An Imieliński–Lipski algebras is an extension of relational algebra onto tables with different types of null values. It is used to operate on relations with incomplete information. Imieliński–Lipski algebras are defined to satisfy precise conditions for semantically meaningful extension of the usual relational operators, such as projection, selection, union, and join, from operators on relations to operators on relations with various kinds of "null values". Take SQL query Q SELECT NAME FROM T WHERE (CLASS = 'Networks' AND SEMESTER = 'Spring') OR (GRADE = 'A' AND SEMESTER <> 'Spring')