What is the difference between UNIQUE CONSTRAINT and
PRIMARY KEY?
1. There is no difference.
2. A PRIMARY KEY cannot be declared on multiple columns.
3. A UNIQUE CONSTRAINT cannot be declared on multiple
columns.
4. A table can have multiple PRIMARY KEYS but only one
UNIQUE CONSTRAINT.
5. A table can have multiple UNIQUE CONSTRAINTs but only
one PRIMARY KEY.
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Answer Posted / andsens
Fields in UNIQUE CONSTRAINTs can be NULL. They cannot with PRIMARY KEYs.
You can define more than one UNIQUE KEY in a table.
Because of this, the behaviour of UNIQUE CONSTRAINTs is very different from PRIMARY KEY constraints. As long as one of the fields in a UNIQUE CONSTRAINT is NULL the constraint itself does not apply and you can set the other fields to whatever you want without getting an error.
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