Are there some mathematical theories/theorems/... behind modern development of database management systems and in particular of SQL ?

I am refreshing my knowledge of these things which are quite down-to-earth "how to use" (create table ..., insert..., select * from ...), but I think some deeper understanding what is behind would be helpful.

In particular in Wikipedia one may find some relations with 3-valued logic:

Along with True and False, the Unknown resulting from direct comparisons with Null thus brings a fragment of three-valued logic to SQL. The truth tables SQL uses for AND, OR, and NOT correspond to a common fragment of the Kleene and Lukasiewicz three-valued logic (which differ in their definition of implication, however SQL defines no such operation).

But it is not very clear for me what it means and how deep it is ?