select unique(columnA), columnB from tableC where ...

Won't work. Consider
columnA  columnB  

-----------------

10       AAA

10       BBB
Which row should it return, (10, AAA) or (10, BBB)?

Besides, I think that ANSI standard is distinct, not unique.

Dragi "Bonzi" Raos
Zagreb, Croatia
Q#5196, MkII#80000376, 18GB green
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Dragi "Bonzi" Raos Q#5196 MkII #080000376, 18GB green MkIIa #040103247, 60GB blue