Sobre a linguagem SQL, um campo com valor nulo é um campo se...

SELECT <column_names>

FROM <table_name>

WHERE NOT NULL (<column_name>);

SELECT <column_names>

FROM <table_name>

WHERE <column_name>= NULL;

SELECT <column_names>

FROM <table_name>

WHERE <column_name> <> 0 AND <column_name> <> '';

SELECT <column_names>

FROM <table_name>

WHERE NULL (<column_name>) = 1;

SELECT <column_names>

FROM <table_name>

WHERE <column_name> IS NULL;