![]() CREATE TABLE MyTableĬreate a table-level check constraint. ExamplesĬreate a composite non-clustered primary key. For example, a cascading constraint may have CASCADE for UPDATE, but NO ACTION for DELETE. You can customize these actions independently of others in the same constraint. All columns that are part of the foreign key are set to their DEFAULT value when the parent is deleted or updated. All columns that are part of the foreign key are set to NULL when the parent is deleted or updated. Values in a child table are updated with values from the parent table when they are updated or deleted along with the parent. When the constraint is violated due to a DML operation, an error is raised and the operation is rolled back.ĬASCADE. ![]() You can use CRI to enforce constraints and define action paths for DML statements that violate the constraints. For example, you can’t delete an order having associated order items.įoreign keys support Cascading Referential Integrity (CRI). The constraint limits INSERT and UPDATE to the child table and UPDATE and DELETE to the parent table. The purpose of a constraint is to guarantee that no orphan rows, which don’t have corresponding matching values in the parent table exist in the referencing table. A best practice is to create appropriate indexes to support joins and constraint enforcement.įoreign key constraints impose DML limitations for the referencing child and parent tables. The set of values that you can use for the referencing table is the set of values that exist in the referenced table.Īlthough the columns referenced in the parent table are indexed because they have either a primary key or unique constraint, no indexes are automatically created for the referencing columns in the child table. FOREIGN KEY ()įoreign keys reference columns in other tables, which must be either primary keys or have unique constraints. Similar to check constraints, foreign keys limit the values stored in a column or set of columns. Foreign Key Constraintsįoreign key constraints enforce domain referential integrity. The default index type for primary keys is a clustered index. All columns that comprise a primary key must also have a NOT NULL constraint. Primary keys might consist of one or more columns. Primary Key ConstraintsĪ primary key is a candidate key serving as the unique identifier of a table row. 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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