These conditions require that the system be safe in the sense that no incorrect conclusion is derivable by using a specified subset F of the relational operators; and that it be complete in the sense that all valid conclusions expressible by relational expressions using operators in F are in fact derivable in this system.
Indeed, regardless what semester Igor took the Networks class (no matter what was the unknown value of NULL), the selection condition will be true.
[2] It is also argued there that 3-valued logic (TRUE, FALSE, MAYBE) can never provide guarantee of complete answer for tables with incomplete information.
In order to be able to extend more relational operators, more expressive form of null value representation is needed in tables which are called V-table.
V-tables algebra is based on many different ("marked") null values or variables allowed to appear in a table.
It has additional column “con” which is a Boolean condition involving variables, null values – same as in V-tables.
Imieliński–Lipski algebras were introduced by Tomasz Imieliński and Witold Lipski Jr. in Incomplete Information in Relational Databases.