Defensive Database Programming - Red Gate Software

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Chapter 7: Advanced Use of ConstraintsChangeQuantity ,CurrentQuantityFROM dbo.InventoryLogWHERE ItemID = 2 ;-- restoring test dataROLLBACK ;(1 row(s) affected)TotalQuantity-------------14(1 row(s) affected)ChangeDateChangeQuantity CurrentQuantity----------------------- -------------- ---------------2010-01-01 5 52010-01-02 -1 42010-01-04 10 10(3 row(s) affected)Listing 7-34: We have managed to start a new history trail for item #2.In order to solve this problem, we need to find a way to prevent two rows with the sameItemID having a NULL value for PreviousChangeDate. Listing 7-35 creates a UNIQUEconstraint to solve this problem.ALTER TABLE dbo.InventoryLogADD CONSTRAINT UNQ_InventoryLog_OneHistoryTrailPerItemUNIQUE(ItemID,PreviousChangeDate ) ;Listing 7-35: Only one history trail per item is allowed.When we rerun Listing 7-34, the INSERT fails, as it should, and the problem hasbeen fixed.244
Chapter 7: Advanced Use of ConstraintsSQL Server UNIQUE constraints and the ANSI StandardIn SQL Server, a UNIQUE constraint disallows duplicate NULLs. However, accordingto the ANSI standard it should allow them. Therefore, the UNIQUE constraint inListing 7-35 will work on SQL Server but will not port to a RDBMS that implementsANSI-compliant UNIQUE constraints.Updating existing rowsIn all previous examples, we have used INSERT commands to add new inventoryrows. Now we need to consider how our system will behave if we allow UPDATEs ofexisting rows in the inventory trail for a given item. First of all, let's add another historyof changes for item #2.INSERT INTO dbo.InventoryLog( ItemID ,ChangeDate ,ChangeQuantity ,CurrentQuantity ,PreviousChangeDate ,PreviousQuantity)VALUES ( 2 ,'20100105' ,-3 ,1 ,'20100102' ,4);Listing 7-36: More history for item #2.Suppose it is discovered that the initial inventory for item #2, entered on January 1st,2010, should have reflected a quantity of 3, rather than 5, so we need to decrease theamount added for that item.245
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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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Defensive Database Programming - Red Gate Software

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