·Domain: A (usually named) set/universe of atomic values, where by "atomic" we mean simply that, from the point of view of the database, each value in the domain is indivisible (i.e., cannot be broken down into component parts).
Examples of domains oSSN: string of digits of length nine
oName: string of characters beginning with an upper case letter
oGPA: a real number between 0.0 and 4.0
oSex: a member of the set { female, male }
oDept_Code: a member of the set { CMPS, MATH, ENGL, PHYS, PSYC, ... }
These are all logical descriptions of domains. For implementation purposes, it is necessary to provide descriptions of domains in terms of concrete data types (or formats) that are provided by the DBMS (such as String, int, boolean), in a manner analogous to how programming languages have intrinsic data types.
·Attribute: the name of the role played by some value (coming from some domain) in the context of a relational schema. The domain of attribute A is denoted dom(A).
·Tuple: A tuple is a mapping from attributes to values drawn from the respective domains of those attributes. A tuple is intended to describe some entity (or relationship between entities) in the miniworld. As an example, a tuple for a PERSON entity might be { Name --> "Keerthy", Sex --> Male, IQ --> 786 }
·Relation: A (named) set of tuples all of the same form (i.e., having the same set of attributes). The term table is a loose synonym.
·Relational Schema: used for describing (the structure of) a relation. E.g., R(A1, A2, ..., An) says that R is a relation with attributes A1, ... An. The degree of a relation is the number of attributes it has, here n. Example: STUDENT(Name, SSN, Address) One would think that a "complete" relational schema would also specify the domain of each attribute.
·Relational Database: A collection of relations, each one consistent with its specified relational schema.
Characteristics of Relations
Ordering of Tuples: A relation is a set of tuples; hence, there is no order associated with them. That is, it makes no sense to refer to, for example, the 5th tuple in a relation. When a relation is depicted as a table, the tuples are necessarily listed in some order, of course, but you should attach no significance to that order. Similarly, when tuples are represented on a storage device, they must be organized in some fashion, and it may be advantageous, from a performance standpoint, to organize them in a way that depends upon their content.
Ordering of Attributes: A tuple is best viewed as a mapping from its attributes (i.e., the names we give to the roles played by the values comprising the tuple) to the corresponding values. Hence, the order in which the attributes are listed in a table is irrelevant. (Note that, unfortunately, the set theoretic operations in relational algebra (at least how Elmasri& Navathe define them) make implicit use of the order of the attributes. Hence, E & N view attributes as being arranged as a sequence rather than a set.)
The Null value: used for don't know, not applicable. Interpretation of a Relation: Each relation can be viewed as a predicate and each tuple an assertion that that predicate is satisfied (i.e., has value true) for the combination of values in it. In other words, each tuple represents a fact.
Keep in mind that some relations represent facts about entities whereas others represent facts about relationships (between entities).
