Contents
- 1 Why we need to create index if primary key is already present in table?
- 2 Can we have more than one unique key?
- 3 What is the difference between index and unique key?
- 4 Can clustered index have duplicate values?
- 5 Can a unique index be created with a primary key constraint?
- 6 When is a unique index cannot be created?
Why we need to create index if primary key is already present in table?
32) Why we need to create an index if the primary key is already present in a table? Primary key can store null value, whereas a unique key cannot store null value.
Can we have more than one unique key?
Unique key constraints can accept only one NULL value for column….Comparison Chart:
| Parameter | PRIMARY KEY | UNIQUE KEY |
|---|---|---|
| Number of keys that can be defined in the table | Only one primary key | More than one unique key |
Does unique index allow duplicates?
No, you cannot create a Unique Index on a table that has duplicate values.
What is the difference between index and unique key?
A unique index ensures that the values in the index key columns are unique. A unique constraint also guarantees that no duplicate values can be inserted into the column(s) on which the constraint is created. When a unique constraint is created a corresponding unique index is automatically created on the column(s).
Can clustered index have duplicate values?
Yes, you can create a clustered index on key columns that contain duplicate values. For example, you might decide to create a clustered index on the LastName column of a table that contains customer data.
Is the SQL Server unique key also an index?
Unique Key: Unique Key enforces uniqueness of the column on which they are defined. Unique Key creates a non-clustered index on the column. Unique Key allows only one NULL Value. Alter table to add unique constraint to column:
Can a unique index be created with a primary key constraint?
Limitations and Restrictions. A unique index, UNIQUE constraint, or PRIMARY KEY constraint cannot be created if duplicate key values exist in the data. A unique nonclustered index can contain included nonkey columns. For more information, see Create Indexes with Included Columns.
When is a unique index cannot be created?
A unique index, UNIQUE constraint, or PRIMARY KEY constraint cannot be created if duplicate key values exist in the data. A unique nonclustered index can contain included nonkey columns. For more information, see Create Indexes with Included Columns. Requires ALTER permission on the table or view.
What are the benefits of a unique index?
Benefits of a Unique Index. Multicolumn unique indexes guarantee that each combination of values in the index key is unique. For example, if a unique index is created on a combination of LastName, FirstName, and MiddleName columns, no two rows in the table could have the same combination of values for these columns.