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Does index have to be unique SQL?
So, when you create the clustered index – it must be unique. But, SQL Server doesn’t require that your clustering key is created on a unique column. You can create it on any column(s) you’d like. Internally, if the clustering key is not unique then SQL Server will “uniquify” it by adding a 4-byte integer to the data.
What does index constraint mean?
Advertisements. The INDEX is used to create and retrieve data from the database very quickly. An Index can be created by using a single or group of columns in a table. When the index is created, it is assigned a ROWID for each row before it sorts out the data.
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.
When to create unique index in primary key?
PRIMARY KEY or UNIQUE constraint When you create a PRIMARY KEY constraint, a unique clustered index on the column or columns is automatically created if a clustered index on the table does not already exist and you do not specify a unique nonclustered index. The primary key column cannot allow NULL values.
How to check unique indexes in SQL Server?
Execute the above script, and it creates the table, primary key and unique key constraint. It creates the SQL Server index for primary key constraints and unique key constraints. We can check existing indexes on a table using sys.sp_helpindex system stored procedure.
How are unique indexes used in query optimizer?
Query optimizer also uses the Unique Index to create cost-optimized execution plans. The only difference is that you cannot directly drop the unique Index created by the unique constraint in SQL Server. You also get a few additional index options once you directly create a unique Index.