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January 3, 2013

Null and Not Null value in Primary and unique keys in sql

Primary key should contain unique value and not null because if attribute value is null than how should maintain the uniqueness..

one more think i want to add that Primary key is a type of constraint and foreign key is also a type of
constraint and pk is null then u can't use the null value as a foreign key.
So, Primary key should be a unique value and not null.

Well first of all null is not a value its a state, null is undefined , so we cannot have null as primary key as we do not know its value also value of one null is different from another null, so if we allow null values each null is unique in a sense , but since we do not know what it stands for we do not allow it in primary key.

Though in case of foreign key we do allow null value reason being
a child(foreign) cannot point towards an non existent parent(primary key) but a child can be orphan it self hence we can allow null in foreign key.

As primary key column is the candidate column in the table where the data is going to be identified by this column itself so it should have a value and the unique value .

Primary key is a unique key which should not accept any null values.& it doesn't allow any duplicates also.whenever table is created the first record will be filled with null values. whenever i create a primary key constraint on table internally an unique index should be created.which doesn't allow any nulls.Null is a missing or unknown value