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Implementing Declarative Data Integrity
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Declarative data integrity is implemented using constraints. There are five types of constraints: PRIMARY KEY, UNIQUE, FOREIGN KEY, CHECK, and DEFAULT.
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PRIMARY KEY AND UNIQUE CONSTRAINTS
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Both primary keys and unique constraints identify a column or combination of columns that uniquely identifies a row in a table. This is enforced through the creation of a unique index; that is, an index that does not allow duplicate values. Because of this, a primary key and unique constraints have the same size limitations as the key of an index, that is, it cannot contain more than 16 columns or 900 bytes of data. If nothing else is specifed, the index that is created for a primary key is a clustered index and the index for a unique constraint is a non-clustered index. However, you can change this behavior by specifying the type of index to create in the ALTER TABLE or CREATE TABLE statement, as follows:
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-- Primary key as a clustered index. ALTER TABLE MyTable ADD PRIMARY KEY (MyTableID);
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-- Primary key as a nonclustered index. ALTER TABLE MyTable ADD PRIMARY KEY NONCLUSTERED (MyTableID);
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Because primary keys and unique constraints are both constraints and indexes, you can find information about them in both the sys.key_constraints and sys.indexes catalog views.
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You can create both primary key and unique constraints on computed columns.
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Tables, Data Types, and Declarative Data Integrity
FOREIGN KEY CONSTRAINTS
Foreign key constraints identify a column or combination of columns whose values must exist in another column or combination of columns in the same table or another table in the same database. Foreign key constraints manage referential integrity between tables or within a single table. To implement a foreign key constraint, you must follow these rules:
The columns being referenced must have exactly the same data type (and collation, for string columns) as the local columns. The columns being referenced must have a unique index created on them. This is typically implemented using either a primary key or a unique constraint. Because the foreign key must reference a unique index, the foreign key columns have the same size limitations as that of the primary key and unique constraints.
You can also create foreign key constraints on computed columns. You can find information about which foreign key constraints exist in your database by querying the sys.foreign_keys and sys.foreign_key_columns catalog views. Foreign keys are usually queried frequently in user queries and in joins, as well as when SQL Server needs to verify referential integrity when deleting or updating primary key rows. This means that foreign keys usually greatly benefit from being indexed. Indexing is covered in greater detail in 6. When a foreign key constraint notices a referential integrity violation because of a DELETE or an UPDATE of a row that it references, the default reaction is to raise an error message and roll back the statement that violated the constraint. If this is not the result you want, you can change the default action for the foreign key to delete the referenced row, update the referenced column, or both. There are four actions to choose from:
n n n n
NO ACTION (the default) SET NULL SET DEFAULT CASCADE
An example implementation is shown here:
CREATE TABLE Test.Customers ( CustomerID INT PRIMARY KEY ); CREATE TABLE Test.Orders ( OrderID INT PRIMARY KEY ,CustomerID INT NULL REFERENCES Test.Customers ON DELETE SET NULL ON UPDATE CASCADE );
Lesson 2: Declarative Data Integrity
The default behavior of the foreign key is NO ACTION. If the foreign key finds a violation and NO ACTION is specified, SQL Server rolls back the statement that violated the constraint and raises an error message. SET NULL and SET DEFAULT cause all the referenced values to be set to either NULL (for SET NULL) or DEFAULT (for SET DEFAULT; that is, the default defined on the column) instead of raising an error and rolling back the statement. In the relationship between the Orders and Customers tables shown in the code sample, if a customer is deleted, the CustomerID column is set to NULL for all orders belonging to that customer and no error message is sent to the calling application. The CASCADE action causes SQL Server to delete referenced rows for a DELETE statement (ON DELETE) and update the referenced values (ON UPDATE) for an UPDATE statement. Using the same code sample, if the CustomerID column is changed for a row in the Customers table, all corresponding rows in the Orders table are updated with the same CustomerID to reflect the change. If ON DELETE CASCADE is specified for the foreign key constraint and a row in the Customers table is deleted, all referencing rows in the Orders table are deleted. This might sound reasonable, but it might not be possible to implement CASCADE for all foreign key constraints because cyclic references are not supported. For example, in the following script, an error is raised when you try to add the foreign key FKCustomersLastOrder because it introduces a cyclic reference. If a customer is deleted, all referencing orders must be deleted, and all customers referencing those orders through the LastOrderID column must also be deleted:
CREATE TABLE Test.Customers ( CustomerID INT PRIMARY KEY ,LastOrderID INT NULL );
CREATE TABLE Test.Orders ( OrderID INT PRIMARY KEY ,CustomerID INT NOT NULL REFERENCES Test.Customers ON DELETE CASCADE ON UPDATE NO ACTION );
ALTER TABLE Test.Customers ADD CONSTRAINT FKCustomersLastOrder FOREIGN KEY (LastOrderID) REFERENCES Test.Orders (OrderID) ON DELETE CASCADE ON UPDATE NO ACTION;
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