The column holding the primary key constraint cannot accept
null values.whereas colum 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
First difference is that primary key doesn't accept null
values whereas unique accepts one or multiple.
second difference is that Clustered index is created on
Primary key constraint and non clustered unique indexes is
created on Unique key constraint.
we can't insert a NULL value into to coloumn holding
primary key constrain.
where as we can insert a NULL value to a unique key feild.
Remember, both keys are used to identify the
rows/tuple/entry of the table uniquely.
so, only once u can enter a NULL value into the unique key
the basic difference is that primary key can not have null
value whereas Unique key can have null values.two null
value are not same.One table may have more than one Unique
columns, but should have only one Primary Key Constraint
7. Primary Key doesnt allow the Null Values. where as in
Unique accept the Null values.
One table may have more than one Unique Key Constraint.
But it sholud have only one Primary Key Constraint column.
* Only one primary key is allowed per table (any number of
unique keys are allowed).
* Primary key columns automatically become NOT NULL (unique
keys can contain nulls).
* UNIQUE and DISTINCT are synonymous in a SELECT list.
Each table having only one PRIMARY KEY.And my contain many
PRIMARY KEY is identity of each row. UNIQUE KEY also
identity of each row.
BOTH keys will not contain repeated value.
BUT UNIQUE KEY will contain NULL value but PRIMAY key will
not contain NULL value.
5. Display full details from the ORDER_LINE table where the
item number is (first condition) between 1 and 200 (no > or
< operators) OR the item number is greater than 1000 AND
(second condition) the item cost is not in the list 1000,
2000, 3000 OR the order number is not equal to 1000.