Which theory is used to select date from data base

How date is used in database?
SQL Server comes with the following data types for storing a date or a date/time value in the database: DATE - format -MM-DD.
...
SQL Date Data Types
DATE - format -MM-DD.
DATETIME - format: -MM-DD HH:MI:SS.
TIMESTAMP - format: -MM-DD HH:MI:SS.
YEAR - format or YY.

How is set theory used in databases?

Relational databases are based almost entirely upon set theory. ... A SQL INNER JOIN is just the intersection of two sets. Finding the LEFT JOIN of two tables is nothing more than finding the set difference or the relative complement of the two tables.

What are the choices?

Answer the lady gave a violent start and stared in bewilderment at my companion;