Relational Algebra Basic Operations Algebra of Bags 1

Relational Algebra Basic Operations Algebra of Bags 1

What is an “Algebra” u. Mathematical system consisting of: w Operands --- variables or values from which new values can be constructed. w Operators --- symbols denoting procedures that construct new values from given values. 2

What is Relational Algebra? u. An algebra whose operands are relations or variables that represent relations. u. Operators are designed to do the most common things that we need to do with relations in a database. w The result is an algebra that can be used as a query language for relations. 3

Core Relational Algebra u. Union, intersection, and difference. w Usual set operations, but both operands must have the same relation schema. u. Selection: picking certain rows. u. Projection: picking certain columns. u. Products and joins: compositions of relations. u. Renaming of relations and attributes. 4

Selection u. R 1 : = σC (R 2) w C is a condition (as in “if” statements) that refers to attributes of R 2. w R 1 is all those tuples of R 2 that satisfy C. 5

Example: Selection Relation Sells: bar Joe’s Sue’s Joe. Menu : = bar Joe’s beer Bud Miller σbar=“Joe’s”(Sells): beer Bud Miller price 2. 50 2. 75 2. 50 3. 00 price 2. 50 2. 75 6

Projection u. R 1 : = πL (R 2) w L is a list of attributes from the schema of R 2. w R 1 is constructed by looking at each tuple of R 2, extracting the attributes on list L, in the order specified, and creating from those components a tuple for R 1. w Eliminate duplicate tuples, if any. 7

Example: Projection Relation Sells: bar Joe’s Sue’s beer Bud Miller Prices : = πbeer, price(Sells): beer price Bud 2. 50 Miller 2. 75 Miller 3. 00 price 2. 50 2. 75 2. 50 3. 00 8

Extended Projection u Using the same πL operator, we allow the list L to contain arbitrary expressions involving attributes: 1. Arithmetic on attributes, e. g. , A+B>C. 2. Duplicate occurrences of the same attribute. 9

Example: Extended Projection R= (A 1 3 B) 2 4 πA+B->C, A, A (R) = C 3 7 A 1 1 3 A 2 1 3 10

Product u. R 3 : = R 1 Χ R 2 w Pair each tuple t 1 of R 1 with each tuple t 2 of R 2. w Concatenation t 1 t 2 is a tuple of R 3. w Schema of R 3 is the attributes of R 1 and then R 2, in order. w But beware attribute A of the same name in R 1 and R 2: use R 1. A and R 2. A. 11

Example: R 3 : = R 1 Χ R 2 R 1( A, 1 3 B) 2 4 R 2( B, 5 7 9 C) 6 8 10 R 3( A, 1 1 1 3 3 3 R 1. B, 2 2 2 4 4 4 R 2. B, C ) 5 6 7 8 9 10 12

Theta-Join u. R 3 : = R 1 ⋈C R 2 w Take the product R 1 Χ R 2. w Then apply σC to the result. u. As for σ, C can be any boolean-valued condition. w Historic versions of this operator allowed only A B, where is =, <, etc. ; hence the name “theta-join. ” 13

Example: Theta Join Sells( bar, Joe’s Sue’s beer, Bud Miller Bud Coors price ) 2. 50 2. 75 2. 50 3. 00 Bars( name, addr ) Joe’s Maple St. Sue’s River Rd. Bar. Info : = Sells ⋈Sells. bar = Bars. name Bars Bar. Info( bar, Joe’s Sue’s beer, Bud Miller Bud Coors price, 2. 50 2. 75 2. 50 3. 00 name, addr ) Joe’s Maple St. Sue’s River Rd. 14

Natural Join u. A useful join variant (natural join) connects two relations by: w Equating attributes of the same name, and w Projecting out one copy of each pair of equated attributes. u. Denoted R 3 : = R 1 ⋈ R 2. 15

Example: Natural Join Sells( bar, Joe’s Sue’s beer, Bud Miller Bud Coors price ) 2. 50 2. 75 2. 50 3. 00 Bars( bar, addr ) Joe’s Maple St. Sue’s River Rd. Bar. Info : = Sells ⋈ Bars Note: Bars. name has become Bars. bar to make the natural join “work. ” Bar. Info( bar, beer, price, addr ) Joe’s Bud 2. 50 Maple St. Joe’s Milller 2. 75 Maple St. Sue’s Bud 2. 50 River Rd. 16 Sue’s Coors 3. 00 River Rd.

Renaming u. The ρ operator gives a new schema to a relation. u. R 1 : = ρR 1(A 1, …, An)(R 2) makes R 1 be a relation with attributes A 1, …, An and the same tuples as R 2. u. Simplified notation: R 1(A 1, …, An) : = R 2. 17

Example: Renaming Bars( name, addr ) Joe’s Maple St. Sue’s River Rd. R(bar, addr) : = Bars R( bar, addr ) Joe’s Maple St. Sue’s River Rd. 18

Building Complex Expressions u Combine operators with parentheses and precedence rules. u Three notations, just as in arithmetic: 1. Sequences of assignment statements. 2. Expressions with several operators. 3. Expression trees. 19

Sequences of Assignments u. Create temporary relation names. u. Renaming can be implied by giving relations a list of attributes. u. Example: R 3 : = R 1 written: ⋈C R 2 can be R 4 : = R 1 Χ R 2 R 3 : = σC (R 4) 20

Expressions in a Single Assignment u Example: theta-join R 3 : = R 1 ⋈C R 2 can be written: R 3 : = σC (R 1 Χ R 2) u Precedence of relational operators: 1. [σ, π, ρ] (highest). 2. [Χ, ⋈]. 3. ∩. 4. [∪, —] 21

Expression Trees u. Leaves are operands --- either variables standing for relations or particular, constant relations. u. Interior nodes are operators, applied to their child or children. 22

Example: Tree for a Query u. 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. 23

As a Tree: ∪ ρ π σ π name addr = “Maple St. ” Bars R(name) σ bar price<3 AND beer=“Bud” Sells 24

Example: Self-Join u. Using Sells(bar, beer, price), find the bars that sell two different beers at the same price. u. Strategy: by renaming, define a copy of Sells, called S(bar, beer 1, price). The natural join of Sells and S consists of quadruples (bar, beer 1, price) such that the bar sells both beers at this price. 25

The Tree π σ bar beer != beer 1 ⋈ ρ S(bar, beer 1, price) Sells 26

Schemas for Results u. Union, intersection, and difference: the schemas of the two operands must be the same, so use that schema for the result. u. Selection: schema of the result is the same as the schema of the operand. u. Projection: list of attributes tells us the schema. 27

Schemas for Results --- (2) u. Product: schema is the attributes of both relations. w Use R. A, etc. , to distinguish two attributes named A. u. Theta-join: same as product. u. Natural join: union of the attributes of the two relations. u. Renaming: the operator tells the schema. 28

Relational Algebra on Bags u. A bag (or multiset ) is like a set, but an element may appear more than once. u. Example: {1, 2, 1, 3} is a bag. u. Example: {1, 2, 3} is also a bag that happens to be a set. 29

Why Bags? u. SQL, the most important query language for relational databases, is actually a bag language. u. Some operations, like projection, are more efficient on bags than sets. 30

Operations on Bags u. Selection applies to each tuple, so its effect on bags is like its effect on sets. u. Projection also applies to each tuple, but as a bag operator, we do not eliminate duplicates. u. Products and joins are done on each pair of tuples, so duplicates in bags have no effect on how we operate. 31

Example: Bag Selection R( A, 1 5 1 B ) 2 6 2 σA+B < 5 (R) = A 1 1 B 2 2 32

Example: Bag Projection R( A, 1 5 1 B ) 2 6 2 πA (R) = A 1 5 1 33

Example: Bag Product R( R A, 1 5 1 ΧS= B ) 2 6 2 A 1 1 5 5 1 1 R. B 2 2 6 6 2 2 S( B, 3 7 S. B 3 7 3 7 C 4 8 4 8 C ) 4 8 34

Example: Bag Theta-Join R( R ⋈ A, 1 5 1 B ) 2 6 2 R. B<S. B S= S( B, 3 7 C ) 4 8 A R. B S. B C 1 1 5 1 1 2 2 6 2 2 3 7 7 3 7 4 8 8 4 8 35

Bag Union u. An element appears in the union of two bags the sum of the number of times it appears in each bag. u. Example: {1, 2, 1} ∪ {1, 1, 2, 3, 1} = {1, 1, 1, 2, 2, 3} 36

Bag Intersection u. An element appears in the intersection of two bags the minimum of the number of times it appears in either. u. Example: {1, 2, 1, 1} ∩ {1, 2, 1, 3} = {1, 1, 2}. 37

Bag Difference u. 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. w But never less than 0 times. u. Example: {1, 2, 1, 1} – {1, 2, 3} = {1, 1}. 38

Beware: Bag Laws != Set Laws u. Some, but not all algebraic laws that hold for sets also hold for bags. u. Example: the commutative law for union (R ∪S = S ∪R ) does hold for bags. w 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. 39

Example: A Law That Fails u. Set union is idempotent, meaning that S ∪ S = S. u. However, for bags, if x appears n times in S, then it appears 2 n times in S ∪ S. u. Thus S ∪S != S in general. w e. g. , {1} ∪ {1} = {1, 1} != {1}. 40