Entity–relationship model - Wikipedia
An entity–relationship model (ER model for short) describes interrelated things of interest in a specific domain of knowledge. m:n is used to denote a many-to-many relationship (m objects on the other side . two components); fixed cardinality (m:n where m and n are fixed constants or. Degrees of Relationship (Cardinality). The degree of relationship (also known as cardinality) is the number of occurrences in one entity which are associated (or.
It occurs with a master table that links to multiple tables in a one-to-many relationship.
The issue derives its name from the way the model looks when it's drawn in an entity—relationship diagram: This type of model looks similar to a star schemaa type of model used in data warehouses.
When trying to calculate sums over aggregates using standard SQL over the master table, unexpected and incorrect results. The solution is to either adjust the model or the SQL. This issue occurs mostly in databases for decision support systems, and software that queries such systems sometimes includes specific methods for handling this issue.
The second issue is a 'chasm trap'. A chasm trap occurs when a model suggests the existence of a relationship between entity types, but the pathway does not exist between certain entity occurrences.
For example, a Building has one-or-more Rooms, that hold zero-or-more Computers. One would expect to be able to query the model to see all the Computers in the Building. However, Computers not currently assigned to a Room because they are under repair or somewhere else are not shown on the list.
Cardinality (data modeling) - Wikipedia
Another relation between Building and Computers is needed to capture all the computers in the building. This last modelling issue is the result of a failure to capture all the relationships that exist in the real world in the model. See Entity-Relationship Modelling 2 for details.
Entity—relationships and semantic modeling[ edit ] Semantic model[ edit ] A semantic model is a model of concepts, it is sometimes called a "platform independent model". There is a many-to-many relationship between the records in the doctor table and records in the patient table because doctors have many patients, and a patient could have several doctors; There is a one-to-many relationship between the department table and the doctor table because each doctor may work for only one department, but one department could have many doctors.
A "one-to-one" relationship is mostly used to split a table in two in order to provide information concisely and make it more understandable.
Discrete Mathematics/Set theory
In the hospital example, such a relationship could be used to keep apart doctors' own unique professional information from administrative details. In data modelingcollections of data elements are grouped into "data tables" which contain groups of data field names called "database attributes". Tables are linked by "key fields".Attributes to relationships
A "primary key" assigns a field to its "special order table". For example, the "Doctor Last Name" field might be assigned as a primary key of the Doctor table with all people having same last name organized alphabetically according to the first three letters of their first name. So A is a subset of B, and the diagram should be re-drawn like Fig 2 above. The diagram should then be re-drawn like Fig 1 above. Example 2 a Draw a Venn diagram to represent three sets A, B and C, in the general case where nothing is known about possible relationships between the sets.
In each case, the Venn diagram can be re-drawn so that empty regions are no longer included.
mysql - Meaning of "n:m" and "1:n" in database design - Stack Overflow
If region ii is empty, the loop representing A should be made smaller, and moved inside B and C to eliminate region ii. If regions ii, iii and iv are empty, make A and B smaller, and move them so that they are both inside C thus eliminating all three of these regionsbut do so in such a way that they still overlap each other thus retaining region vi.
If regions iii and vi are empty, 'pull apart' loops A and B to eliminate these regions, but keep each loop overlapping loop C.
Drawing Venn diagrams for each of the above examples is left as an exercise for the reader. Example 3 The following sets are defined: The technique is as follows: Draw a 'general' 3-set Venn diagram, like the one in Example 2.
Go through the elements of the universal set one at a time, once only, entering each one into the appropriate region of the diagram. Re-draw the diagram, if necessary, moving loops inside one another or apart to eliminate any empty regions.
Don't begin by entering the elements of set A, then set B, then C — you'll risk missing elements out or including them twice! Solution After drawing the three empty loops in a diagram looking like Fig.