Why Study the Relational Model? The Relational Model Most widely used model. Vendors: IBM, Informix, Microsoft, Oracle, Sybase, etc. Legacy systems in older models E.G., IBM s IMS Recent competitor: object-oriented model ObjectStore, Versant, Ontos A synthesis emerging: object-relational model Informix Universal Server, UniSQL, O2, Oracle, DB2 Database Management Systems 1 Database Management Systems 2 Relational Database: Definitions Example Instance of Students Relation Relational database: a set of relations Relation: made up of 2 parts: Instance : a table, with rows and columns. #Rows = cardinality, #fields = degree / arity. Schema : specifies of relation, plus and type of each column. E.G. Students(sid: string, : string, login: string, age: integer, gpa: real). Can think of a relation as a set of rows or tuples (i.e., all rows are distinct). sid login age gpa 53666 Jones jones@cs 18 3.4 53688 Smith smith@eecs 18 3.2 53650 Smith smith@math 19 3.8 Cardinality = 3, degree = 5, all rows distinct Do all columns in a relation instance have to be distinct? Database Management Systems 3 Database Management Systems 4 Relational Query Languages The SQL Query Language A major strength of the relational model: supports simple, powerful querying of data. Queries can be written intuitively, and the DBMS is responsible for efficient evaluation. The key: precise semantics for relational queries. Allows the optimizer to extensively re-order operations, and still ensure that the answer does not change. Developed by IBM (system R) in the 1970s Need for a standard since it is used by many vendors Standards: SQL-86 SQL-89 (minor revision) SQL-92 (major revision) SQL-99 (major extensions, current standard) Database Management Systems 5 Database Management Systems 6
The SQL Query Language To find all 18 year old students, we can write: SELECT * FROM Students S WHERE S.age=18 sid login age gpa 53666 Jones jones@cs 18 3.4 53688 Smith smith@ee 18 3.2 To find just s and logins, replace the first line: SELECT S., S.login Querying Multiple Relations What does the following query compute? SELECT S., E.cid FROM Students S, Enrolled E WHERE S.sid=E.sid AND E.grade= A Given the following instance of Enrolled (is this possible if the DBMS ensures referential integrity?): sid cid grade 53831 Carnatic101 C 53831 Reggae203 B 53650 Topology112 A 53666 History105 B we get: S. Smith E.cid Topology112 Database Management Systems 7 Database Management Systems 8 Creating Relations in SQL Creates the Students relation. Observe that the type (domain) of each field is specified, and enforced by the DBMS whenever tuples are added or modified. As another example, the Enrolled table holds information about courses that students take. CREATE TABLE Students (sid: CHAR(20), : CHAR(20), login: CHAR(10), age: INTEGER, gpa: REAL) CREATE TABLE Enrolled (sid: CHAR(20), cid: CHAR(20), grade: CHAR(2)) Database Management Systems 9 Destroying and Altering Relations DROP TABLE Students Destroys the relation Students. The schema information and the tuples are deleted. ALTER TABLE Students ADD COLUMN firstyear: integer The schema of Students is altered by adding a new field; every tuple in the current instance is extended with a null value in the new field. Database Management Systems 10 Adding and Deleting Tuples Can insert a single tuple using: INSERT INTO Students (sid,, login, age, gpa) VALUES (53688, Smith, smith@ee, 18, 3.2) Can delete all tuples satisfying some condition (e.g., = Smith): DELETE FROM Students S WHERE S. = Smith * Powerful variants of these commands are available; more later! Database Management Systems 11 Integrity Constraints (ICs) IC: condition that must be true for any instance of the database; e.g., domain constraints. ICs are specified when schema is defined. ICs are checked when relations are modified. A legal instance of a relation is one that satisfies all specified ICs. DBMS should not allow illegal instances. If the DBMS checks ICs, stored data is more faithful to real-world meaning. Avoids data entry errors, too! Database Management Systems 12
Primary Key Constraints A set of fields is a key for a relation if : 1. No two distinct tuples can have same values in all key fields, and 2. This is not true for any subset of the key. Part 2 false? A superkey. If there s >1 key for a relation, one of the keys is chosen (by DBA) to be the primary key. E.g., sid is a key for Students. (What about?) The set {sid, gpa} is a superkey. Database Management Systems 13 Primary and Candidate Keys in SQL Possibly many candidate keys (specified using UNIQUE), one of which is chosen as the primary key. For a given student and course, there is a single grade. vs. Students can take only one course, and receive a single grade for that course; further, no two students in a course receive the same grade. Used carelessly, an IC can prevent the storage of database instances that arise in practice! CREATE TABLE Enrolled (sid CHAR(20) cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid,cid) ) CREATE TABLE Enrolled (sid CHAR(20) cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid), UNIQUE (cid, grade) ) Database Management Systems 14 Foreign Keys, Referential Integrity Foreign key : Set of fields in one relation that is used to `refer to a tuple in another relation. (Must correspond to primary key of the second relation.) Like a `logical pointer. E.g. sid is a foreign key referring to Students: Enrolled(sid: string, cid: string, grade: string) If all foreign key constraints are enforced, referential integrity is achieved, i.e., no dangling references. Can you a data model w/o referential integrity? Links in HTML! Foreign Keys in SQL Only students listed in the Students relation should be allowed to enroll for courses. CREATE TABLE Enrolled (sid CHAR(20), cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid,cid), FOREIGN KEY (sid) REFERENCES Students ) Enrolled sid cid grade 53666 Carnatic101 C 53666 Reggae203 B 53650 Topology112 A 53666 History105 B Students sid login age gpa 53666 Jones jones@cs 18 3.4 53688 Smith smith@eecs 18 3.2 53650 Smith smith@math 19 3.8 Database Management Systems 15 Database Management Systems 16 Enforcing Referential Integrity Consider Students and Enrolled; sid in Enrolled is a foreign key that references Students. What should be done if an Enrolled tuple with a non-existent student id is inserted? (Reject it!) What should be done if a Students tuple is deleted? Also delete all Enrolled tuples that refer to it. Disallow deletion of a Students tuple that is referred to. Set sid in Enrolled tuples that refer to it to a default sid. (In SQL, also: Set sid in Enrolled tuples that refer to it to a special value null, denoting `unknown or `inapplicable.) Similar if primary key of Students tuple is updated. Database Management Systems 17 Referential Integrity in SQL SQL/92 and SQL:1999 support all 4 options on deletes and updates. Default is NO ACTION (delete/update is rejected) CASCADE (also delete all tuples that refer to deleted tuple) SET NULL / SET DEFAULT (sets foreign key value of referencing tuple) CREATE TABLE Enrolled (sid CHAR(20), cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid,cid), FOREIGN KEY (sid) REFERENCES Students ON DELETE CASCADE ON UPDATE SET DEFAULT ) Database Management Systems 18
Where do ICs Come From? Logical DB Design: ER to Relational ICs are based upon the semantics of the real-world enterprise that is being described in the database relations. We can check a database instance to see if an IC is violated, but we can NEVER infer that an IC is true by looking at an instance. An IC is a statement about all possible instances! From example, we know is not a key, but the assertion that sid is a key is given to us. Key and foreign key ICs are the most common; more general ICs supported too. Entity sets to tables: CREATE TABLE ( CHAR(11), CHAR(20), INTEGER, PRIMARY KEY ()) Database Management Systems 19 Database Management Systems 20 Relationship Sets to Tables Review: Key Constraints In translating a relationship set to a relation, attributes of the relation must include: Keys for each participating entity set (as foreign keys). This set of attributes forms a superkey for the relation. All descriptive attributes. CREATE TABLE Works_In( CHAR(1), did INTEGER, since DATE, PRIMARY KEY (, did), FOREIGN KEY () REFERENCES, FOREIGN KEY (did) REFERENCES Departments) Each dept has at most one manager, according to the key constraint on Manages. since Manages d did budget Departments Translation to relational model? 1-to-1 1-to Many Many-to-1 Many-to-Many Database Management Systems 21 Database Management Systems 22 Translating ER Diagrams with Key Constraints Review: Participation Constraints Map relationship to a table: Note that did is the key now! Separate tables for and Departments. Since each department has a unique manager, we could instead combine Manages and Departments. CREATE TABLE Manages( CHAR(11), did INTEGER, since DATE, PRIMARY KEY (did), FOREIGN KEY () REFERENCES, FOREIGN KEY (did) REFERENCES Departments) CREATE TABLE Dept_Mgr( did INTEGER, d CHAR(20), budget REAL, CHAR(11), since DATE, PRIMARY KEY (did), FOREIGN KEY () REFERENCES ) Does every department have a manager? If so, this is a participation constraint: the participation of Departments in Manages is said to be total (vs. partial). Every did value in Departments table must appear in a row of the Manages table (with a non-null value!) since Manages Works_In did d Departments budget Database Management Systems 23 Database Management Systems since 24
Participation Constraints in SQL We can capture participation constraints involving one entity set in a binary relationship, but little else (without resorting to CHECK constraints). CREATE TABLE Dept_Mgr( did INTEGER, d CHAR(20), budget REAL, CHAR(11) NOT NULL, since DATE, PRIMARY KEY (did), FOREIGN KEY () REFERENCES, ON DELETE NO ACTION) Database Management Systems 25 Review: Weak Entities A weak entity can be identified uniquely only by considering the primary key of another (owner) entity. Owner entity set and weak entity set must participate in a one-to-many relationship set (1 owner, many weak entities). Weak entity set must have total participation in this identifying relationship set. Policy p Dependents Database Management Systems 26 cost age Translating Weak Entity Sets Weak entity set and identifying relationship set are translated into a single table. When the owner entity is deleted, all owned weak entities must also be deleted. CREATE TABLE Dep_Policy ( p CHAR(20), age INTEGER, cost REAL, CHAR(11) NOT NULL, PRIMARY KEY (p, ), FOREIGN KEY () REFERENCES, ON DELETE CASCADE) Review: ISA Hierarchies hourly_wages As in C++, or other PLs, attributes are inherited. If we declare A ISA B, every A entity is also considered to be a B entity. hours_worked Hourly_Emps ISA contractid Contract_Emps Overlap constraints: Can Joe be an Hourly_Emps as well as a Contract_Emps entity? (Allowed/disallowed) Covering constraints: Does every entity also have to be an Hourly_Emps or a Contract_Emps entity? (Yes/no) Database Management Systems 27 Database Management Systems 28 Translating ISA Hierarchies to Relations General approach: 3 relations:, Hourly_Emps and Contract_Emps. Hourly_Emps: Every employee is recorded in. For hourly emps, extra info recorded in Hourly_Emps (hourly_wages, hours_worked, ); must delete Hourly_Emps tuple if referenced tuple is deleted). Queries involving all employees easy, those involving just Hourly_Emps require a join to get some attributes. Alternative: Just Hourly_Emps and Contract_Emps. Hourly_Emps:,,, hourly_wages, hours_worked. Each employee must be in one of these two subclasses. Database Management Systems 29 Review: Binary vs. Ternary Relationships What are the additional constraints in the 2nd diagram? Bad design Covers p age Dependents policyid cost Database Management Systems 30 Policies policyid cost p age Dependents Purchaser Better design Policies Beneficiary
Binary vs. Ternary Relationships (Contd.) The key constraints allow us to combine Purchaser with Policies and Beneficiary with Dependents. Participation constraints lead to NOT NULL constraints. What if Policies is a weak entity set? CREATE TABLE Policies ( policyid INTEGER, cost REAL, CHAR(11) NOT NULL, PRIMARY KEY (policyid). FOREIGN KEY () REFERENCES, ON DELETE CASCADE) CREATE TABLE Dependents ( p CHAR(20), age INTEGER, policyid INTEGER, PRIMARY KEY (p, policyid). FOREIGN KEY (policyid) REFERENCES Policies, ON DELETE CASCADE) Database Management Systems 31 Views A view is just a relation, but we store a definition, rather than a set of tuples. CREATE VIEW YoungActiveStudents (, grade) AS SELECT S., E.grade FROM Students S, Enrolled E WHERE S.sid = E.sid and S.age<21 Views can be dropped using the DROP VIEW command. How to handle DROP TABLE if there s a view on the table? DROP TABLE command has options to let the user specify this. Database Management Systems 32 Views and Security Relational Model: Summary Views can be used to present necessary information (or a summary), while hiding details in underlying relation(s). Given YoungStudents, but not Students or Enrolled, we can find students s who have are enrolled, but not the cid s of the courses they are enrolled in. A tabular representation of data. Simple and intuitive, currently the most widely used. Integrity constraints can be specified by the DBA, based on application semantics. DBMS checks for violations. Two important ICs: primary and foreign keys In addition, we always have domain constraints. Powerful and natural query languages exist. Rules to translate ER to relational model Database Management Systems 33 Database Management Systems 34 Relational Query Languages Relational Algebra Query languages: Allow manipulation and retrieval of data from a database. Relational model supports simple, powerful QLs: Strong formal foundation based on logic. Allows for much optimization. Query Languages!= programming languages! QLs not expected to be Turing complete. QLs not intended to be used for complex calculations. QLs support easy, efficient access to large data sets. Database Management Systems 35 Database Management Systems 36
Formal Relational Query Languages Two mathematical Query Languages form the basis for real languages (e.g. SQL), and for implementation: Relational Algebra: More operational, very useful for representing execution plans. Relational Calculus: Lets users describe what they want, rather than how to compute it. (Nonoperational, declarative.) Database Management Systems 37 Preliminaries A query is applied to relation instances, and the result of a query is also a relation instance. Schemas of input relations for a query are fixed (but query will run regardless of instance!) The schema for the result of a given query is also fixed! Determined by definition of query language constructs. Positional vs. d-field notation: Positional notation easier for formal definitions, d-field notation more readable. Both used in SQL Database Management Systems 38 Example Instances Sailors and Reserves relations for our examples. We ll use positional or d field notation, assume that s of fields in query results are `inherited from s of fields in query input relations. S1 S2 sid bid day 22 101 10/10/96 58 103 11/12/96 sid s rating age 22 dustin 7 45.0 31 lubber 8 55.5 58 rusty 10 35.0 sid s rating age 28 yuppy 9 35.0 31 lubber 8 55.5 44 guppy 5 35.0 58 rusty 10 35.0 Database Management Systems 39 R1 Relational Algebra Basic operations: σ π Selection ( ) Selects a subset of rows from relation. Projection ( ) Deletes unwanted columns from relation. Cross-product ( ) Allows us to combine two relations. Set-difference ( ) Tuples in reln. 1, but not in reln. 2. Union ( ) Tuples in reln. 1 and in reln. 2. Additional operations: Intersection, join, division, renaming: Not essential, but (very!) useful. Since each operation returns a relation, operations can be composed! (Algebra is closed.) Database Management Systems 40 Projection Deletes attributes that are not in projection list. Schema of result contains exactly the fields in the projection list, with the same s that they had in the (only) input relation. Projection operator has to eliminate duplicates! (Why??) Note: real systems typically don t do duplicate elimination unless the user explicitly asks for it. (Why not?) s rating yuppy 9 lubber 8 guppy 5 rusty 10 π ( S2) s, rating age 35.0 55.5 π age ( S2) Selection Selects rows that satisfy selection condition. No duplicates in result! (Why?) Schema of result identical to schema of (only) input relation. Result relation can be the input for another relational algebra operation! (Operator composition.) sid s rating age 28 yuppy 9 35.0 58 rusty 10 35.0 π σ S rating >8 ( 2) s rating yuppy 9 rusty 10 σ ( ( S )) s, rating rating>8 2 Database Management Systems 41 Database Management Systems 42
Union, Intersection, Set-Difference All of these operations take two input relations, which must be union-compatible: Same number of fields. `Corresponding fields have the same type. What is the schema of result? sid s rating age 22 dustin 7 45.0 S1 S2 sid s rating age 22 dustin 7 45.0 31 lubber 8 55.5 58 rusty 10 35.0 44 guppy 5 35.0 28 yuppy 9 35.0 S1 S2 sid s rating age 31 lubber 8 55.5 58 rusty 10 35.0 S1 S2 Database Management Systems 43 Cross-Product Each row of S1 is paired with each row of R1. Result schema has one field per field of S1 and R1, with field s `inherited if possible. Conflict: Both S1 and R1 have a field called sid. (sid) s rating age (sid) bid day 22 dustin 7 45.0 22 101 10/10/96 22 dustin 7 45.0 58 103 11/12/96 31 lubber 8 55.5 22 101 10/10/96 31 lubber 8 55.5 58 103 11/12/96 58 rusty 10 35.0 22 101 10/10/96 58 rusty 10 35.0 58 103 11/12/96 Renaming operator: ρ ( C( 1 sid15, sid2), S1 R1) Database Management Systems 44 Joins Condition Join: R c S = σ c ( R S) (sid) s rating age (sid) bid day 22 dustin 7 45.0 58 103 11/12/96 31 lubber 8 55.5 58 103 11/12/96 S 1 R S1. sid < R1. sid 1 Result schema same as that of cross-product. Fewer tuples than cross-product, might be able to compute more efficiently Sometimes called a theta-join. Database Management Systems 45 Joins Equi-Join: A special case of condition join where the condition c contains only equalities. sid s rating age bid day 22 dustin 7 45.0 101 10/10/96 58 rusty 10 35.0 103 11/12/96 S1 R1 sid Result schema similar to cross-product, but only one copy of fields for which equality is specified. Natural Join: Equijoinon all common fields. Database Management Systems 46 Division Not supported as a primitive operator, but useful for expressing queries like: Find sailors who have reserved all boats. Let A have 2 fields, x and y; B have only field y: A/B = x x, y A y B { } i.e., A/B contains all x tuples (sailors) such that for every y tuple (boat) in B, there is an xy tuple in A. Or: If the set of y values (boats) associated with an x value (sailor) in A contains all y values in B, the x value is in A/B. In general, x and y can be any lists of fields; y is the list of fields in B, and x y is the list of fields of A. Database Management Systems 47 Examples of Division A/B sno s2 s2 s3 s4 s4 pno p1 p3 p4 p1 p4 A pno B1 sno s2 s3 s4 pno p4 B2 sno s4 pno p1 p4 B3 sno A/B1 A/B2 A/B3 Database Management Systems 48
Expressing A/B Using Basic Operators Find s of sailors who ve reserved boat #103 Division is not essential op; just a useful shorthand. (Also true of joins, but joins are so common that systems implement joins specially.) Idea: For A/B, compute all x values that are not `disqualified by some y value in B. x value is disqualified if by attaching y value from B, we obtain an xy tuple that is not in A. Disqualified x values: A/B: π x (( π x ( A) B) A) π x ( A) all disqualified tuples Database Management Systems 49 Solution 1: πs(( σ Re serves) Sailors) bid=103 Solution 2: ρ ( Temp1, σ Re serves) bid =103 ρ ( Tem, Temp1 Sailors) π s ( Tem) Solution 3: π s( σ (Re serves Sailors)) bid =103 Database Management Systems 50 Find s of sailors who ve reserved a red boat Information about boat color only available in Boats; so need an extra join: π s (( σ Boats serves Sailors color ' red ' ) Re ) = A more efficient solution: π s ( π π σ sid (( Boats s Sailors bid color ' red ' ) Re ) ) = A query optimizer can find this, given the first solution! Database Management Systems 51 Find sailors who ve reserved a red or a green boat Can identify all red or green boats, then find sailors who ve reserved one of these boats: ρ ( Tempboats,( σ )) color = ' red ' color = ' green' Boats π s ( Tempboats Re serves Sailors) Can also define Tempboats using union! (How?) What happens if is replaced by in this query? Database Management Systems 52 Find sailors who ve reserved a red and a green boat Find the s of sailors who ve reserved all boats Previous approach won t work! Must identify sailors who ve reserved red boats, sailors who ve reserved green boats, then find the intersection (note that sid is a key for Sailors): ρ ( Tempred, π (( σ Boats) Re serves)) sid color = ' red' ρ ( Tempgreen, π (( σ Boats) Re serves)) sid color = ' green ' π s (( Tempred Tempgreen) Sailors) Database Management Systems 53 Uses division; schemas of the input relations to / must be carefully chosen: ρ ( Tempsids,( π Re serves) / ( π )) sid, bid bid Boats π s ( Tempsids Sailors) To find sailors who ve reserved all Interlake boats:... / π ( σ ) bid b= ' Interlake' Boats Database Management Systems 54
Summary The relational model has rigorously defined query languages that are simple and powerful. Relational algebra is more operational; useful as internal representation for query evaluation plans. Several ways of expressing a given query; a query optimizer should choose the most efficient version. Database Management Systems 55