A Primary Key Especially An Auto-generated Surrogate Key Is Sufficient

Posted on 03.08.2020by admin

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Surrogate keys are unique. Because surrogate keys are system-generated, it is impossible for the system to create and store a duplicate value. Surrogate keys apply uniform rules to all records. The surrogate key value is the result of a program, which creates the system-generated value. ON DUPLICATE KEY) MySQL will always perform a lookup against ALL keys of a table - not just the PRIMARY KEY. So if you add a surrogate key in addition to a unique constraint/index on 5 columns then performance will likely be worse and the database will be more complex (more columns, more indexes, more lookups to handle the surrogate key processing when data is inserted/updated). By default, a primary key is created as a clustered index, but you can choose non-clustered instead, preferably after creating the clustered index. So you can have an integer identity generated column as clustered index, and, say, file name nvarchar(128 characters) as primary key. Jul 14, 2019 The main difference between surrogate key and primary key is that surrogate key is a type of primary key that helps to identify each record uniquely, while the primary key is a set of minimal columns that helps to identify each record uniquely. RDBMS is a DBMS designed using the relational data model. It helps to store and manage data in databases. Mar 23, 2011 10 tips for choosing between a surrogate and natural primary key by Susan Harkins in 10 Things, in After Hours on March 23, 2011, 4:16 AM PST.

Primary Key Generation Using Oracle's Sequence

Oracle provides the 'sequence' utility to automatically generate unique primary keys. To use this utility to auto-generate primary keys for a CMP entity bean, you must create a sequence table and use the ejbgen:automatic-key-generation tag to point to this table.

In your Oracle database, you must create a sequence table that will create the primary keys, like is shown in the following example:

This creates a sequences of primary key, starting with 1, followed by 2, 3, and so forth. The sequence table in the example uses the default increment 1, but you can change this by specifying the increment keyword, such as increment by 3. When you do the latter, you must specify the exact same value in the cache-size attribute of the ejbgen:automatic-key-generation tag:

If you have specified automatic table creation in the CMP bean's project settings, the sequence table will be created automatically when the entity bean is deployed. For more information, see @ejbgen:jar-settings Annotation. For more information on the definition of a CMP entity bean, see below.

Primary Key Generation Using SQL Server's IDENTITY

In SQL Server (2000) you can use the 'IDENTITY' keyword to indicate that a primary-key needs to be auto-generated. The following example shows a common scenario where the first primary key value is 1, and the increment is 1:

In the CMP entity bean definition you need to specify SQLServer(2000) as the type of automatic key generator you are using. You can also provide a cache size:

Note. The SQLServer2000 option is the same as SQLServer, except that SQLServer uses @@IDENTITY column to get the generated key value and SQLServer2000 uses the SCOPE_IDENTITY() function instead.

Primary Key Generation Using a Named Sequence Table

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A named sequence table is similar to the Oracle sequence functionality in that a dedicated table is used to generate primary keys. However, the named sequence table approach is vendor-neutral. To auto-generate primary keys this way, create a named sequence table using the two SQL statements shown in the example:

In the CMP entity bean definition you need to specify the named sequence table as the type of automatic key generator you are using. You can also provide a cache size:

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Note. When you specify a cache-size for a named sequence table, a series of unique values are reserved for entity bean creation. When a new cache is necessary, a second series of unique values is reserved, under the assumption that the first series of unique values was entirely used. This guarantees that primary key values are always unique, although it leaves open the possibility that primary key values are not necessarily sequential. For instance, when the first series of values is 10..20, the second series of values is 21-30, even if not all values in the first series were actually used to create entity beans.

Defining the CMP Entity Bean

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When defining a CMP entity bean that uses one of the primary key generators, you point to the name of the primary key generator table to obtain primary keys, using the ejbgen:automatic-key-generation tag. Also, you must define a primary key field of type Integer or Long, to set and get the auto-generated primary key. However, the ejbCreate method does not take a primary key value as an argument. Instead the EJB container adds the correct primary key to the entity bean record.

The following example shows what the entity bean might look like. Notice that the bean uses the named sequence option describe above, and that ejbCreate method does not take a primary key: