Reducing Conflicts with Custom Update Commands in .NET framework

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Reducing Conflicts with Custom Update Commands
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Depending on your application s requirements, you can often reduce update conflicts by means of custom commands assigned to the DataAdapter s UpdateCommand property. For example, consider two users who update a given record with the same values and the conflict that occurs when the second user attempts to send the same record to the database. On the companion CD, you can find a file, Resolving Conflicts on a Row-byRow Basis, in which I discuss how you can use the RowUpdated event to solve this unnecessary conflict, but you can achieve the same effect by modifying the UpdateCom mand property:
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UPDATE Publishers SET pub_id=@newPub_id, pub_name=@newPub_name, city=@newCity, state@=newState, country=@newCountry WHERE pub_id=@pub_id AND ( (pub_name IS NULL AND (@pub_name IS NULL OR @newPub_name Is NULL)) OR (pub_name=@pub_name OR pub_name=@newPub_name) ) AND ( (city IS NULL AND (@city IS NULL OR @newCity Is NULL)) OR (city=@city OR city=@newCity) ) AND ( (state IS NULL AND (@state IS NULL OR @newState Is NULL)) OR (state=@state OR state=@newState) ) AND ( (country IS NULL AND (@country IS NULL OR @newCountry Is NULL)) OR (country=@country OR country=@newCountry) )
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The UPDATE statement must account for the fact that all fields except pub_id are nul lable, which makes tests for equality more verbose. I ll omit here the source code that creates a parameterized command based on this SQL statement, but you can find it in the demo program that comes on the companion CD. Custom Update comments can be useful if you have a data entry form that lets you modify a subset of the columns in a record. For example, let s say that a form in your application lets you modify only the price, advance, and royalty columns of the Titles table in the Pubs database. The custom command that you need must consider only the three fields in the SET clause and the primary key pub_id plus these three columns in the WHERE clause:
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UPDATE Titles SET price=@newPrice, advance=@newAdvance, royalty=@newRoyalty WHERE title_id=@title_id AND ( ((price IS NULL AND @price IS NULL) OR price=@price) OR ((@newP rice IS NULL AND @price IS NULL) OR @newPrice=@price) ) AND ( ((advance IS NULL AND @advance IS NULL) OR advance=@advance) OR ((@newAdvance IS NULL AND @advance IS NULL) OR @newAdvance=@advance) ) AND ( ((royalty IS NULL AND @royalty IS NULL) OR royalty=@royalty) OR ((@newRoyalty IS NULL AND @royalty IS NULL) OR @newRoyalty=@royalty) )
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See Also Reducing Conflicts with the RowUpdating Events.doc on the companion CD describes a technique that allows you to use the RowUpdating event to build a custom com mand with which you can prevent conflicts whenever two users update two different groups of columns in the same application.
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All the examples shown so far are based on rather simple queries. Possibly they retrieved all the records in a database table or perhaps they attempted to reduce the network traffic and database activity by selecting a subset of rows with a WHERE clause or a subset of columns, as in these two examples:
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-- Only the titles published on or after 10/1/1992.
SELECT title_id, title, pub_id, pubdate FROM Titles WHERE pubdate> 10/1/1992
-- Only the publishers from the U.S.A.
SELECT pub_id, pub_name, city FROM Publishers WHERE country= USA
Alas, in the real world few queries are that simple, as all database programmers know. For example, consider a query that must return all (and only) the titles published after October 1, 1991, from all (and only) the publishers based in the United States. No prob lem, you might say: just fill two DataTable objects using the preceding two SELECT queries, and then create a relationship between them. Well, this can work with tables with a few hundred rows, but you aren t going to use this naive technique with tables of 100,000 rows, are you The point is, you would read a lot of records that you don t really want such as titles published by publishers not in the United States and U.S. publishers who haven t published any books since October 1991. Obviously, you must filter rows before the resultset leaves the server if you want to reduce both network traffic and the load on the database engine. The ideal solution would be to issue a JOIN command like this:
-- QUERY A: Retrieve data on titles and publishers satisfying -- the query criteria in one resultset, sorted by Publishers. SELECT pub_name, city, Publishers.pub_id, title_id, title, pubdate FROM Publishers INNER JOIN Titles ON Publishers.pub_id = Titles.pub_id WHERE country = USA AND pubdate > 10/1/1991 ORDER BY Publishers.pub_id
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