Defensive Database Programming - Red Gate Software

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Chapter 7: Advanced Use of ConstraintsSELECT ItemID ,ChangeDate ,ChangeQuantityFROM dbo.InventoryLog ;ROLLBACK ;Listing 7-21: Examples of possible and impossible changes to the inventory.For the tiny number of rows in Listing 7-21, it is not a problem to perform a SUM on theChangeQuantity column every time we change the inventory. However, for a realinventory system, with hundreds of thousands of transactions per item, this becomes ahuge performance drain.To eliminate this expensive querying of historical inventory changes, it is very temptingto store the new stock quantity of a given item that results from an inventory change,along with the change amount. Let's add one more column to InventoryLog table tostore the current amount, as shown in Listing 7-22. At the same time, we add a CHECKconstraint to make sure that CurrentQuantity is never negative.ALTER TABLE dbo.InventoryLogADD CurrentQuantity INT NOT NULL ;GOALTER TABLE dbo.InventoryLogADDCONSTRAINT CHK_InventoryLog_NonnegativeCurrentQuantityCHECK( CurrentQuantity >= 0) ;Listing 7-22: Adding the CurrentQuantity column to the InventoryLog table.Now, instead of querying the history of changes, we need only look up the Current-Quantity value for the most recent row, and add it to the proposed ChangeQuantity,as shown in Listing 7-23.232
Chapter 7: Advanced Use of ConstraintsSET NOCOUNT ON ;BEGIN TRANSACTION ;DECLARE @ItemID INT ,@ChangeQuantity INT ,@ChangeDate DATETIME ;SET @ItemID = 1 ;SET @ChangeQuantity = 5 ;SET @ChangeDate = '20100101' ;-- this is a valid change:-- we can always add to InventoryLogINSERT INTO dbo.InventoryLog( ItemID ,ChangeDate ,ChangeQuantity ,CurrentQuantity)SELECT @ItemID ,@ChangeDate ,@ChangeQuantity ,COALESCE(( SELECT TOP (1)CurrentQuantityFROM dbo.InventoryLogWHERE ItemID = @ItemIDAND ChangeDate < @ChangeDateORDER BY ChangeDate DESC), 0) + @ChangeQuantity-- we do not have log entries for later daysWHERE NOT EXISTS ( SELECT *FROM dbo.InventoryLogWHERE ItemID = @ItemIDAND ChangeDate > @ChangeDate ) ;SET @ItemID = 1 ;SET @ChangeQuantity = -3 ;SET @ChangeDate = '20100105' ;233
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Chapter 2: Code Vulnerabilities due
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Enforcing Data Integrity in the App
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About the AuthorAlex Kuznetsov has
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IntroductionResilient T-SQL code is
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IntroductionCh. 02: Code Vulnerabil
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IntroductionSpecific examples demon
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IntroductionWhat this book does not
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Chapter 1: Basic Defensive Database
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Chapter 3: Surviving Changes to Dat
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Chapter 5: Reusing T-SQL CodeBEGINS
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Defensive Database Programming - Red Gate Software

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