Primary keys can be created using either numeric or character data types, with one exception: Oracle does not have the same concept of identity columns as MS SQL Server.
What is an Identity Column?
It is a property that automatically generates a unique sequential value when it is assigned to a numeric data type. Oracle handles this concept using a database object called sequence.
The Sequence database object enables you to generate a unique sequence number. Each user of the sequence can increment it and obtain numbers for their use. Because multiple users can obtain sequence numbers, there is no guarantee that the numbers you get will not have gaps.
A sequence does not have to be related to a single table, and therefore could be used to provide unique numbers to multiple tables. As a general rule, having one sequence per table or at least one for each major table results in easier diagnostics and a better overall experience.
The following examples show how you can add an identity column for two tables: categories and publishers.
Each table will use a IDENTITY column as PRIMARY KEY.
Fig 1. The categories and publishers catalog.
Fig 2. Script to create both tables.
The following code shows an example of how to select the NEXTVAL from a sequence. Upon executing this trigger, the sequence value is incremented by 1.
Fig 3. Script to create database objects: triggers and sequences.
Each time we add a new row, the trigger will automatically create a new unique number for that row.
With this concept, the first row ID would be 1, and the next ID would be last ID number plus one.
Fig 4. Inserting rows without primary key.
Fig 5.Querying the tables.
A delegate is similar to a function pointer in C or C++ except that delegates are type-safe. The term type-safe means that code is specified in a well-defined manner that can be recognized by a compiler. In this case it means that an incorrect use of a delegate is a compile-time error. This is quite different than in C++, where an incorrect use of a function pointer may not cause an error until the program is running.
Delegates allow you to write code that can dynamically change the methods that it calls.
A delegate contains a reference to a method rather than the method name.
By using delegates, you can invoke a method without knowing its name. Calling the delegate will actually execute the method referenced by the delegate.
To use a delegate, you must follow these steps:
- First, define it using the reserved keyword delegate.
- Second, instantiate it.
- Third, write the implementation with the same return value and signature of your delegate.
Fig 1. Steps to use a delegate.
A delegate is similar to an interface. It specifies a contract between a caller and an implementer.
The following code shows how to define, create and call delegates, It creates an array of delegates with instances of delegates that refers to the methods that represent each arithmetic operation.
We execute each delegate with a foreach keyword that iterates through this array.
Fig 2. Sample using delegates.
Using delegates is a solution much simpler than using function pointer.
Fig 3. Testing the program.
Fig 4. Another test.