I am currently undertaking a review of the primary keys in a SQL Server 2000 database with a view to improving performance of queries. I have heard that, in the case of compound primary keys, it is important to select the correct order for the columns within the key. For instance, imagine a table called OrderLine which has primary key.
You can define a primary key in SQL Server by using SQL Server Management Studio or Transact-SQL. Creating a primary key automatically creates a corresponding unique clustered index, or a nonclustered index if specified as such.
A table can contain only one PRIMARY KEY constraint.
All columns defined within a PRIMARY KEY constraint must be defined as NOT NULL. If nullability is not specified, all columns participating in a PRIMARY KEY constraint have their nullability set to NOT NULL.
Creating a new table with a primary key requires CREATE TABLE permission in the database and ALTER permission on the schema in which the table is being created.
Creating a primary key in an existing table requires ALTER permission on the table.
Caution
If you want to redefine the primary key, any relationships to the existing primary key must be deleted before the new primary key can be created. A message will warn you that existing relationships will be automatically deleted as part of this process.
A primary key column is identified by a primary key symbol in its row selector.
If a primary key consists of more than one column, duplicate values are allowed in one column, but each combination of values from all the columns in the primary key must be unique. Generate ssh key centos 6.
If you define a compound key, the order of columns in the primary key matches the order of columns as shown in the table. However, you can change the order of columns after the primary key is created. For more information, see Modify Primary Keys.
The following example creates a primary key on the column TransactionID
in the AdventureWorks database.
The following example creates a table and defines a primary key on the column TransactionID
in the AdventureWorks database.
The following example creates a table and defines a primary key on the column CustomerID
and a clustered index on TransactionID
in the AdventureWorks database.