Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Chapter 3 The Relational Data Model and Relational Database ConstraintsCopyright © 2011 Ramez Elmasri and Shamkant Navathe Chapter 3 Outline The Relational Data Model and Relational Database Constraints Relational Model Constraints and Relational Database Schemas Update Operations, Transactions, and Dealing with Constraint ViolationsCopyright © 2011 Ramez Elmasri and Shamkant Navathe The Relational Data Model and Relational Database Constraints Relational model First commercial implementations available in early 1980s Has been implemented in a large number of commercial system Hierarchical and network models Preceded the relational modelCopyright © 2011 Ramez Elmasri and Shamkant Navathe Relational Model Concepts Represents data as a collection of relations Table of values Row • Represents a collection of related data values • Fact that typically corresponds to a real-world entity or relationship • Tuple Table name and column names • Interpret the meaning of the values in each row attributeCopyright © 2011 Ramez Elmasri and Shamkant Navathe Relational Model Concepts (cont’d.)Copyright © 2011 Ramez Elmasri and Shamkant Navathe Domains, Attributes, Tuples, and Relations Domain D Set of atomic values Atomic Each value indivisible Specifying a domain Data type specified for each domainCopyright © 2011 Ramez Elmasri and Shamkant Navathe Domains, Attributes, Tuples, and Relations (cont’d.) Relation schema R Denoted by R(A1, A2, ...,An) Made up of a relation name R and a list of attributes, A1, A2, ..., An Attribute Ai Name of a role played by some domain D in the relation schema R Degree (or arity) of a relation Number of attributes n of its relation schemaCopyright © 2011 Ramez Elmasri and Shamkant Navathe Domains, Attributes, Tuples, and Relations (cont’d.) Relation (or relation state) Set of n-tuples r = {t1, t2, ..., tm} Each n-tuple t • Ordered list of n values t =<v1, v2, ..., vn • Each value vi, 1 ≤ i ≤ n, is an element of dom(Ai) or is a special NULL valueCopyright © 2011 Ramez Elmasri and Shamkant Navathe Domains, Attributes, Tuples, and Relations (cont’d.) Relation (or relation state) r(R) Mathematical relation of degree n on the domains dom(A1), dom(A2), ..., dom(An) Subset of the Cartesian product of the domains that define R: • r(R) ⊆ (dom(A1) × dom(A2) × ... × dom(An))Copyright © 2011 Ramez Elmasri and Shamkant Navathe Domains, Attributes, Tuples, and Relations (cont’d.) Cardinality Total number of values in domain Current relation state Relation state at a given time Reflects only the valid tuples that represent a particular state of the real world Attribute names Indicate different roles, or interpretations, for the domainCopyright © 2011 Ramez Elmasri and Shamkant Navathe Characteristics of Relations Ordering of tuples in a relation Relation defined as a set of tuples Elements have no order among them Ordering of values within a tuple and an alternative definition of a relation Order of attributes and values is not that important As long as correspondence between attributes and values maintainedCopyright © 2011 Ramez Elmasri and Shamkant Navathe Characteristics of Relations (cont’d.) Alternative definition of a relation Tuple considered as a set of (<attribute>, <value>) pairs Each pair gives the value of the mapping from an attribute Ai to a value vi from dom(Ai) Use the first definition of relation Attributes and the values within tuples are ordered Simpler notationCopyright © 2011 Ramez Elmasri and Shamkant Navathe Characteristics of Relations (cont’d.)Copyright © 2011 Ramez Elmasri and Shamkant Navathe Characteristics of Relations (cont’d.) Values and NULLs in tuples Each value in a tuple is atomic Flat relational model • Composite and multivalued attributes not allowed • First normal form assumption Multivalued attributes • Must be represented by separate relations Composite attributes • Represented only by simple component attributes in basic relational modelCopyright © 2011 Ramez Elmasri and Shamkant Navathe Characteristics of Relations (cont’d.) NULL values Represent the values of attributes that may be unknown or may not apply to a tuple Meanings for NULL values • Value unknown • Value exists but is not available • Attribute does not apply to this tuple (also known as value undefined)Copyright © 2011 Ramez Elmasri and Shamkant Navathe Characteristics of Relations (cont’d.) Interpretation (meaning) of a relation Assertion • Each tuple in the relation is a fact or a particular instance of the assertion Predicate • Values in each tuple interpreted as values that satisfy predicateCopyright © 2011 Ramez Elmasri and Shamkant Navathe Relational Model Notation Relation schema R of degree n Denoted by R(A1, A2, ..., An) Uppercase letters Q, R, S Denote relation names Lowercase letters q, r, s Denote relation states Letters t, u, v Denote tuplesCopyright © 2011 Ramez Elmasri and Shamkant Navathe Relational Model Notation Name of a relation schema: STUDENT Indicates the current set of tuples in that relation Notation: STUDENT(Name, Ssn, ...) Refers only to relation schema Attribute A can be qualified with the relation name R to which it belongs Using the dot notation R.ACopyright © 2011 Ramez Elmasri and Shamkant Navathe Relational Model Notation n-tuple t in a relation r(R) Denoted by t = <v1, v2, ..., vn> vi is the value corresponding to attribute Ai Component values of tuples: t[Ai] and t.Ai refer to the value vi in t for attribute Ai t[Au, Aw, ..., Az] and t.(Au, Aw, ..., Az) refer to the subtuple of values <vu, vw, ..., vz> from t corresponding to the attributes specified in the listCopyright © 2011 Ramez Elmasri and Shamkant Navathe Relational Model Constraints Constraints
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