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UNIQUE constraints When you create a UNIQUE constraint, a unique nonclustered index is created to enforce a UNIQUE constraint by default. You can specify a unique clustered index if a clustered index on the table does not already exist. For more information, see Unique Constraints and Check Constraints.
Both clustered and nonclustered indexes can be unique. This means no two rows can have the same value for the index key. Otherwise, the index is not unique and multiple rows can share the same key value. For more information, see Create unique indexes.
PostgreSQL automatically creates a unique index when a unique constraint or primary key is defined for a table. The index covers the columns that make up the primary key or unique constraint (a multicolumn index, if appropriate), and is the mechanism that enforces the constraint.
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.
I am new to SQL Server and while learning about clustered index, I got confused!
Is unique key clustered or a non-clustered index? Unique key holds only unique values in the column including null, so according to this concept, unique key should be a clustered index, right? But when I went through this article I got confused MSDN
When you create a UNIQUE constraint, a unique nonclustered index is created to enforce a UNIQUE constraint by default. You can specify a unique clustered index if a clustered index on the table does not already exist.
Please help me to understand the concept in a better manner, Thank you.
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