Added indexes, loops, transactions, trigger

Author: nidea1

Indexes/Examples.sql

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+-- CREATE INDEX
+
+-- Syntax
+
+CREATE INDEX index_name
+ON table_name (column1, column2, ...);
+
+-- Example
+
+CREATE INDEX idx_Orders_CustomerID
+ON Orders (CustomerID);
+
+-- Now we can do faster queries by CustomerID
+
+SELECT OrderID, OrderDate
+FROM Orders
+WHERE CustomerID = 'ALFKI';
+
+
+-- DROP INDEX
+
+-- Syntax
+
+DROP INDEX table_name.index_name;
+
+-- Example
+
+DROP INDEX Orders.idx_Orders_CustomerID;

Indexes/README.md

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+# Indexes
+
+Indexes are used for faster reach to data and the users can't see the indexes.
+
+![img1](https://i.ytimg.com/vi/YuRO9-rOgv4/maxresdefault.jpg)

Loops/Examples.sql

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+-- WHILE LOOP
+
+-- Syntax
+
+WHILE <condition>
+    SQL Statement | statement_block | BREAK | CONTINUE
+
+
+-- Example 1
+
+DECLARE @i INT = 1;
+
+WHILE @i <= 5
+BEGIN
+    PRINT(@i);
+    SET @i = @i + 1;
+END;
+
+-- Example 2
+
+CREATE PROCEDURE CountOrders
+    @EmployeeID INT,
+    @OrderCount INT OUTPUT
+as
+BEGIN
+    SET @OrderCount = 0;
+
+    DECLARE @CurrentOrderID INT, @CurrentEmployeeID INT; -- New local variables
+
+    DECLARE order_cursor CURSOR FOR
+    SELECT OrderID, EmployeeID
+    FROM Orders; -- Creating order_cursor for choosing per row in orders
+
+    OPEN order_cursor; 
+
+    FETCH NEXT FROM order_cursor
+    INTO @CurrentOrderID, @CurrentEmployeeID; -- Opening order_cursor for taking first row and setting orderID and employeeID to our variables from current row
+
+    WHILE @@FETCH_STATUS = 0 -- If fetch is success
+    BEGIN
+        IF @CurrentEmployeeID = @EmployeeID
+        BEGIN
+            SET @OrderCount = @OrderCount + 1;
+        END;
+
+        FETCH NEXT FROM order_cursor 
+        INTO @CurrentOrderID, @CurrentEmployeeID;
+    END;
+
+    CLOSE order_cursor;
+    DEALLOCATE order_cursor;
+END;
+
+
+-- Test
+
+DECLARE @OrderCount INT;
+
+EXEC
+CountOrders 4,
+@OrderCount OUTPUT;
+
+SELECT @OrderCount as 'Order Count';
+
+
+-- We can too do this as
+SELECT COUNT(*) as 'Order Count'
+FROM Orders
+WHERE EmployeeID = 4;        

Loops/README.md

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+# Loops
+
+Loops are the technique where a set of SQL statements are executed repeatedly until a condition is met.
+
+![img1](https://s33046.pcdn.co/wp-content/uploads/2019/10/flow-chart-of-the-sql-while-loop-1.png)

Transactions/Examples.sql

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+-- Transaction Expressions
+
+-- BEGIN TRANSACTION -> starting a new transaction.
+-- COMMIT -> makes all changes in the transaction permanent.
+-- ROLLBACK -> rolls back all changes in the transaction since start. If a error occured or transaction is not completed, used ROLLBACK expression.
+
+-- Example
+
+BEGIN TRANSACTION;
+
+UPDATE Products
+SET UnitsInStock = UnitsInStock - 5
+WHERE ProductID = 1;
+
+IF @@ERROR = 0
+    COMMIT;
+ELSE
+    ROLLBACK;
+
+
+-- Example 2
+
+BEGIN TRANSACTION;
+
+BEGIN TRY
+    INSERT INTO Orders (CustomerID, EmployeeID, OrderDate)
+    VALUES ('VINET', 5, GETDATE())
+
+    DECLARE @NewOrderID INT;
+    SELECT @NewOrderID = SCOPE_IDENTITY()
+
+    INSERT INTO [Order Details] (OrderID, ProductID, UnitPrice, Quantity, Discount)
+    VALUES (@NewOrderID, 3, 15, 3, 0)
+
+    COMMIT;
+END TRY
+BEGIN CATCH
+    ROLLBACK;
+
+    PRINT ERROR_MESSAGE();
+END CATCH;

Transactions/README.md

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+# Transactions
+
+In SQL, a process (transaction) is that grouping one or more sql statements as one statemenet. Transactions ensure that statements are performed correctly and error management.
+
+## 4 Features of Transactions (ACID)
+- **Atomicity:** A transaction is completely successes or never successes. If there is a error in any transaction, transaction is not completed and all changes in the database are rolled back.
+- **Consistency:** Each transaction is moves the database from one consistent state to another. So a transaction complies with rules of the database before and after.
+- **Isolation:** Concurrent processes are isolated from each other. The results of a transaction are not visible to other transactions until the transaction is complete.
+- **Durability:** After a transaction is completed, the results of the transaction are permanent and are not lost even in the event of a system failure.
+
+![img1](https://f4n3x6c5.stackpathcdn.com/UploadFile/f0b2ed/transaction-management-in-sql/Images/Transaction%20Control.jpg)

Triggers/Examples.sql

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+-- DML Triggers
+-- CREATE TRIGGER
+--Syntax
+
+CREATE TRIGGER trigger_name
+ON table_name
+{FOR | AFTER | INSTEAD OF} {[INSERT] [,] [UPDATE] [,] [DELETE]}
+AS
+{sql_statements}
+
+-- Example 1
+
+CREATE TRIGGER NewOrderTrigger
+ON Orders
+AFTER INSERT
+as
+BEGIN
+	PRINT 'A new order has been placed.';	
+END;
+
+-- Users when add a data to the orders will see print message
+
+
+-- Example 2
+
+-- Procedure
+CREATE PROCEDURE GetProductByID
+(
+    @ProductID INT
+)
+as
+BEGIN
+    SELECT ProductID,
+        ProductName,
+        UnitsInStock
+    FROM Products
+    WHERE ProductID = @ProductID
+END;
+
+CREATE PROCEDURE DecraseStock
+(
+    @ProductID INT,
+    @Quantity INT
+)
+as
+BEGIN
+    UPDATE Products
+    SET UnitsInStock = UnitsInStock - @Quantity
+    WHERE ProductID = @ProductID
+END;
+
+-- Trigger
+CREATE TRIGGER UpdateStock
+ON [Order Details]
+AFTER INSERT
+as
+BEGIN
+    DECLARE @Product INT
+    DECLARE @Qty INT
+
+    SELECT @Product = ProductID,
+        @Qty = Quantity
+    FROM INSERTED;
+
+    EXEC DecraseStock
+    @ProductID = @Product,
+    @Quantity = @Qty
+
+    EXEC GetProductByID
+    @ProductID = @Product
+END;
+
+-- Test
+INSERT INTO [Order Details]
+(
+	OrderID,
+	ProductID,
+	Quantity,
+	UnitPrice
+)
+VALUES
+(
+	10501,
+	2,
+	1,
+	5
+)
+
+
+-- ALTER TRIGGER
+-- Syntax
+
+ALTER TRIGGER trigger_name
+ON table_name
+{FOR | AFTER | INSTEAD OF} {[INSERT] [,] [UPDATE] [,] [DELETE]}
+AS
+{sql_statements}
+
+-- Example
+ALTER TRIGGER UpdateStock
+ON [Order Details]
+AFTER INSERT
+as
+BEGIN
+    DECLARE @Product INT
+    DECLARE @Qty INT
+
+    SELECT @Product = ProductID,
+        @Qty = Quantity
+    FROM INSERTED;
+
+    EXEC DecraseStock
+    @ProductID = @Product,
+    @Quantity = @Qty
+END;
+
+
+-- Now lets update the UpdateStock to more complex
+
+-- 1 more procedure
+CREATE PROCEDURE IncreaseStock
+(
+    @ProductID INT,
+    @Quantity INT
+)
+as
+BEGIN
+    UPDATE Products
+    SET UnitsInStock = UnitsInStock + @Quantity
+    WHERE ProductID = @ProductID
+END;
+
+-- Now updating our trigger to insert and delete events cause actived the trigger
+
+ALTER TRIGGER UpdateStock
+ON [Order Details]
+FOR INSERT, DELETE
+as
+BEGIN
+    DECLARE @insertedProduct INT, @insertedQty INT, @deletedProduct INT, @deletedQty INT
+
+    SELECT @insertedProduct = ProductID,
+        @insertedQty = Quantity
+    FROM INSERTED;
+
+    SELECT @deletedProduct = ProductID,
+        @deletedQty = Quantity
+    FROM DELETED;
+
+    IF @insertedProduct IS NOT NULL
+    BEGIN
+        EXEC DecraseStock
+        @ProductID = @insertedProduct,
+        @Quantity = @insertedQty
+
+        EXEC GetProductByID
+        @ProductID = @insertedProduct
+    END;
+
+    IF @deletedProduct IS NOT NULL
+    BEGIN
+        EXEC IncreaseStock
+        @ProductID = @deletedProduct,
+        @Quantity = @deletedQty
+
+        EXEC GetProductByID
+        @ProductID = @deletedProduct
+    END;
+END;
+
+-- Test
+-- Insert
+INSERT INTO [Order Details]
+(
+	OrderID,
+	ProductID,
+	Quantity,
+	UnitPrice
+)
+VALUES
+(
+	10503,
+	2,
+	1,
+	5
+)
+
+-- Delete
+DELETE FROM [Order Details]
+WHERE OrderID = 10503 AND ProductID = 2;
+
+
+-- DROP TRIGGER
+DROP TRIGGER UpdateStock
+
+
+-- DDL Trigger Example
+
+CREATE TRIGGER PreventTableCreation
+ON example
+FOR CREATE_TABLE
+AS
+BEGIN
+    PRINT 'You cannot create a table in this database.';
+    ROLLBACK;
+END;
+
+
+-- LOGON Trigger Example
+
+CREATE TRIGGER RestrictLogon
+ON ALL SERVER WITH EXECUTE AS 'sa'
+FOR LOGON
+AS
+BEGIN
+    IF ORIGINAL_LOGIN() != 'sa' AND ORIGINAL_LOGIN() != 'dbo'
+    BEGIN
+        PRINT 'You cannot access this database.';
+        ROLLBACK;
+    END
+END;
+

Triggers/README.md

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+# Triggers
+
+An SQL Trigger is database object that is associated with a table. A trigger is automatically invokes whenever a special event in the database occurs.
+
+## Different Types of Triggers
+- **DDL Trigger:** The Data Definition Language (DDL) command events such as Create Table, Create View, Drop Table, Drop View and Alter Table cause the DDL Triggers to be activated.
+- **DML Trigger:** The Data Manipulation Language (DML) command events that begin with Insert, Update and Delete set off the DML Triggers.
+- **Logon Triggers:** Logon triggers are fires in response to a LOGON event. When a user session is created with a SQL Server instance after the authentication process of logging is finished but before establishing a user session, the LOGON event takes place.
+
+![img1](https://d1jnx9ba8s6j9r.cloudfront.net/blog/wp-content/uploads/2019/10/Triggers-in-SQL.png)