Export (0) Print
Expand All

Designing INSTEAD OF Triggers

The primary advantage of INSTEAD OF triggers is that they enable views that would not be updatable to support updates. A view based on multiple base tables must use an INSTEAD OF trigger to support inserts, updates, and deletes that reference data in more than one table. Another advantage of INSTEAD OF triggers is that they enable you to code logic that can reject parts of a batch while letting other parts of a batch to succeed.

An INSTEAD OF trigger can take actions such as:

  • Ignoring parts of a batch.

  • Not processing a part of a batch and logging the problem rows.

  • Taking an alternative action when an error condition is encountered.

    NoteNote

    INSTEAD OF DELETE and INSTEAD OF UPDATE triggers cannot be defined on a table that has a foreign key that is defined by using a DELETE or UPDATE cascading action.

Coding this logic as part of an INSTEAD OF trigger prevents all applications that access the data from having to reimplement the logic.

In the following sequence of Transact-SQL statements, an INSTEAD OF trigger updates two base tables from a view. Additionally, the following approaches to handling errors are shown:

  • Duplicate inserts to the Person table are ignored, and the information from the insert is logged in the PersonDuplicates table.

  • Inserts of duplicates to the EmployeeTable are turned into an UPDATE statement that retrieves the current information into the EmployeeTable without generating a duplicate key violation.

The Transact-SQL statements create two base tables, a view, a table to record errors, and the INSTEAD OF trigger on the view. The following tables separate personal and business data and are the base tables for the view.

CREATE TABLE Person
   (
    SSN         char(11) PRIMARY KEY,
    Name        nvarchar(100),
    Address     nvarchar(100),
    Birthdate   datetime
   )

CREATE TABLE EmployeeTable
   (
    EmployeeID       int PRIMARY KEY,
    SSN              char(11) UNIQUE,
    Department       nvarchar(10),
    Salary           money,
    CONSTRAINT FKEmpPer FOREIGN KEY (SSN)
    REFERENCES Person (SSN)
   )

The following view reports all relevant data from the two tables for a person.

CREATE VIEW Employee AS
SELECT P.SSN as SSN, Name, Address,
       Birthdate, EmployeeID, Department, Salary
FROM Person P, EmployeeTable E
WHERE P.SSN = E.SSN

You can record attempts to insert rows with duplicate social security numbers. The PersonDuplicates table logs the inserted values, the name of the user who tried the insert, and the time of the insert.

CREATE TABLE PersonDuplicates
   (
    SSN           char(11),
    Name          nvarchar(100),
    Address       nvarchar(100),
    Birthdate     datetime,
    InsertSNAME   nchar(100),
    WhenInserted  datetime
   )

The INSTEAD OF trigger inserts rows into multiple base tables from a single view. Attempts to insert rows with duplicate social security numbers are recorded in the PersonDuplicates table. Duplicate rows in the EmployeeTable are changed to update statements.

CREATE TRIGGER IO_Trig_INS_Employee ON Employee
INSTEAD OF INSERT
AS
BEGIN
SET NOCOUNT ON
-- Check for duplicate Person. If there is no duplicate, do an insert.
IF (NOT EXISTS (SELECT P.SSN
      FROM Person P, inserted I
      WHERE P.SSN = I.SSN))
   INSERT INTO Person
      SELECT SSN,Name,Address,Birthdate
      FROM inserted
ELSE
-- Log an attempt to insert duplicate Person row in PersonDuplicates table.
   INSERT INTO PersonDuplicates
      SELECT SSN,Name,Address,Birthdate,SUSER_SNAME(),GETDATE()
      FROM inserted
-- Check for duplicate Employee. If no there is duplicate, do an INSERT.
IF (NOT EXISTS (SELECT E.SSN
      FROM EmployeeTable E, inserted
      WHERE E.SSN = inserted.SSN))
   INSERT INTO EmployeeTable
      SELECT EmployeeID,SSN, Department, Salary
      FROM inserted
ELSE
--If there is a duplicate, change to UPDATE so that there will not
--be a duplicate key violation error.
   UPDATE EmployeeTable
      SET EmployeeID = I.EmployeeID,
          Department = I.Department,
          Salary = I.Salary
   FROM EmployeeTable E, inserted I
   WHERE E.SSN = I.SSN
END
Was this page helpful?
(1500 characters remaining)
Thank you for your feedback

Community Additions

ADD
Show:
© 2014 Microsoft