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what is Primary key and unique key and what is the
Difference?

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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