Relational model - Wikipedia
This chapter will present the main concepts of database design and query, and will help In Section 4 we present the Relational Model of data that is used to .. The total constraint on participation is sometimes referred to as existential dependence. . This clause is used to filter the information in a query. Chapter 7: outline. SQL. The Entity-Relationship Model. The Entity- Relationship Model Creating database tables using PRIMARY KEY and FOREIGN KEY constraints. ▫ The SQL CREATE . The WHERE clause match criteria may include. ▫ Equals “=” . either table, depending on whether RIGHT. JOIN or LEFT. dependent of the details of data representation and storage. . For each situation , draw an ER diagram that describes it (assuming that no further constraints .. If the clause WHERE mephistolessiveur.info >= 2 is added to the original query, what is the set of.
Normalisation is often misconceived as forbidding the storage of calculated values, leading people to avoid appropriate summary fields. There are two costs to consider when contemplating using a summary field: Some typical summary fields which you may encounter in an accounting system are: Summary Tables A summary table is a table whose records summarise large amounts of related data or the results of a series of calculations.
The entire table is maintained to optimise reporting, querying, and generating cross-table selections. Summary tables contain derived data from multiple records and do not necessarily violate the rules of normalisation. People often overlook summary tables based on the misconception that derived data is necessarily denormalised.
In order for a summary table to be useful it needs to be accurate. This means you need to update summary records whenever source records change. This task can be taken care of in the program code, or in a database trigger preferredor in a batch process. You must also make sure to update summary records if you change source data in your code.
Keeping the data valid requires extra work and introduces the possibility of coding errors, so you should factor this cost in when deciding if you are going to use this technique.
Entity–relationship model - Wikipedia
Optional Attributes that exist as a group As mentioned in the guidelines for developing relations in 3rd normal form all relations which share the same primary key are supposed to be combined into the same table. However, there are circumstances where is is perfectly valid to ignore this rule. Take the following example which I encountered in A finance company gives loans to customers, and a record is kept of each customer's repayments.
If a customer does not meet a scheduled repayment then his account goes into arrears and special action needs to be taken. This means that withcustomers there will be roughly 5, in arrears. This is not very efficient.Entity Relationship Diagram (ERD) Tutorial - Part 1
A solution in these circumstances is to extract all the attributes which deal with arrears and put them in a separate table. Thus if there are 5, customers in arrears you can reference a table which contains only 5, records. As the arrears data is subordinate to the customer data the arrears table must be the 'child' in the relationship with the customer 'parent'. It would be possible to give the arrears table a different primary key as well as the foreign key to the customer table, but this would allow the customer arrears relationship to be one-to-many instead of one-to-one.
To enforce this constraint the foreign key and the primary key should be exactly the same. This situation can be expressed using the following structure: Attribute K is the primary key. Attributes A B C exist all the time.
Attributes X Y Z exist some of the time but always as a group under the same circumstances. Attributes X Y Z require special processing. After denormalising the result is two separate relations, as follows: I have come across many different implementation tips and techniques over the years, and some that have worked in one database system have been successfully carried forward into a new database system.
Some tips, on the other hand, may only be applicable to a particular database system. For particular options and limitations you must refer to your database manual.
- Basic Concepts of ER Model in DBMS
- Entity–relationship model
- Relational model
Database names should be short and meaningful, such as products, purchasing and sales. Short, but not too short, as in prod or purch. Meaningful but not verbose, as in 'the database used to store product details'. Do not waste time using a prefix such as db to identify database names.
The SQL syntax analyser has the intelligence to work that out for itself - so should you. If your DBMS allows a mixture of upper and lowercase names, and it is case sensitive, it is better to stick to a standard naming convention such as: Leading uppercase, remainder lowercase.
Inconsistencies may lead to confusion, confusion may lead to mistakes, mistakes can lead to disasters. If a database name contains more than one word, such as in sales orders and purchase orders, decide how to deal with it: Separate the words with a single space, as in sales orders note that some DBMSs do not allow embedded spaces, while most languages will require such names to be enclosed in quotes.
Separate the words with a hyphen, as in sales-orders. Use camel caps, as in SalesOrders. Rather than putting all the tables into a single database it may be better to create separate databases for each logically related set of tables. The next figure shows a ternary relationship type we say relationship type of level 3 that relates patients, samples and projects: There can be relationship types of any level, but level 2 relationship types are more common.
Cardinality Ratio The cardinality ratio specifies the number of relationship instances in which an entity can participate. This means that for every Patient entity, we can associate any number of Sample entities. However, a Sample entity can only be related to a single Patient.
The Entity-Relationship Model
The most common cardinality ratios for binary relationship types are 1: N one-to-"N" and M: Constraints on Relationship Types The cardinality ratio of a relationship establishes two types of constraints on these relationships: A nice feature of DBMS software is that they automatically enforce these constraints, that is, entities and relationships cannot be entered into the database if the constraints are not satisfied.
The next figure illustrated a relationship with cardinality 1: Constraint on Participation The constraint on participation specifies whether the existence of an entity defines its participation on a relationship. There are two types of constraints on participation: For example, if the clinical database establishes a rule that every registered Sample must belong to a Patient, then the entity Sample can only exist if it participates in a relationship instance HAS.
The total constraint on participation is sometimes referred to as existential dependence. Hence, the patient's participation in the HAS relationship is partial. This means that some entities in the group of entities Patient may be related to an entity Sample via HAS, but not necessarily all of them.
Structural Constraint The Structural Constraint specifies the minimum and maximum number of relationships on which an entity should participate.
However, a Patient can have as many samples as necessary depending on the project. Therefore, the structural constraint of Sample in relation to the relationship type HAS is 1,1. Relationship Attribute Relationship Types can also have Attributes. For example, the date when the sample was collected Collection Date from a given patient should be represented in the relationship type HAS. The creation and specification of the Entity-Relationship model is called the Conceptual Design. During this process we identify entity types, their attributes, the key-attributes, the relationships their cardinalities and constraints.
In our example we have three entity types key attributes are underlined: The relationship types were also identified, together with the cardinality ratios, constraint on participation and attributes: Partial, as the patient is not obliged to have a sample in order to be registered in the database. Total, as a sample necessarily must be associated to a Patient in order to be registered in the database. Partial, as a Sample can be registered although there may be no Sequence to associate it with.
Total, as there must be an existing Sample in the database from which the sequence have been generated. It is one of the most common forms of documenting a database's structure. This is a concept used when discussing relational data which states that table relationships must always be consistent. Different RDBMSes might enforce referential integrity rules differently, but the concept is the same.
The constraints we've defined for our addresses table enforce the one to one relationship we want between it and our users table, whereby a user can only have one address and an address must have one, and only one, user. This is an example of referential integrity. Let's demonstrate how this works. What happens if we try to add another address for a user who already has one?
How about if we try to add an address for a user who doesn't exist? Here we get a different error. If you're wondering why we can add a user without an address but can't add an address without a user, this is down to the modality of the relationship between the two entities.
Don't worry about exactly what this means for now, just think of it as another aspect of entity relationships. Adding this clause, and setting it to CASCADE basically means that if the row being referenced is deleted, the row referencing it is also deleted.
Determining what to do in situations where you delete a row that is referenced by another row is an important design decision, and is part of the concept of maintaining referential integrity.
One-to-Many Okay, time to get back to our different table relationship types with a look at one-to-many. A one-to-many relationship exists between two entities if an entity instance in one of the tables can be associated with multiple records entity instances in the other table.
Understanding Database Design - Bioinformatics in Tropical Disease Research - NCBI Bookshelf
The opposite relationship does not exist; that is, each entity instance in the second table can only be associated with one entity instance in the first table. A book has many reviews. A review belongs to only one book.