Can a unique index have included columns?
A unique nonclustered index can contain included nonkey columns. For more information, see Create Indexes with Included Columns.
Which columns should be included in index?
Primary key columns are typically great for indexing because they are unique and are often used to lookup rows.
How many unique index can a table have in Db2?
Each table can have only one primary index. However, the table can have additional unique indexes. No keywords are required in the CREATE INDEX statement.
How do you create a unique index in Db2?
Db2 UNIQUE Index
- First, specify the name of the unique index in the CREATE UNIQUE INDEX clause.
- Second, specify the name of the table to which the unique index belongs and a list of columns that will be included in the index.
What is the difference between key columns and included columns in indexes?
In SQL Server 2000 and earlier, for a nonclustered index, all columns defined for an index were key columns, which meant they were part of every level of the index, from the root down to the leaf level. When a column is defined as an included column, it is part of the leaf level only.
How many columns should be in an index?
Number of Columns In SQL Server, you can include up-to 1023 columns per each non-clustered index. But you have to add minimum one key column to your non-clustered index in order to create it.
What is unique index in Db2?
Db2 uses unique indexes to ensure that no identical key values are stored in a table. When you create a table that contains a primary key or a unique constraint, you must create a unique index for the primary key and for each unique constraint.
What is the difference between primary key and unique index in Db2?
Primary key will not accept NULL values whereas Unique key can accept NULL values. A table can have only one primary key whereas there can be multiple unique key on a table. A Clustered index automatically created when a primary key is defined whereas Unique key generates the non-clustered index.
What is the difference between primary key and unique index?
Primary key will not accept NULL values whereas Unique key can accept NULL values….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 |
| Index | Creates clustered index | Creates non-clustered index |
What is the use of unique index in Db2?
What is included index?
Indexes with included columns provide the greatest benefit when covering the query. This means that the index includes all columns referenced by your query, as you can add columns with data types, number or size not allowed as index key columns.
What is the difference between unique index and unique column in DB2?
If you include one column in the unique index, Db2 will enforce the uniqueness of values in this column. In case you define a unique index that includes multiple columns, Db2 will enforce the uniqueness of values in these columns. Any attempt to insert or update data into the unique indexed columns that cause the duplicate will result in an error.
What happens if I have multiple columns in a DB2 index?
In case you define a unique index that includes multiple columns, Db2 will enforce the uniqueness of values in these columns. Any attempt to insert or update data into the unique indexed columns that cause the duplicate will result in an error.
What happens when you create a unique index for a table?
When you create a unique index for the table that already has existing data, Db2 will check the data in the indexed key columns for the uniqueness. If the table contains duplicate key values, the index creation process will fail.
Do I need an include clause for a unique index?
Note that the include clause is not strictly required for this behavior: databases that make a proper distinction between unique constraints and unique indexes just need an index with the unique key columns as the leftmost columns—additional columns are fine. For the Oracle Database, the corresponding syntax is this: