INSTEAD OF Trigger

For people who are familiar with database triggers, you must be aware with INSERT, UPDATE & DELETE Triggers which get fired whenever the said action takes place. MS SQL provides a new type of trigger to the set called INSTEAD OF triggers. INSTEAD OF trigger help us override the action itself by our very own custom made SQL operations. For example, whenever a delete operation is performed on a table, I do not want the record to be deleted but instead just set a flag-‘Deleted’ as true in the record. This can be achieved using INSTEAD OF DELETE Trigger.

Below is the extract from msdn (http://msdn.microsoft.com/en-us/library/aa175158%28SQL.80%29.aspx) regarding workings of INSTEAD OF Triggers:

Designing INSTEAD OF Triggers

The primary advantage of INSTEAD OF triggers is that they allow views that would not be updatable support updates. A view comprising multiple base tables must use an INSTEAD OF trigger to support inserts, updates and deletes that reference data in the tables. Another advantage of INSTEAD OF triggers is that they allow you to code logic that can reject parts of a batch while allowing other parts of a batch 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 if an error condition is encountered.

Note INSTEAD OF DELETE and INSTEAD OF UPDATE triggers cannot be defined on a table that has a foreign key defined with a DELETE or UPDATE action.

Coding this logic as part of an INSTEAD OF trigger prevents all applications accessing 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. In addition, two 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. These 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)
   )

This 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 attempted 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 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 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 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 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

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