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SELECT DISTINCT

A SELECT DISTINCT statement considers all NULLs to be equal (NOT DISTINCT FROM each other), so if the SELECT is on a single column it will return at most one NULL.

As mentioned earlier, Firebird 2.0 has a bug that causes the NULLS FIRST | LAST directive to fail under certain circumstances with SELECT DISTINCT. For more details, see the bugs list.

See also:
Data retrieval

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