define tables

Hi,
An example:
primary key(personId),
unique id(personId),)

I often see primary key and unique id go together when define a table in some articles. Why unique id(personId) is needed? Doesn't primary key is already unique?

Thanks,
John

Hi
actually any one field can be primary key and unique key
but they have different meaning

primary key is used in relations with other tables ( basically to
link tables)

unique key need not be a primary key it is used when you need
some field to be unique but you dont want it to link to any
table

Hope this helps
Cheers

Jayesh Jain

Thanks, but why declare primary key and unique key for the same field?

You dont have to, but older versions of MySQL don't like it when you don't call both. If your using a fairly recent version of MySQL you will only have to call:

primary key(personId),

i would only define a unique key if i didnt want to have multiple records of something. Say you have a field called 'color' and you didnt want to have the same color listed twice, you would make the color field unique as JayeshJain said.

OH, that's why. Thanks Ben.