DML trigger statements use two special tables: the deleted table and the inserted tables. SQL Server 2005 automatically creates and manages these tables. You can use these temporary, memory-resident tables to test the effects of certain data modifications and to set conditions for DML trigger actions. You cannot directly modify the data in the tables or perform data definition language (DDL) operations on the tables, such as CREATE INDEX.
In DML triggers, the inserted and deleted tables are primarily used to perform the following:
1. Extend referential integrity between tables.
2. Insert or update data in base tables underlying a view.
3. Test for errors and take action based on the error.
4. Find the difference between the state of a table
before and after a data modification and
take actions based on that difference.
The deleted table stores copies of the affected rows during DELETE and UPDATE statements. During the execution of a DELETE or UPDATE statement, rows are deleted from the trigger table and transferred to the deleted table. The deleted table and the trigger table ordinarily have no rows in common.
The inserted table stores copies of the affected rows during INSERT and UPDATE statements. During an insert or update transaction, new rows are added to both the inserted table and the trigger table. The rows in the inserted table are copies of the new rows in the trigger table.
An update transaction is similar to a delete operation followed by an insert operation; the old rows are copied to the deleted table first, and then the new rows are copied to the trigger table and to the inserted table.
When you set trigger conditions, use the inserted and deleted tables appropriately for the action that fired the trigger. Although referencing the deleted table when testing an INSERT or the inserted table when testing a DELETE does not cause any errors, these trigger test tables do not contain any rows in these cases.
SQL Server 2005 does not allow for text, ntext, or image column references in the inserted and deleted tables for AFTER triggers. However, these data types are included for backward compatibility purposes only. The preferred storage for large data is to use the varchar(max), nvarchar(max), and varbinary(max) data types. Both AFTER and INSTEAD OF triggers support varchar(max), nvarchar(max), and varbinary(max) data in the inserted and deleted tables. For more information, see CREATE TRIGGER (Transact-SQL).
An Example of Using the inserted Table in a Trigger to Enforce Business Rules
Because CHECK constraints can reference only the columns on which the column-level or table-level constraint is defined, any cross-table constraints (in this case, business rules) must be defined as triggers.
The following example creates a DML trigger. This trigger checks to make sure the credit rating for the vendor is good when an attempt is made to insert a new purchase order into the PurchaseOrderHeader table. To obtain the credit rating of the vendor corresponding to the purchase order that was just inserted, the Vendor table must be referenced and joined with the inserted table. If the credit rating is too low, a message is displayed and the insertion does not execute.
IF OBJECT_ID ('Purchasing.LowCredit','TR') IS NOT NULL
DROP TRIGGER Purchasing.LowCredit
CREATE TRIGGER LowCredit ON Purchasing.PurchaseOrderHeader
DECLARE @creditrating tinyint,
SELECT @creditrating = v.CreditRating, @vendorid = p.VendorID
FROM Purchasing.PurchaseOrderHeader p INNER JOIN inserted i ON p.PurchaseOrderID =
i.PurchaseOrderID JOIN Purchasing.Vendor v on v.VendorID = i.VendorID
IF @creditrating = 5
RAISERROR ('This vendor''s credit rating is too low to accept new
purchase orders.', 16, 1)
Using the inserted and deleted Tables in INSTEAD OF Triggers
The inserted and deleted tables passed to INSTEAD OF triggers defined on tables follow the same rules as the inserted and deleted tables passed to AFTER triggers. The format of the inserted and deleted tables is the same as the format of the table on which the INSTEAD OF trigger is defined. Each column in the inserted and deleted tables maps directly to a column in the base table.
The following rules regarding when an INSERT or UPDATE statement referencing a table with an INSTEAD OF trigger must supply values for columns are the same as if the table did not have an INSTEAD OF trigger:
1. Values cannot be specified for computed columns or
columns with a timestamp data type.
2. Values cannot be specified for columns with an IDENTITY
property, unless IDENTITY_INSERT is ON for that table.
When IDENTITY_INSERT is ON, INSERT statements must
supply a value.
3. INSERT statements must supply values for all NOT NULL
columns that do not have DEFAULT constraints.
4. For any columns except computed, identity, or
timestamp columns, values are optional for any
column that allows nulls, or any NOT NULL column that
has a DEFAULT definition.
When an INSERT, UPDATE, or DELETE statement references a view that has an INSTEAD OF trigger, the SQL Server 2005 Database Engine calls the trigger instead of taking any direct action against any table. The trigger must use the information presented in the inserted and deleted tables to build any statements required to implement the requested action in the base tables, even when the format of the information in the inserted and deleted tables built
The format of the inserted and deleted tables passed to an INSTEAD OF trigger defined on a view matches the select list of the SELECT statement defined for the view. For example:
CREATE VIEW EmployeeNames (EmployeeID, LName, FName)
SELECT e.EmployeeID, c.LastName, c.FirstName
FROM AdventureWorks.HumanResources.Employee e
JOIN AdventureWorks.Person.Contact c
ON e.ContactID = c.ContactID
The result set for this view has three columns: an int column and two nvarchar columns. The inserted and deleted tables passed to an INSTEAD OF trigger defined on the view also have an int column named EmployeeID, an nvarchar column named LName, and an nvarchar column named FName.
The select list of a view can also contain expressions that do not directly map to a single base-table column. Some view expressions, such as a constant or function invocation, may not reference any columns and can be ignored. Complex expressions can reference multiple columns, yet the inserted and deleted tables have only one value for each inserted row. The same issues apply to simple expressions in a view if they reference a computed column that has a complex expression. An INSTEAD OF trigger on the view must handle these types of expressions. For more information, see Expressions and Computed Columns in INSTEAD OF Triggers.
This article is copied from MSDN for my personal learning.
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