In practical sense there is no difference between UNIQUE INDEX and UNIQUE CONSTRAINT. If you want more detailed explanation, google

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>I am changing the FK constraint from referencing just one column to two columns (was: CATEGORY, now SITE_NO, CATEGORY).
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>Before the change, I had to DROP UNIQUE FIELD CONSTRAIN on the parent table. Then I created a UNIQUE INDEX (not CONSTRAINT) and then created (on the parent table) the FK_CONSTRANT on the two columns (on the child table). All works.
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>What I don't understand is in which case I would need the UNIQUE FIELD CONSTRAINT (on the parent table)? I had it before, dropped it, created unique index and all works. I am just trying to see if I will miss something by not having the UNIQUE FIELD CONSTRAINT.
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>TIA