Before you begin to establish relationships between tables in the database, you must know what types of relationships can exist between a given pair of tables. In Kafka's ''The Metamorphosis,'' there's a kind of inverse relationship between A powerful, judgmental, abusive father, and a quiet, reclusive, abused son: ''The. A scientist needs a list of all toxic chemicals from a file of chemical data. Moreover, the database definition establishes relationships among the data. Thus.
When you don't have source code to the application writing to the database and you need to modify the behaviour.
Triggers are your only option. If you are performing CRUD operations on a view. I tend to perform basic validation in the app.
This way the user is given immediate feedback that something is wrong. Complex validation that requires looking up related tables is probably best done in the database as well as the simple validation that the app does. I would argue that some forms of validation are almost impossible to guarantee at the application level, without using complicated locking strategies.
If you have multiple applications, possibly written in different languages on different platforms, then the case for putting more of the validation into the database layer is strengthened.
The liklihood of two or more applications, written by different programmers, performing identical validation is fairly remote. Best do it in one place. The Jeff Atwoods of this world would suggest that you write a web service that all the apps use to communicate with. The web service performs the data validation. Doing this allows the database to remain a dumb storage container, thus enabling you to switch database engines. In this case, there is a crow's foot symbol located next to each table.
Problems with Many-to-Many Relationships A many-to-many relationship has an inherent peculiarity that you must address before you can effectively use the data from the tables involved in the relationship. The issue is this: How do you easily associate records from the first table with records in the second table in order to establish the relationship?
This is an important question because you'll encounter problems such as these if you do not establish the relationship properly: It will be tedious and somewhat difficult to retrieve information from one of the tables. One of the tables will contain a large amount of redundant data. Duplicate data will exist within both tables. It will be difficult for you to insert, update, and delete data. There are two common methods that novice and inexperienced developers use in a futile attempt to address this situation.
Note As this example unfolds, keep in mind that every many-to-many relationship you encounter will exhibit these same issues. As you can see, there is no actual connection between the two tables, so you have no way of associating records in one table with records in the other table. The first method you might use to attempt to establish a connection involves taking a field from one table and incorporating it a given number of times within the other table.
This approach usually appeals to people who are accustomed to working with spreadsheets. Do these structures look vaguely familiar? All you've done using this method is introduce a "flattened" multivalued field into the table structure.
In doing so, you've also introduced the problems associated with a multivalued field. If necessary, review Chapter 7. Although you know how to resolve a multivalued field, this is not a good or proper way to establish the relationship. The second method you might attempt to use is simply a variation of the first method.
Types of Relationships
In this case, you take one or more fields from one table and incorporate a single instance of each field within the other table. This may seem to be a distinct improvement over the first method, but you'll see that there are problems that arise from such modifications when you load the revised STUDENTS table with sample data. The table contains unnecessary duplicate fields.
You learned all about unnecessary duplicate fields and the problems they pose back in Chapter 7, so you know that using them here is not a good idea.
There is a large amount of redundant data.
What is a Relationship? - Definition from Techopedia
It is difficult to insert a new record. This will automatically trigger a violation of the Elements of a Primary Key because the primary key cannot be null; therefore, you cannot insert the record into the table until you can provide a proper primary key value. It is difficult to delete a record. This is especially true if the only data about a new class has been recorded in the particular student record you want to delete. Note the record for Diana Barlet, for example.7 Early Signs of A Toxic Relationship
If Diana decides not to attend any classes this year and you delete her record, you will lose the data for the "Introduction to Database Design" class.
Fortunately, you will not have to worry about any of these problems because you're going to learn the proper way to establish a many-to-many relationship. Self-Referencing Relationships This particular type of relationship does not exist between a pair of tables, which is why it isn't mentioned at the beginning of this section. It is instead a relationship that exists between the records within a table.
Ironically, you'll still regard this throughout the design process as a table relationship. A table bears a self-referencing relationship also known as a recursive relationship to itself when a given record in the table is related to other records within the table.
Similar to its dual-table counterpart, a self-referencing relationship can be one-to-one, one-to-many, or many-to-many. One-to-One A self-referencing one-to-one relationship exists when a given record in the table can be related to only one other record within the table.
In this case, a given member can sponsor only one other member within the organization; the SPONSOR ID field stores the member identification number of the member acting as a sponsor. Example of a self-referencing one-to-one relationship.
Diagramming a self-referencing one-to-one relationship. One-to-Many A table bears a self-referencing one-to-many relationship to itself when a given record in the table can be related to one or more other records within the table. Example of a self-referencing one-to-many relationship.
Diagramming a self-referencing one-to-many relationship. Many-to-Many A self-referencing many-to-many relationship exists when a given record in the table can be related to one or more other records within the table and one or more records can themselves be related to the given record.