What is the difference between primary key and a unique key?

Both primary key and unique key will unique,but in unique key it will allow one value to be null

Primary Key:

1. When created, an index gets created for the same.

2. Cannot hold NULL values

3. No duplicates allowed


Unique key

1. When created, index gets created for unique key also

2. Can hold NULL values.

3. no duplicates allowed

There can be Single Null value in Unique key but Not in Primary Key

One more point..
There will be only primary key in a table and you can have any number of unique keys.

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Sireesha

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

Both should have unique keys to identify the rows but unique key can have null value

Hi,

Primary key doesnot allow null value but unique key allows null value.

We can declare only one primary key in a table but a table can have multiple unique key(column assign).

Thanks &Regards,
Riju