How DML triggers are used

Building and Optimizing Triggers in SQL Server

Florin Angelescu

Instructor

Why should we use DML triggers?

Initiating actions when manipulating data
Preventing data manipulation
Tracking data or database object changes
User auditing and database security

Deciding between AFTER and INSTEAD OF

CREATE TRIGGER MyFirstAfterTrigger
ON Table1
-- Triggered after 
-- the firing event (UPDATE)
AFTER UPDATE
AS
{trigger_actions_section};

CREATE TRIGGER MyFirstInsteadOfTrigger
ON Table2
-- Triggered instead of 
-- the firing event (UPDATE)
INSTEAD OF UPDATE
AS
{trigger_actions_section};

Deciding between AFTER and INSTEAD OF

AFTER trigger

Initial event fires the trigger

Initial event executes

The trigger actions execute

How an AFTER Trigger Works

INSTEAD OF trigger

Initial event fires the trigger

Initial event is not executed anymore

The trigger actions execute

How an INSTEAD OF Trigger Works

AFTER trigger usage example

Data is inserted into a sales table
Start a data cleansing procedure
Generate a table report with the procedure results
Notify the database administrator

CREATE TRIGGER SalesNewInfoTrigger
ON Sales
AFTER INSERT
AS

EXEC sp_cleansing @Table = 'Sales';

EXEC sp_generateSalesReport;

EXEC sp_sendnotification;

INSTEAD OF trigger usage example

| Brand | Model    | Power | Stock |
|-------|----------|-------|-------|
| Ecco  | Standard | 30W   | 30    |
| Miry  | Buma     | 45W   | 0     |
| Lume  | Ultra    | 50W   | 0     |

The power changes for some models

CREATE TRIGGER BulbsStockTrigger
ON Bulbs
INSTEAD OF INSERT
AS

INSTEAD OF trigger usage example

| Brand | Model    | Power | Stock |
|-------|----------|-------|-------|
| Ecco  | Standard | 30W   | 30    |
| Miry  | Buma     | 50W   | 100   |
| Lume  | Ultra    | 52W   | 100   |

The power changes for some models
Update only the products with no stock

CREATE TRIGGER BulbsStockTrigger
ON Bulbs
INSTEAD OF INSERT
AS

IF EXISTS (SELECT * FROM Bulbs AS b
INNER JOIN inserted AS i
           ON b.Brand = i.Brand
          AND b.Model = i.Model
WHERE b.Stock = 0)
BEGIN
    UPDATE b
    SET b.Power = i.Power,
        b.Stock = i.Stock
    FROM Bulbs AS b
    INNER JOIN inserted AS i
                ON b.Brand = i.Brand
                  AND b.Model = i.Model
    WHERE b.Stock = 0
END

INSTEAD OF trigger usage example

| Brand | Model    | Power | Stock |
|-------|----------|-------|-------|
| Ecco  | Standard | 30W   | 30    |
| Miry  | Buma     | 50W   | 100   |
| Lume  | Ultra    | 52W   | 100   |
| Ecco  | Standard | 35W   | 100   |

The power changes for some models
Update only the products with no stock
Add new rows for the products with stock

-- First part was truncated for spacing reasons
IF EXISTS (SELECT * FROM Bulbs AS b
INNER JOIN inserted AS i
           ON b.Brand = i.Brand
          AND b.Model = i.Model
WHERE b.Stock = 0)
BEGIN
    UPDATE b
    SET b.Power = i.Power,
        b.Stock = i.Stock
    FROM Bulbs AS b
    INNER JOIN inserted AS i
                ON b.Brand = i.Brand
                  AND b.Model = i.Model
    WHERE b.Stock = 0
END
ELSE
    INSERT INTO Bulbs
    SELECT * FROM inserted;

Practice questions

Building and Optimizing Triggers in SQL Server