data matrix barcode generator java Generating primary keys in Java

Printer Data Matrix in Java Generating primary keys

8.2.7 Generating primary keys
DataMatrix Printer In Java
Using Barcode maker for Java Control to generate, create Data Matrix image in Java applications.
www.OnBarcode.com
Reading DataMatrix In Java
Using Barcode recognizer for Java Control to read, scan read, scan image in Java applications.
www.OnBarcode.com
When we identify a column or set of columns as the primary key, we essentially ask the database to enforce uniqueness. Primary keys that consist of business data are called natural keys. The classic example of this is SSN as the primary key for an EMPLOYEE table. CATEGORY_ID or EMPLOYEE_ID, on the other hand, are examples of
Barcode Generator In Java
Using Barcode printer for Java Control to generate, create Barcode image in Java applications.
www.OnBarcode.com
Generating Data Matrix In Java
Using Barcode generator for Java Control to generate, create Data Matrix ECC200 image in Java applications.
www.OnBarcode.com
Object-relational mapping
Encode Barcode In Java
Using Barcode generator for Java Control to generate, create Barcode image in Java applications.
www.OnBarcode.com
Linear Printer In Java
Using Barcode encoder for Java Control to generate, create Linear Barcode image in Java applications.
www.OnBarcode.com
surrogate keys. Essentially, surrogate keys are columns created explicitly to function as primary keys. Surrogate keys are popular and we highly recommend them, especially over compound keys. There are three popular ways of generating primary key values: identities, sequences, and tables. Fortunately, all three strategies are supported via the @GeneratedValue annotation and switching is as easy as changing the configuration. Let s start our analysis with the simplest case: using identities. Identity columns as generators Many databases such as Microsoft SQL Server support the identity column. You can use an identity constraint to manage the primary key for the User entity as follows:
Encoding UPC - 13 In Java
Using Barcode printer for Java Control to generate, create EAN13 image in Java applications.
www.OnBarcode.com
Code 93 Extended Drawer In Java
Using Barcode creation for Java Control to generate, create Code 93 Full ASCII image in Java applications.
www.OnBarcode.com
@Id @GeneratedValue(strategy=GenerationType.IDENTITY) @Column(name="USER_ID") protected Long userId;
Data Matrix ECC200 Generation In .NET
Using Barcode generation for Reporting Service Control to generate, create Data Matrix ECC200 image in Reporting Service applications.
www.OnBarcode.com
Paint Data Matrix ECC200 In None
Using Barcode printer for Office Word Control to generate, create Data Matrix 2d barcode image in Office Word applications.
www.OnBarcode.com
This code assumes that an identity constraint exists on the USERS.USER_ID column. Note that when using IDENTITY as the generator type, the value for the identity field may not be available before the entity data is saved in the database because typically it is generated when a record is committed. The two other strategies, SEQUENCE and TABLE, both require the use of an externally defined generator: a SequenceGenerator or TableGenerator must be created and set for the GeneratedValue. You ll see how this works by first taking a look at the sequence generation strategy. Database sequences as generators To use sequence generators, first define a sequence in the database. The following is a sample sequence for the USER_ID column in an Oracle database:
Drawing Matrix Barcode In Visual C#
Using Barcode printer for .NET framework Control to generate, create Matrix image in VS .NET applications.
www.OnBarcode.com
Barcode Creator In Java
Using Barcode creator for Android Control to generate, create Barcode image in Android applications.
www.OnBarcode.com
CREATE SEQUENCE USER_SEQUENCE START WITH 1 INCREMENT BY 10;
Painting Denso QR Bar Code In None
Using Barcode printer for Office Excel Control to generate, create QR-Code image in Microsoft Excel applications.
www.OnBarcode.com
Recognizing Data Matrix 2d Barcode In VS .NET
Using Barcode scanner for .NET framework Control to read, scan read, scan image in .NET framework applications.
www.OnBarcode.com
We are now ready to create a sequence generator in EJB 3:
EAN128 Generation In Objective-C
Using Barcode encoder for iPad Control to generate, create EAN / UCC - 13 image in iPad applications.
www.OnBarcode.com
Painting UPCA In None
Using Barcode generator for Excel Control to generate, create GTIN - 12 image in Office Excel applications.
www.OnBarcode.com
@SequenceGenerator(name="USER_SEQUENCE_GENERATOR", sequenceName="USER_SEQUENCE", initialValue=1, allocationSize=10)
Create GS1 DataBar In VS .NET
Using Barcode printer for VS .NET Control to generate, create GS1 DataBar image in VS .NET applications.
www.OnBarcode.com
Print USS Code 128 In None
Using Barcode encoder for Font Control to generate, create Code-128 image in Font applications.
www.OnBarcode.com
The @SequenceGenerator annotation creates a sequence generator named USER_ SEQUENCE_GENERATOR referencing the Oracle sequence we created and matching its setup. Naming the sequence is critical since it is referred to by the @GeneratedValue annotation. The initialValue element is pretty self-explanatory: allocationSize specifies by how much the sequence is incremented each time a value is generated.
Barcode Decoder In Visual Basic .NET
Using Barcode Control SDK for VS .NET Control to generate, create, read, scan barcode image in Visual Studio .NET applications.
www.OnBarcode.com
Read Code 3 Of 9 In Java
Using Barcode recognizer for Java Control to read, scan read, scan image in Java applications.
www.OnBarcode.com
Mapping entities
The default values for initialValue and allocationSize are 0 and 50, respectively. It s handy that the sequence generator need not be created in the same entity in which it is used. As a matter of fact, any generator is shared among all entities in the persistence module and therefore each generator must be uniquely named in a persistence module. Finally, we can reimplement the generated key for the USER_ID column as follows:
@Id @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="USER_SEQUENCE_GENERATOR") @Column(name="USER_ID") protected Long userId;
Sequence tables as generators Using table generators is just as simple as with a sequence generator. The first step is creating a table to use for generating values. The table must follow a general format like the following one created for Oracle:
CREATE TABLE SEQUENCE_GENERATOR_TABLE (SEQUENCE_NAME VARCHAR2(80) NOT NULL, SEQUENCE_VALUE NUMBER(15) NOT NULL, PRIMARY KEY (SEQUENCE_NAME));
The SEQUENCE_NAME column is meant to store the name of a sequence, and the SEQUENCE_VALUE column is meant to store the current value of the sequence. The next step is to prepare the table for use by inserting the initial value manually as follows:
INSERT INTO SEQUENCE_GENERATOR_TABLE (SEQUENCE_NAME, SEQUENCE_VALUE) VALUES ('USER_SEQUENCE', 1);
In a sense, these two steps combined are the equivalent of creating the Oracle sequence in the second strategy. Despite the obvious complexity of this approach, one advantage is that the same sequence table can be used for multiple sequences in the application. We are now prepared to create the TableGenerator utilizing the table:
@TableGenerator (name="USER_TABLE_GENERATOR", table="SEQUENCE_GENERATOR_TABLE", pkColumnName="SEQUENCE_NAME", valueColumnName="SEQUENCE_VALUE", pkColumnValue="USER_SEQUENCE")
If you need to, you can specify the values for initialValue and allocationSize as well. Finally, we can use the table generator for USER_ID key generation:
Copyright © OnBarcode.com . All rights reserved.