Difference between primary key and unique key

rajesh822 · New User Joined: 22 Sep 2005 Posts: 5

what is the difference between primary key and unique key?

radhakrishnan82 · Posted: Mon Oct 10, 2005 10:21 am

Primary Key is Unique and NotNull
UniqueKey --The Values should be Unique and can be null even

for further info on this one,please search the previous posts in this forum.

hope this helps.

SWAPNA LATHA · New User Joined: 13 Jun 2005 Posts: 3

one more addition to the above question......
if unique key allows null values and more than one record has null entries,then how is uniqueness maintained?????

radhakrishnan82 · Posted: Sat Oct 15, 2005 4:35 pm

swapna latha,

Unique can have only one null value.

Pls refer this previous posts.
http://ibmmainframes.com/about5855.html&highlight=unique+key

hope this helps.

SWAPNA LATHA · New User Joined: 13 Jun 2005 Posts: 3

thanks radhakrishna

this answers my query...

nijugopalan · New User Joined: 15 Oct 2005 Posts: 15 Location: pune

all the keys used in the table which have unique values are unique keys.while the unique key which is selected to identify records is the primary key while all other keys are alternate keys.

ALL PRIMARY KEYS ARE UNIQUE KEYS WHILE ALL PRIMARY KEYS CANNOT BE AN UNIQUE KEYS.

THIS IS CORRECT NAA. MY FREINDS.

ravisan · New User Joined: 13 Sep 2005 Posts: 13 Location: chennai

One more differance is there......

A table may contains no of unique key. But it must contains only a primery key

Regards,
Ravisan

khamarutheen · Active Member Joined: 23 Aug 2005 Posts: 677 Location: NJ

hi frnd,
the primary key is let active when any the unique index is create. its same like unique value. ie., the unique key can have only one null value and it cannot have any foreign key ref. as the primary key cannot have a null value and can have ref to any foreign key. hope enough