
Server version:
Firebird/linux AMD64 (access method), version "LI-V2.5.3.26744 Firebird 2.5"
Firebird/linux AMD64 (remote server), version "LI-V2.5.3.26744 Firebird 2.5/tcp (oel64)/P12"
Firebird/linux AMD64 (remote interface), version "LI-V2.5.3.26744 Firebird 2.5/tcp (oel64)/P12"
on disk structure version 11.2
SQL> recreate table test(id int, guid varchar(36));
SQL> recreate table seminar(id int);
SQL> recreate table course_seminar(seminar_id int, Course_id int);
SQL> recreate table course_test(course_id int, test_id int);
SQL> commit;
SQL> set planonly;
SQL> SELECT seminar.* ,
CON>  (SELECT test.guid
CON>   from test
CON>   where test.id
CON>   IN (SELECT course_test.test_id
CON>         FROM course_test
CON>         WHERE course_test.course_id = 2
CON>       )
CON>   ) AS test_guid
CON> FROM seminar
CON> WHERE seminar.id IN (
CON> SELECT course_seminar.seminar_id
CON> FROM course_seminar
CON> WHERE course_seminar.Course_id = 2
CON> );

PLAN (COURSE_SEMINAR NATURAL)
PLAN (COURSE_TEST NATURAL)
PLAN (TEST NATURAL)
PLAN (SEMINAR NATURAL)
SQL>