Complex sentence: Thus, in order for a unique key to uniquely identify each row in a table, NULL values must not be used, however a column defined as unique key column allows only one null value, which in turn can uniquely identify that row/tuple.
Split into simpler sentences: Thus, in order for a unique key to uniquely identify each row in a table, NULL values must not be used. However, a column defined as a unique key column allows only one NULL value, which in turn can uniquely identify that row/tuple.