need answer

unique key enforces only uniqueness--ie--no duplicate value

primary key enforces both uniqueness and not null
an index is also created on primary key field.

In addition:-
Table shoud have only one promary key where as it can have more than one unique key.

A unique key must uniquely identify all possible rows that exist in a table and not only the currently existing rows. Examples of unique keys are Social Security numbers (associated with a specific person[1]) or ISBNs (associated with a specific book). Telephone books and dictionaries cannot use names or words or Dewey Decimal system numbers as candidate keys because they do not uniquely identify telephone numbers or words.

A primary key is a special case of unique keys. The major difference is that for unique keys the implicit NOT NULL constraint is not automatically enforced, while for primary keys it is. Thus, the values in a unique key columns may or may not be NULL. Another difference is that primary keys must be defined using another syntax.

For more information refer to the following link

Unique key - Wikipedia, the free encyclopedia

Thanks
Sushma

Hi Sushma, Please don't mind but people want explanation in our words, you need not work like an agent who does wiki search for questions.

hai,
The column holding the primary key constraint cannot accept
null values.whereas column holding the unique constraint can
accept null values assume that t3 is a table with two
columns t1 having primary key constraint and t2 having
unique constraint if u try to insert null into t2 it will
accept that values whereas column t1 will not accept null
Regards,
Nithya.R

In a table there can be multiple unique keys but there can be only one primary key.
Unique key can be defined as null but primary key cann't be set to null.

Unique key--The data in the column is unique. i.e if you specify NULL that should be only one NULL value

Primary key--No NULL at all

This is perfect!!!

perfect ........

both are using uniquely identify a row
but
a table can have more than 1 unique keys.a tabe can have only one primary key
and unique key allows NULL values but primary key never allows NULL values

diffrence between SQL UNIQUE Constraint and sql primary key

you need to go through other posts in this thread.

primary key--> primary key is uniqe column in particular table and null value is not allow or other words duplicate value not allow as well as null value is not accepted.. in one table have only one primary key it called primary key

unique key--> unique key is same as primary key but in unique key allow null value

difference between unique and primary key is


unique identifies only unique values(including null values )

primary key identifies only unique(but not null values)

unique should contain the null value but primary key cannot take a null value

1. Primary Key doesn't allow a null value but Unique key can have at most one null value.
2. Only one Primary key can be declared in a table but there can be multiple unique keys in a table.

unique key can have more than one null value! primary key cannot have null values at all... both can be referenced by foreign keys...but u can hav only 1 primary key but any number of unique keys...