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Is Unique index same as primary key?
Primary key will not accept NULL values whereas Unique key can accept NULL values. A table can have only 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.
Does primary key must be unique?
Primary keys must contain UNIQUE values, and cannot contain NULL values. A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).
Can we create a foreign key without primary key?
If you really want to create a foreign key to a non-primary key, it MUST be a column that has a unique constraint on it.
Is primary key automatically indexed in SQL Server?
Primary keys are always indexed by default. You can define a primary key in SQL Server 2012 by using SQL Server Management Studio or Transact-SQL. Creating a primary key automatically creates a corresponding unique, clustered or nonclustered index.
Can a unique index be created without a primary key?
Declaring a PRIMARY KEY or UNIQUE constraint causes SQL Server to automatically create an index. An unique index can be created without matching a constraint, but a constraint (either primary key or unique) cannot exist without having a unique index.
Is it good to have PK index for primary key?
But the fact that you can create another doesn’t indicate that the PK index doesn’t also already exist. The additional index does no good, but the only harm (very small) is the additional file size and row-creation overhead. As everyone else have already said, primary keys are automatically indexed.
Is it possible to create a unique index in SQL?
Unique indexes are not part of the SQL standard. The particular implementation of a DBMS will determine what are the consequences of declaring a unique index. In Oracle, declaring a primary key will result in a unique index being created on your behalf, so the question is almost moot.
Is the primary key automatically indexed in SQL Server?
Making it a primary key should also automatically create an index for it. Well in SQL Server, generally, primary key is automatically indexed. This is true, but it not guaranteed of faster query. The primary key will give you excellent performance when there is only 1 field as primary key.