**Advanced SQL

This lesson delves into advanced database programming techniques, focusing on Stored Procedures, Functions, and Triggers. You will learn how to encapsulate complex logic directly within the database, enhancing data integrity, improving performance, and streamlining data manipulation operations.

Learning Objectives

  • Define and differentiate between Stored Procedures, User-Defined Functions (UDFs), and Triggers.
  • Write and execute Stored Procedures to perform complex data operations, including input/output parameters and error handling.
  • Create and utilize User-Defined Functions (UDFs) to encapsulate reusable logic for data transformations and calculations.
  • Implement Triggers to automatically enforce data integrity rules and react to data modification events.

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Lesson Content

Introduction to Stored Procedures

Stored Procedures are precompiled SQL statements stored within the database. They can accept input parameters, return output parameters, and contain control-flow statements like IF/ELSE and loops. They offer several advantages, including reduced network traffic (as logic is executed on the server), improved security (by controlling user access to the underlying tables), and code reusability.

Example (SQL Server syntax):

CREATE PROCEDURE GetCustomerOrders
    @CustomerID INT,
    @OrderCount INT OUTPUT
AS
BEGIN
    SELECT @OrderCount = COUNT(*) FROM Orders WHERE CustomerID = @CustomerID;
    SELECT * FROM Orders WHERE CustomerID = @CustomerID;
END;

-- To execute:
DECLARE @Count INT;
EXEC GetCustomerOrders 1, @Count OUTPUT; -- Assuming CustomerID 1 exists
SELECT @Count AS OrderCount;

User-Defined Functions (UDFs)

UDFs are reusable blocks of code that accept input parameters, perform calculations, and return a single value (scalar UDF) or a table (table-valued UDF). They help in modularizing code and promoting code reuse. They can significantly improve query readability by encapsulating complex calculations.

Example (Scalar UDF - SQL Server):

CREATE FUNCTION CalculateDiscount
    (@Price DECIMAL(10, 2), @DiscountRate DECIMAL(5, 2))
RETURNS DECIMAL(10, 2)
AS
BEGIN
    RETURN @Price * (1 - @DiscountRate);
END;

-- To use in a SELECT statement:
SELECT OrderID, ProductName, Price, dbo.CalculateDiscount(Price, 0.10) AS DiscountedPrice FROM OrderItems;

Example (Table-Valued UDF - SQL Server):

CREATE FUNCTION GetOrdersByDateRange
    (@StartDate DATE, @EndDate DATE)
RETURNS TABLE
AS
RETURN
    SELECT OrderID, OrderDate, CustomerID FROM Orders
    WHERE OrderDate BETWEEN @StartDate AND @EndDate;

-- To use in a SELECT statement:
SELECT * FROM dbo.GetOrdersByDateRange('2023-01-01', '2023-01-31');

Database Triggers

Triggers are special stored procedures that automatically execute in response to certain events on a particular table or view, such as INSERT, UPDATE, or DELETE. They are primarily used to enforce data integrity, implement auditing, or cascading operations. There are two types: AFTER triggers (executed after the event) and INSTEAD OF triggers (executed in place of the event).

Example (SQL Server, AFTER INSERT trigger):

CREATE TRIGGER trg_Orders_AfterInsert
ON Orders
AFTER INSERT
AS
BEGIN
    -- Log the new order in an audit table
    INSERT INTO OrderAudit (OrderID, CustomerID, OrderDate, Action, Timestamp)
    SELECT OrderID, CustomerID, OrderDate, 'INSERT', GETDATE() FROM inserted;
END;

-- Assume OrderAudit table exists with columns: OrderID, CustomerID, OrderDate, Action, Timestamp

Error Handling and Transaction Management

Robust database programming requires proper error handling and transaction management. Stored procedures and functions can include TRY...CATCH blocks to gracefully handle potential errors. Transactions (using BEGIN TRANSACTION, COMMIT TRANSACTION, and ROLLBACK TRANSACTION) ensure atomicity, consistency, isolation, and durability (ACID properties) when performing multiple related operations.

Example (SQL Server):

CREATE PROCEDURE UpdateProductPrice
    @ProductID INT, @NewPrice DECIMAL(10,2)
AS
BEGIN
    BEGIN TRY
        BEGIN TRANSACTION;

        UPDATE Products SET Price = @NewPrice WHERE ProductID = @ProductID;

        -- Simulate a potential error
        -- IF @NewPrice < 0 THROW 50000, 'Price cannot be negative', 1;

        COMMIT TRANSACTION;
        SELECT 'Price updated successfully.';
    END TRY
    BEGIN CATCH
        IF @@TRANCOUNT > 0
            ROLLBACK TRANSACTION;
        -- Log the error and return an error message.
        SELECT ERROR_MESSAGE() AS ErrorMessage;
    END CATCH
END;

Best Practices

When working with Stored Procedures, Functions, and Triggers consider these guidelines:
* Keep it Modular: Design procedures and functions to do one specific task to ensure reusability.
* Parameterize all input: Prevent SQL injection vulnerabilities.
* Use Descriptive names: Create clear and concise procedure and function names.
* Thoroughly Test: Validate procedures, function and triggers with sufficient testing to ensure data integrity and query optimization.
* Consider Performance: Avoid complex logic within triggers which can impact application performance.
* Document: Add comments within the code, that explain function and procedure usage.

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