One-to-One Relationships in Software

Creating Code 128 Code Set C in Software One-to-One Relationships
One-to-One Relationships
Create Code 128A In Visual C#
Using Barcode generator for .NET framework Control to generate, create Code 128 Code Set A image in VS .NET applications.
One-to-one relationships are implemented in exactly the same way as oneto-many relationships sort of You still create a primary key and a foreign key; the problem is that at this point SQL Server still allows users to insert many rows into the foreign key table that reference the primary key table
Code 128 Code Set A Printer In Visual Studio .NET
Using Barcode generation for ASP.NET Control to generate, create Code 128C image in ASP.NET applications.
Referential Integrity
Painting Code 128B In .NET
Using Barcode creation for .NET framework Control to generate, create Code 128 Code Set A image in .NET applications.
There is no way, by default, to constrain the data to one-to-one To implement a one-to-one relationship that is enforced, you must get a little creative The first option is to write a stored procedure (more on stored procedures later in this chapter) to do all your inserting, and then add logic to prevent a second row from being added to the table This method works in most cases, but what if you need to load data directly to tables without a stored procedure Another option to implement one-to-one relationships is to use a trigger, which we also look at shortly Basically, a trigger is a piece of code that can be executed after or instead of the actual insert statement Using this method, you could roll back any insert that would violate the one-to-one relationship Additionally and this is probably the easiest method you can add a unique constraint on the foreign key columns This would mean that the data in the foreign key would have to be a value from the primary key, and each value could appear only once in the referencing table This approach effectively creates a one-to-one relationship that is managed and enforced by SQL Server
Code 128 Code Set A Encoder In Visual Basic .NET
Using Barcode maker for .NET framework Control to generate, create Code 128 Code Set B image in Visual Studio .NET applications.
Many-to-Many Relationships
Read Barcode In .NET Framework
Using Barcode reader for .NET framework Control to read, scan read, scan image in Visual Studio .NET applications.
One of the most complex relationships when it comes to implementation is the many-to-many relationship Even though you can have a many-tomany relationship between two entities, you cannot create a many-to-many relationship between only two tables To implement this relationship, you must create a third table, called a junction table, and two one-to-many relationships Let s walk through an example to see how it works You have two tables one called Student and one called Class and both contain an identity called objid as their PK In this situation you need a many-to-many relationship, because each student can be in more than one class and each class will have more than one student To implement the relationship, you create a junction table that has only two columns: one containing the student_objid, and the other containing the class_objid You then create a one-to-many relationship from this junction table to the Student table, and another to the Class table Figure 33 shows how this relationship looks You will notice a few things about this configuration First, in addition to being foreign keys, these columns are used together as the primary key for the Student_Class junction table How does this implement a many-tomany relationship The junction table can contain rows as long as they do
Recognize Code128 In .NET
Using Barcode reader for .NET Control to read, scan read, scan image in Visual Studio .NET applications.
3
Draw Code 39 In Java
Using Barcode generation for Java Control to generate, create Code 39 Extended image in Java applications.
Physical Elements of Data Models
Code 3 Of 9 Printer In C#.NET
Using Barcode creator for .NET Control to generate, create ANSI/AIM Code 39 image in Visual Studio .NET applications.
FIGURE 33 Many-to-many relationship between a Student and a Class table not violate the primary key This means that you can relate each student to all the classes he attends, and you can relate all the students in a particular class to that class This gives you a many-to-many relationship It may sound complex, but once you create a many-to-many relationship and add some data to the tables, it becomes pretty clear The best way to really understand it is to do it When we build our physical model in 9, we look more closely at many-to-many relationships, including ways to make them most useful
Barcode Creation In .NET Framework
Using Barcode generator for .NET framework Control to generate, create barcode image in .NET framework applications.
Implementing Advanced Cardinality
Code 128C Generation In Java
Using Barcode creator for Java Control to generate, create USS Code 128 image in Java applications.
In 2, we talk about cardinality Cardinality simply describes the number of rows in a table that can relate to rows in another table Cardinality is often derived from your customer s business rules As with one-to-one relationships, SQL Server does not have a native method to support advanced cardinality Using primary and foreign keys, you can easily enforce one-or-more-to-many, zero-or-more-to-many, or one-to-one cardinality as we have described previously What if you want to create a relationship whereby each parent can contain only a limited number of child records For example, using our employee and vehicle tables, you might want to limit your data so that each employee can have no more than five cars assigned Additionally, employees are not required to have a car at all The cardinality of this relationship is said to be zero-to-five-to-many To enforce this requirement, you need to be creative In this scenario you could use a trigger that counts the number of cars assigned to an employee If the additional car would put the employee over five, the insert could be reversed or rolled back Each situation is unique In some cases you might be able to use check constraints or another combination of PKs, FKs, and constraints to implement your cardinality You need to examine your requirements closely to decide on the best approach
ANSI/AIM Code 39 Generator In .NET Framework
Using Barcode printer for ASP.NET Control to generate, create Code 39 image in ASP.NET applications.
Make USS Code 39 In VB.NET
Using Barcode encoder for Visual Studio .NET Control to generate, create Code 39 image in Visual Studio .NET applications.
DataMatrix Creation In .NET Framework
Using Barcode maker for ASP.NET Control to generate, create Data Matrix ECC200 image in ASP.NET applications.
Encoding Bar Code In VB.NET
Using Barcode drawer for VS .NET Control to generate, create barcode image in .NET framework applications.
Painting Bar Code In .NET Framework
Using Barcode generation for ASP.NET Control to generate, create bar code image in ASP.NET applications.
Painting EAN 128 In Visual Studio .NET
Using Barcode maker for .NET Control to generate, create USS-128 image in Visual Studio .NET applications.