![]() The default index type for unique constraints is non-clustered. Otherwise, every INSERT and UPDATE would require a full table scan to verify that the table doesn’t include duplicates. To improve the efficiency, SQL Server creates a unique index to support unique constraints. You can use a NOT NULL constraint in addition to a unique constraint to address this limitation. However, in SQL Server, you can use a NULL value only for a single row. A column may be a part of more than one constraint.Īccording to the ANSI SQL standard, you can have multiple rows with NULL values for unique constraints. Composite constraints are composed of multiple columns. Simple constraints are composed of a single column. Unique constraints guarantee that no rows with duplicate column values exist in a table.Ī unique constraint can be simple or composite. ![]() A candidate key is an attribute or a set of attributes or columns that uniquely identify each row in the relation (table data). You can use unique constraints for all candidate keys. In SQL Server, you can use user-defined functions in constraints to access other rows, tables, or databases. In ANSI SQL, check constraints can’t access other rows as part of the expression. In this case, it works as a table-level check constraint. Also, you can apply a single check constraint to multiple columns. You can assign multiple check constraints to a column. For check constraints, an expression that evaluates to UNKNOWN is functionally equivalent to TRUE because the value is permitted by the constraint. For example, in a WHERE clause, a logical expression that evaluates to UNKNOWN is functionally equivalent to FALSE and the row is filtered out. Check constraint expressions behave differently than predicates in other query clauses.
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