How can I do a BEFORE UPDATED trigger with sql server?

sql

best-practices

data-integrity

trigger-optimization

byNikita Barsukov·Nov 15, 2024

To emulate a BEFORE UPDATE trigger in SQL Server, use the INSTEAD OF UPDATE trigger. It sets off before the proposed update and can decide whether to pass or block it:

CREATE TRIGGER trgBeforeUpdateExample
ON Employees
INSTEAD OF UPDATE
AS
BEGIN
    -- Why not Salary<0? Because we are not energy-drink-loving programmers who work for free 
    IF (SELECT COUNT(*) FROM inserted WHERE Salary < 0) > 0
    BEGIN
        RAISERROR('Negative salary not allowed. Maybe in a parallel universe!', 16, 1);
        RETURN;
    END

    -- Proceed with actual update if it passed our "Before Update" test
    UPDATE e
    SET e.Name = i.Name, e.Salary = i.Salary
    FROM Employees e
    JOIN inserted i ON e.EmployeeID = i.EmployeeID;
END

This trigger blocks updates that assign a negative salary, exemplifying a typical BEFORE UPDATE action.

Implementing INSTEAD OF UPDATE Triggers

The INSTEAD OF UPDATE trigger serves as SQL Server's closest equivalent to a BEFORE UPDATE trigger. Correctly implementing it is essential to maintain data integrity and handle conditional logic.

Understanding trigger nuances in SQL Server

When crafting SQL Server triggers, consider these important caveats:

Cascading triggers: They might remind you of falling dominos, but watch out! SQL Server safeguards against infinite recursion to prevent infinite loops of triggers.
Performance: While nifty for data manipulation, triggers can tax your system's performance. Always gauge their impact on transaction time.
Error handling: Get your safety nets ready! Employ comprehensive error checking and plan for rollback if your pre-update validation goes haywire.

Tricky AFTER triggers

Let's not ignore AFTER UPDATE triggers! Take them as the responsible sibling who keeps a record of all the naughtiness:

CREATE TRIGGER trgAfterUpdate
ON Employees
AFTER UPDATE
AS
BEGIN
    -- Here we take a "memory" snapshot of all changes for future "I told you so" moments
    INSERT INTO AuditTable(UserID, ChangeDate, OldValue, NewValue)
    SELECT i.EmployeeID, GETDATE(), d.Salary, i.Salary
    FROM inserted i
    JOIN deleted d ON i.EmployeeID = d.EmployeeID
    WHERE i.Salary <> d.Salary;
END

This trigger logs every change after an update operation, providing an audit trail.

Optimizing for SQL Server Express

If you're working with SQL Server Express, adapt your trigger strategy to fit its resource constraints and size limitations.

Advanced Scenarios and Best Practices

For a thorough understanding of using triggers in SQL Server, explore these advanced practices and best practices:

Type 2 Dimension Table Updates

Triggers can assist with managing type 2 dimension updates in data warehousing. Imagine being Harry Potter and control how entries appear and age in the data!

Track historical entries: Before updating a record, wave your magic wand and move old values to place them in a history table. Voila! You've just performed a data time travel.
Uphold SCD type 2 compliance: Flag old records as retired, and insert the updated records as new fresh entries.

Complex Validation Scenarios

Think of triggers as the Sherlock Holmes of SQL Server - solving intricate validation cases:

Cross-Reference Check: Analyze related records in another table before approving an update. The game is on!
Dependency Validation: Be the smart detective and ensure the children (records) have valid alibis before updating the parents.

Optimization and Hygiene Practices

To ensure triggers remain efficient and tidy:

Less is More: Keep the logic lean to avoid performance penalties.
Beware of Extraneous Data Access: Only manipulate affected rows, accessed via the inserted and deleted tables. Don't interrogate every bystander in the crowd!

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