By Alex Kuznetsov
Resilient T-SQL code is code that's designed to final, and to be competently reused by means of others. The objective of protective database programming, the aim of this booklet, is that will help you to supply resilient T-SQL code that robustly and gracefully handles situations of unintentional use, and is resilient to universal alterations to the database surroundings. Too frequently as builders, we cease paintings once our code passes a number of simple assessments to substantiate that it produces the 'right end result' in a given use case. we don't cease to think about what different attainable ways that the code could be utilized in the longer term, or how our code will reply to universal adjustments to the database surroundings, comparable to a transformation within the database language environment, or a metamorphosis to the nullability of a desk column, and so forth. within the momentary, this method is appealing; we get issues performed quicker. even though, if our code is designed for use for greater than quite a few months, then it's very most likely that such alterations can and may ensue, and the inevitable result's damaged code or, even worse, code that silently starts off to act in a different way, or produce diversified effects. whilst this occurs, the integrity of our information is threatened, as is the validity of the reviews on which severe company judgements are frequently dependent. At this aspect, months or years later, and lengthy after the unique developer has left, starts off the painstaking means of troubleshooting and solving the matter. wouldn't it be more uncomplicated to avoid all this troubleshooting from taking place? wouldn't it be greater to spend a bit extra effort and time in the course of unique improvement, to avoid wasting significantly extra time on troubleshooting, computer virus solving, retesting, and redeploying? this is often what protective programming is all approximately: we research what can get it wrong with our code, and we proactively follow this information in the course of improvement. This booklet is stuffed with useful, reasonable examples of the types of difficulties that beset database courses, together with: - adjustments in database gadgets, equivalent to tables, constraints, columns, and kept techniques. - alterations to concurrency and isolation degrees. - enhancements to new models of SQL Server. - alterations in requisites. - Code reuse. - difficulties inflicting lack of information integrity. - issues of mistakes dealing with in T-SQL. In each one case, it demonstrates ways to help you comprehend and implement (or put off) the assumptions on which your answer is predicated, and to enhance its robustness. finally, the ebook teaches you ways to imagine and improve defensively, and the way to proactively establish and dispose of strength vulnerabilities in T-SQL code.
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Additional info for Defensive Database Programming with SQL Server
WeightInPounds ; Listing 1-10: Logging updates to the Objects table. Please note that my approach to all examples in this book is to keep them as simple as they can be, while still providing a realistic demonstration of the point, which here is the effect of SET ROWCOUNT. So, in this case, I have omitted: • a "real" key on the ObjectsChangeLog table, enforced by a UNIQUE constraint (ObjectID, ChangedColumnName, ChangedAt), in addition to the surrogate key on ObjectsChangeLogID • the equivalent insert and delete triggers to log INSERT and DELETE modifications, as well as UPDATEs.
Suppose that we have not detected or resolved this duplicate, and that we are updating our Codes table from the staging table. Codes ; Code ---------AR IN Description ---------------------------------------description for Argentina new description for Indiana (2 row(s) affected) Listing 1-23: An ambiguous UPDATE…FROM, when loading data from a staging table (CodesStaging) into a target table (Codes). Although two descriptions were provided for the AR code, the UPDATE…FROM command did not raise an error; it just silently updated the corresponding row in the Codes table with one of the two provided values.
SET, SELECT, and the dreaded infinite loop We cannot assume that SET and SELECT always change the values of variables. If we rely on that incorrect assumption, our code may not work as expected, so we need to eliminate it. Listing 2-9 demonstrates a case where SELECT leaves the value of a variable unchanged, if the result set is empty. SET NOCOUNT ON ; DECLARE @i INT ; SELECT @i = -1 ; SELECT SELECT WHERE SELECT @i AS [@i before the assignment] ; @i = 1 1 = 2 ; @i AS [@i after the assignment] ; 64 Chapter 2: Code Vulnerabilities due to SQL Server Misconceptions @i before the assignment ------------------------1 @i after the assignment -----------------------1 Listing 2-9: SELECT may leave a variable unchanged if the result set is empty.