barcode generator vb.net Advanced Stored Procedure Programming in Software

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Advanced Stored Procedure Programming
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As -- read next Id Select @intId = SequenceNumber From SequenceNumbers (updlock) Where Tablename = 'a table' -- increment SequenceNumber Update SequenceNumbers Set SequenceNumber = @intId + 1 Where Tablename = 'a table' Return
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NOTE: Unfortunately, although very interesting, a detailed discussion of locks, deadlocks, concurrency, and other such issues is beyond the scope of this book.
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Preserving an Identity Value in a Temporary Table
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I hope you haven t forgotten the original problem we set out to solve. The problem is how to preserve an identity value if a table has a trigger that inserts another record in some other table with the same identity key. The key word is preserve. You can create a private temporary table and preserve the value of the identity key in it. Private tables are not visible to other processes, and you should not have concurrency issues with this solution. In this case, a temporary table needs to be created from the stored procedure that is inserting a record into the table. You should then use a trigger (you can use the same one that is causing the problem) to pick an identity value and preserve it in the temporary table. When the triggers are finished, the stored procedure will be able to read an identity value from a temporary table.
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SQL Server 2000 Stored Procedure Programming
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We will demonstrate this solution by changing an earlier example. You should execute the following against the tempdb database:
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Drop Trigger tr_a_I Drop Table a Drop Table b Create Table a (a_id int identity(1,1), a_desc varchar(20), b_desc varchar(20)) Go
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Create Table b (b_id int identity(1,1), b_desc varchar(20)) Go
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Create Trigger tr_a_I On dbo.a After Insert As If @@Rowcount = 0 Return -- preserve identity value Insert Into #ids (TableName, id) Values ('a', @@identity) -- add inserted leases to total Insert Into b (b_desc) Select b_desc From inserted Go --For Insert
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Advanced Stored Procedure Programming
As you can see, the trigger preserves the identity value in temporary table #ids. This table has to be created from the outer stored procedure or batch that will insert a record.
Create Table #ids( TableName sysname, id int) Insert Into b (b_desc) Values ('1') Insert Into a (a_desc, b_desc) Values ('aaa', 'bbb') -- right identity value Select id [IdentityValue] From #ids Where TableName = 'a' Drop Table #ids
This time, the result is correct:
IdentityValue ------------1
TIP: I do not like either of these solutions. The sequence table is an archaic approach. It requires many manual steps, and the performance of the database will suffer because of concurrency problems. I find the second solution even more distasteful. The trigger is coupled with the code that is calling it. It depends on the existence of a temporary table. The developer might forget to create the trigger, or the user might try to insert records with some other tool. Too many things can go wrong. Let s keep this problem in mind and solve it by avoiding such triggers!
SQL Server 2000 Stored Procedure Programming
GUIDS
Distributed environments have different requirements for the generation of unique keys. A typical example is a database of sales representatives who are carrying notebook computers with local databases installed on them. These users do not have to be connected to a central database. They do the majority of their work locally and then replicate the information in their local database to the central database once in awhile. The use of identity fields in this case will lead to repetition, unless the key is composite and consists of an identity field and another field that is unique to the user. Another solution could be to divide key ranges between users (for example by setting an identity seed differently in each database). Each of these solutions has different limitations. One way to generate unique keys is to use GUID (Global Unique Identification) fields. We discussed the uniqueidentifier datatype in 4. When a column in a table is assigned this datatype, it does not mean that its (unique) value will be generated automatically. Some other database object needs to generate a unique value using the NewID() function. Typically a GUID value is generated as a default value of a table:
Create Table Location( LocationId uniqueidentifier NOT NULL DEFAULT newid(), Location varchar(50) not null, CompanyId int NOT NULL, PrimaryContactName varchar(60) NOT NULL, Address varchar(30) NOT NULL, City varchar(30) NOT NULL, ProvinceId varchar(3) NULL, PostalCode varchar(10) NOT NULL, Country varchar(20) NOT NULL, Phone varchar(15) NOT NULL, Fax varchar(15) NULL ) Go
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