CREATE TABLE assignedto (scientist NUMBER, project CLOB)
CREATE TABLE scientists (ssn NUMBER, name CLOB)
CREATE TABLE projects (code CLOB, name CLOB, hours NUMBER)

Select the project names which are not assigned yet.
SELECT name FROM projects WHERE NOT code IN (SELECT project FROM assignedto)
SELECT "name" FROM "projects" WHERE NOT "code" IN (SELECT "project" FROM "assignedto")