Answer Posted / dqm

In theory, a unique key may not contain any nulls. The
reason is simple: the key, by definition is an identifier
and it is illogical to identify something by an unknown.

In practice, some SQL databases permit nulls in unique
keys. Oracle, in particular, allows nulls as long as no
duplicate indices are created in the underlying unique
index. Since Oracle does not create a indices if all
columns are null, the floodgates are open.

For example, for a single column key, this is acceptable
because only the first row makes it to the index:

1
null
null


For a two column key, this is acceptable because in the
first two rows, the first column is unique and the last two
rows, being entirely null, are not represented in the index.

1, null
2, null
null, null
null, null

However, with the same two-column index, this is
unacceptable because the first two rows produce duplicate
inices:

1, null
1, null
null, null
null, null

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