If clustered or nonclustered is not specified for a primary key constraint, clustered is used if there is no clustered index on the table.Īll columns defined within a primary key constraint must be defined as not null. The index generated by a primary key constraint cannot cause the number of indexes on the table to exceed 999 nonclustered indexes and 1 clustered index. This prevents the insertion of duplicate rows.Ī table can contain only one primary key constraint.Ī primary key cannot exceed 16 columns and a total key length of 900 bytes. This makes sure that every row in the ProductVendor table has a unique combination of ProductID and VendorID. If a primary key constraint is defined on more than one column, values may be duplicated within one column, but each combination of values from all the columns in the primary key constraint definition must be unique.Īs shown in the following illustration, the ProductID and VendorID columns in the Purchasing.ProductVendor table form a composite primary key constraint for this table. This index also permits fast access to data when the primary key is used in queries. When you specify a primary key constraint for a table, the Database Engine enforces data uniqueness by automatically creating a unique index for the primary key columns. Because primary key constraints guarantee unique data, they are frequently defined on an identity column. This column, or columns, is called the primary key (PK) of the table and enforces the entity integrity of the table.
Primary Key ConstraintsĪ table typically has a column or combination of columns that contain values that uniquely identify each row in the table. Primary keys and foreign keys are two types of constraints that can be used to enforce data integrity in SQL Server tables. Applies to: SQL Server 2016 (13.x) and later Azure SQL Database Azure SQL Managed Instance
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