Two attributes multiple Entities
If the total number of database fields across the subtypes is . no relationships at the super type level, you might go with only sub-type tables. Entering Data in Tables in a Supertype/Subtype Relationship* This presentation was prepared by Professor Steve Ross, with the advice of other MIS Faculty. The guide gives the sql to create a Purchase Order (PO) application. You will write (or copy and Identify the Super type/Subtype relationships. a. List the super.
Class table inheritance has one table for the superclass and one table for each subclass. Generalized data goes in the superclass table, and specialized data goes in the appropriate subclass table.
Shared primary key enforces the one to one nature of the relationships between each subclass and the superclass. It also allows a foreign key elsewhere in the database to reference the superclass without knowing or caring which subclass it belongs to.
For a good treatment of this subject, look up Martin Fowler's treatment on the web. Edit based on feedback The question of which ER model is "correct" is usually determined by the database design team by analyzing the subject matter, and coming up with a model that depicts the way it works in "the real world".
This presumes that the database is yet to be designed, but the business itself has already been designed, and the business design is not to be changed. Your comment makes me think that you are designing or redesigning the business itself, not just the database. If so, then the question is outside the expertise of database design. Meaning, just because the E property is shared between entity types A and B does not mean that A and B should be grouped together.
Just because things "look" the same i. I would consider moving E to the base-type table IF: My employers changed the business logic, removing E altogether! But while it is true on a technical level that anything can be refactored, the reality of the situation is a matter of scale. Businesses are constantly setting priorities on projects because those resources are very limited.
While it is frustrating for us technical folks because we understand what the long-term benefits of refactoring projects are, it is just the nature of business that the less-technical, business folks have an easier time seeing the direct relationship between new functionality and new revenue.
What this boils down to is: BUT, if you have hundreds of millions of rows in these tables, and an incredible amount of code referencing the tables code that has to be tested when changes are madethen it usually pays to be more pragmatic than idealistic.
And this is the environment that I had to deal with for years: And so much code hit these tables base-class and sub-class tables that there was a lot of resistance -- mostly from management but sometimes from the rest of the team -- to making any changes due to the amount of development and QA resources that would need to be allocated. However, there are a couple of points that leave me unconvinced.
For example, take centralizing Name-Value pairs into a single table. I side with the anti-OTLT group; these tables introduce numerous issues. We might employ the software development analogy of using a single enumerator to represent all possible values of all possible constants as an equivalent to this practice.
To remind you, the OTLT table typically looks something like this, with entries from multiple domains thrown into the same table.
need help with subtype supertype
I agree with anti-OTLT group; these tables introduce numerous issues. While this is a valid point, it is not one that I usually think of when creating a new data model. Koirala has a couple of OLAP design items that are generally not mentioned in other best practice lists. His inclusion of a dimension and fact design may be useful, but it could also be dangerous for newbie designers. This article is interesting if you are moving from beginning into more advanced data modeling. It will help you consider the analytic versus transactional nature of your future models.
He shows how to check for proper normalization: Over-normalization will have a negative impact on database performance. Also, using natural business keys rather than generated primary keys is sound advice when you want to avoid translating from a business key to a generated row ID for each database access.
MySQL - Supertype/Subtype design - Stack Overflow
Using proper naming standards and column types is also good advice. The point about the overuse of nullable columns is sound: Are all the columns NULLable? Within the database columns definitions good data domains, ranges and values should be analyzed, evaluated and prototyped for the business application. Having good default values, a limited scope of values and always a value are best for performance and application logic. Make sure your database design represents data that is known and only uses a minimum of NULLable columns.
Disjoint and reflexive relationships in Access
In the end, I felt that the article lacked both depth and breadth, and had no examples to clarify the points. However, he does offer valuable advice related to normalization and natural keys. All includes information from seasoned developers, such as Michael Blaha.
He encourages the reuse of your best practices and patterns. These are understood and proven, and in that respect preferable to data models that must be created from scratch. For example, I often reverse engineer databases — databases of an application to be replaced as well as databases of related applications.
These existing databases often do not have an available data model. But a data model is implicit in the database schema and can be at least partially extracted with database reverse engineering techniques. This is a short slide show that data model designers can quickly scan through and glean the tips that resonate with them. For me, the re-use tip is one of my favorites.
These best practices started off just fine, but then got into some sticky issues. I am not convinced the advice offered is always on point. A previous author wrote: Names are not unique, and many seemingly unique identifiers such as Social Security numbers actually are not, due to real-world data reliability problems. This is a rather pat answer to a complex issue, though it is one that will suffice in a number of cases and is at least preferable to having no PrimaryKey at all.