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IS698: Database Management Min Song IS NJIT

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The Relational Data Model

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Data and Its Structure Data is actually stored as bits, but it is difficult to work with data at this level. It is convenient to view data at different levels of abstraction. Schema: Description of data at some level. Each level has its own schema.

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Levels of Abstraction View 3View 2View 1 Physical schema Conceptual schema payrollrecords billing External schemas

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Relational Model A particular way of structuring data (relations) Simple Mathematically based Expressions (queries) can be analyzed by DBMS Transformed to equivalent expressions automatically (query optimization)

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Relation Instance Relation is a set of tuples Tuple ordering immaterial No duplicates Cardinality of relation = number of tuples All tuples in a relation have the same structure; constructed from the same set of attributes

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Relation Instance (Example) Id Name Address Status Student

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Relation Mathematical entity corresponding to a table row ~ tuple column ~ attribute Values in a tuple are related to each other John lives at 123 Main Relation R can be thought of as predicate R R(x,y,z) is true iff tuple (x,y,z) is in R

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Relation Schema Relation name Attribute names and domains Integrity constraints - e.g.,: The values of a particular attribute in all tuples are unique The values of a particular attribute in all tuples are greater than 0

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Relational Database Finite set of relations Each relation consists of a schema and an instance Database schema = set of relation schemas (and other things) Database instance = set of (corresponding) relation instances

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Database Schema (Example) Student (Id: INT, Name: STRING, Address: STRING, Status: STRING) Professor (Id: INT, Name: STRING, DeptId: DEPTS) Course (DeptId: DEPTS, CrsName: STRING, CrsCode: COURSES) Transcript (CrsCode: COURSES, StudId: INT, Grade: GRADES, Semester: SEMESTERS) Department(DeptId: DEPTS, Name: STRING)

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Integrity Constraints Part of schema Restriction on state (or sequence of states) of data base Enforced by DBMS Intra-relational - involve only one relation Part of relation schema e.g., all Ids are unique Inter-relational - involve several relations Part of relation schema or database schema

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Key Constraint Values in a column (or columns) of a relation are unique: at most one row in a relation instance can contain a particular value(s) Key - set of attributes satisfying key constraint e.g., Id in Student, e.g., (StudId, CrsCode, Semester) in Transcript

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Key Constraint (con’t) Minimality - no subset of a key is a key (StudId, CrsCode) is not a key of Transcript Superkey - set of attributes containing key (Id, Name) is a superkey of Student Every relation has a key Relation can have several keys: primary key (Id in Student) – (cannot be null) candidate key ((Name, Address) in Student)

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Foreign Key Constraint Referential integrity => Item named in one relation must correspond to tuple(s) in another that describes the item Transcript (CrsCode) references Course(CrsCode ) Professor(DeptId) references Department(DeptId) a1 is a foreign key of R1 referring to a2 in R2 => if v is a value of a1, there is a unique tuple of R2 in which a2 has value v This is a special case of referential integrity: a2 must be a candidate key of R2 (CrsCode is a key of Course) If no row exists in R2 => violation of referential integrity Not all rows of R2 need to be referenced.: relationship is not symmetric (some course might not be taught) Value of a foreign key might not be specified (DeptId column of some professor might be null)

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Foreign Key Constraint (Example) a2 v3 v5 v1 v6 v2 v7 v4 a1 v1 v2 v3 v4 -- v3 R1R2 Foreign key Candidate key

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Foreign Key (con’t) Names of a1 and a2 need not be the same. With tables: ProfId attribute of Teaching references Id attribute of Professor R1 and R2 need not be distinct. Employee(Id:INT, MgrId:INT, ….) Employee(MgrId) references Employee(Id) Every manager is also an employee and hence has a unique row in Employee Teaching(CrsCode: COURSES, Sem: SEMESTERS, ProfId: INT) Professor(Id: INT, Name: STRING, DeptId: DEPTS)

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Relational Algebra Operators Expression Trees

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What is an “Algebra” Mathematical system consisting of: Operands --- variables or values from which new values can be constructed. Operators --- symbols denoting procedures that construct new values from given values.

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What is Relational Algebra? An algebra whose operands are relations or variables that represent relations. Operators are designed to do the most common things that we need to do with relations in a database. The result is an algebra that can be used as a query language for relations.

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Roadmap There is a core relational algebra that has traditionally been thought of as the relational algebra. But there are several other operators we shall add to the core in order to model better the language SQL --- the principal language used in relational database systems.

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Core Relational Algebra Union, intersection, and difference. Usual set operations, but require both operands have the same relation schema. Selection: picking certain rows. Projection: picking certain columns. Products and joins: compositions of relations. Renaming of relations and attributes.

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Selection R1 := SELECT C (R2) C is a condition (as in “if” statements) that refers to attributes of R2. R1 is all those tuples of R2 that satisfy C.

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Example Relation Sells: barbeerprice Joe’sBud2.50 Joe’sMiller2.75 Sue’sBud2.50 Sue’sMiller3.00 JoeMenu := SELECT bar=“Joe’s” (Sells): barbeerprice Joe’sBud2.50 Joe’sMiller2.75

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Projection R1 := PROJ L (R2) L is a list of attributes from the schema of R2. R1 is constructed by looking at each tuple of R2, extracting the attributes on list L, in the order specified, and creating from those components a tuple for R1. Eliminate duplicate tuples, if any.

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Example Relation Sells: barbeerprice Joe’sBud2.50 Joe’sMiller2.75 Sue’sBud2.50 Sue’sMiller3.00 Prices := PROJ beer,price (Sells): beerprice Bud2.50 Miller2.75 Miller3.00

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Product R3 := R1 * R2 Pair each tuple t1 of R1 with each tuple t2 of R2. Concatenation t1t2 is a tuple of R3. Schema of R3 is the attributes of R1 and R2, in order. But beware attribute A of the same name in R1 and R2: use R1.A and R2.A.

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Example: R3 := R1 * R2 R1(A,B ) 12 34 R2(B,C ) 56 78 910 R3(A,R1.B,R2.B,C ) 1256 1278 12910 3456 3478 34910

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Theta-Join R3 := R1 JOIN C R2 Take the product R1 * R2. Then apply SELECT C to the result. As for SELECT, C can be any boolean-valued condition. Historic versions of this operator allowed only A theta B, where theta was =, <, etc.; hence the name “theta-join.”

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Example Sells(bar,beer,price )Bars(name,addr ) Joe’sBud2.50Joe’sMaple St. Joe’sMiller2.75Sue’sRiver Rd. Sue’sBud2.50 Sue’sCoors3.00 BarInfo := Sells JOIN Sells.bar = Bars.name Bars BarInfo(bar,beer,price,name,addr ) Joe’sBud2.50Joe’sMaple St. Joe’sMiller2.75Joe’sMaple St. Sue’sBud2.50Sue’sRiver Rd. Sue’sCoors3.00Sue’sRiver Rd.

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Natural Join A frequent type of join connects two relations by: Equating attributes of the same name, and Projecting out one copy of each pair of equated attributes. Called natural join. Denoted R3 := R1 JOIN R2.

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Example Sells(bar,beer,price )Bars(bar,addr ) Joe’sBud2.50Joe’sMaple St. Joe’sMiller2.75Sue’sRiver Rd. Sue’sBud2.50 Sue’sCoors3.00 BarInfo := Sells JOIN Bars Note Bars.name has become Bars.bar to make the natural join “work.” BarInfo(bar,beer,price,addr ) Joe’sBud2.50Maple St. Joe’sMilller2.75Maple St. Sue’sBud2.50River Rd. Sue’sCoors3.00River Rd.

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Renaming The RENAME operator gives a new schema to a relation. R1 := RENAME R1(A1,…,An) (R2) makes R1 be a relation with attributes A1,…,An and the same tuples as R2. Simplified notation: R1(A1,…,An) := R2.

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Example Bars(name, addr ) Joe’sMaple St. Sue’sRiver Rd. R(bar, addr ) Joe’sMaple St. Sue’sRiver Rd. R(bar, addr) := Bars

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Building Complex Expressions Algebras allow us to express sequences of operations in a natural way. Example: in arithmetic --- (x + 4)*(y - 3). Relational algebra allows the same. Three notations, just as in arithmetic: 1.Sequences of assignment statements. 2.Expressions with several operators. 3.Expression trees.

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Sequences of Assignments Create temporary relation names. Renaming can be implied by giving relations a list of attributes. Example: R3 := R1 JOIN C R2 can be written: R4 := R1 * R2 R3 := SELECT C (R4)

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Expressions in a Single Assignment Example: the theta-join R3 := R1 JOIN C R2 can be written: R3 := SELECT C (R1 * R2) Precedence of relational operators: 1.Unary operators --- select, project, rename --- have highest precedence, bind first. 2.Then come products and joins. 3.Then intersection. 4.Finally, union and set difference bind last. wBut you can always insert parentheses to force the order you desire.

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Expression Trees Leaves are operands --- either variables standing for relations or particular, constant relations. Interior nodes are operators, applied to their child or children.

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Example Using the relations Bars(name, addr) and Sells(bar, beer, price), find the names of all the bars that are either on Maple St. or sell Bud for less than $3.

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As a Tree: BarsSells SELECT addr = “Maple St.” SELECT price<3 AND beer=“Bud” PROJECT name RENAME R(name) PROJECT bar UNION

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Example Using Sells(bar, beer, price), find the bars that sell two different beers at the same price. Strategy: by renaming, define a copy of Sells, called S(bar, beer1, price). The natural join of Sells and S consists of quadruples (bar, beer, beer1, price) such that the bar sells both beers at this price.

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The Tree Sells RENAME S(bar, beer1, price) JOIN PROJECT bar SELECT beer != beer1

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Relational Algebra on Bags A bag is like a set, but an element may appear more than once. Multiset is another name for “bag.” Example: {1,2,1,3} is a bag. {1,2,3} is also a bag that happens to be a set. Bags also resemble lists, but order in a bag is unimportant. Example: {1,2,1} = {1,1,2} as bags, but [1,2,1] != [1,1,2] as lists.

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Why Bags? SQL, the most important query language for relational databases is actually a bag language. SQL will eliminate duplicates, but usually only if you ask it to do so explicitly. Some operations, like projection, are much more efficient on bags than sets.

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Operations on Bags Selection applies to each tuple, so its effect on bags is like its effect on sets. Projection also applies to each tuple, but as a bag operator, we do not eliminate duplicates. Products and joins are done on each pair of tuples, so duplicates in bags have no effect on how we operate.

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Example: Bag Selection R(A,B )S(B,C ) 1234 5678 12 SELECT A+B<5 (R) =AB 12

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Example: Bag Projection R(A,B )S(B,C ) 1234 5678 12 PROJECT A (R) =A 1 5 1

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Example: Bag Product R(A,B )S(B,C ) 1234 5678 12 R * S =AR.BS.BC 1234 1278 5634 5678 1234 1278

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Example: Bag Theta-Join R(A,B )S(B,C ) 1234 5678 12 R JOIN R.B

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Bag Union Union, intersection, and difference need new definitions for bags. An element appears in the union of two bags the sum of the number of times it appears in each bag. Example: {1,2,1} UNION {1,1,2,3,1} = {1,1,1,1,1,2,2,3}

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Bag Intersection An element appears in the intersection of two bags the minimum of the number of times it appears in either. Example: {1,2,1} INTER {1,2,3} = {1,2}.

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Bag Difference An element appears in the difference A – B of bags as many times as it appears in A, minus the number of times it appears in B. But never less than 0 times. Example: {1,2,1} – {1,2,3} = {1}.

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Beware: Bag Laws != Set Laws Not all algebraic laws that hold for sets also hold for bags. For one example, the commutative law for union (R UNION S = S UNION R ) does hold for bags. Since addition is commutative, adding the number of times x appears in R and S doesn’t depend on the order of R and S.

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An Example of Inequivalence Set union is idempotent, meaning that S UNION S = S. However, for bags, if x appears n times in S, then it appears 2n times in S UNION S. Thus S UNION S != S in general.

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The Extended Algebra 1.DELTA = eliminate duplicates from bags. 2.TAU = sort tuples. 3.Extended projection : arithmetic, duplication of columns. 4.GAMMA = grouping and aggregation. 5.OUTERJOIN: avoids “dangling tuples” = tuples that do not join with anything.

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Duplicate Elimination R1 := DELTA(R2). R1 consists of one copy of each tuple that appears in R2 one or more times.

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Example: Duplicate Elimination R =AB 12 34 12 DELTA(R) =AB 12 34

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Extended Projection Using the same PROJ L operator, we allow the list L to contain arbitrary expressions involving attributes, for example: 1.Arithmetic on attributes, e.g., A+B. 2.Duplicate occurrences of the same attribute.

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Example: Extended Projection R =AB 12 34 PROJ A+B,A,A (R) =A+BA1A2 311 733

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Aggregation Operators Aggregation operators are not operators of relational algebra. Rather, they apply to entire columns of a table and produce a single result. The most important examples: SUM, AVG, COUNT, MIN, and MAX.

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Example: Aggregation R =AB 13 34 32 SUM(A) = 7 COUNT(A) = 3 MAX(B) = 4 AVG(B) = 3

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Grouping Operator R1 := GAMMA L (R2). L is a list of elements that are either: 1.Individual (grouping ) attributes. 2.AGG(A ), where AGG is one of the aggregation operators and A is an attribute.

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Applying GAMMA L (R) Group R according to all the grouping attributes on list L. That is, form one group for each distinct list of values for those attributes in R. Within each group, compute AGG(A ) for each aggregation on list L. Result has grouping attributes and aggregations as attributes. One tuple for each list of values for the grouping attributes and their group’s aggregations.

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Example: Grouping/Aggregation R =ABC 123 456 125 GAMMA A,B,AVG(C) (R) = ?? First, group R : ABC 123 125 456 Then, average C within groups: ABAVG(C) 12 4 45 6

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Outerjoin Suppose we join R JOIN C S. A tuple of R that has no tuple of S with which it joins is said to be dangling. Similarly for a tuple of S. Outerjoin preserves dangling tuples by padding them with a special NULL symbol in the result.

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Example: Outerjoin R = ABS =BC 1223 4567 (1,2) joins with (2,3), but the other two tuples are dangling. R OUTERJOIN S =ABC 123 45NULL NULL67

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