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Can we use more than one null value for unique key?
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Answer Posted / hassan khan
A unique key constraint does not imply the NOT NULL
constraint in practice. Because NULL is not an actual value
(it represents the lack of a value), when two rows are
compared, and both rows have NULL in a column, the column
values are not considered to be equal. Thus, in order for a
unique key to uniquely identify each row in a table, NULL
values must not be used. According to the SQL standard and
Relational Model theory, a unique key (unique constraint)
should accept NULL in several rows/tuples — however not all
RDBMS implement this feature correctly.
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