# CS2Bh: Current Technologies. Introduction to XML and Relational Databases. The Relational Model. The relational model

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1 CS2Bh: Current Technologies Introduction to XML and Relational Databases Spring 2005 The Relational Model CS2 Spring 2005 (LN6) 1 The relational model Proposed by Codd in It is the dominant data model today. Well-known relational DBMS: DB2, Oracle, Sybase, Ingres, ccess, SQLServer, FoxBase. In the relational data model we organize the data into tables. We don t (initially) worry about how these tables are implemented. In the rest of the lectures we are going to discuss Relational databases: Basics: relations, schemas Integrity constraints: keys, foreign keys, referential integrity Relational query languages Relational algebra SQL CS2 Spring 2005 (LN6) 2

2 What is a relational database? cid CS2 grade B Courses: cid cname credits instructor math 3 poe CS2 db 4 fan Question: can a table have duplicate rows? CS2 Spring 2005 (LN6) 3 Some terminology Column names: attributes, fields. Rows: tuple, records. Each attribute has values taken from a domain, e.g. the domain of is string, and the domain of is real. Domains: string, integer, real, date, etc -- atomic types table is called a relation, which is a set of tuples (records). The cardinality of a relation is the number of tuples in it. The degree (arity) of a relation is the number of fields. Questions: What are the cardinality and arity of relation Students? Can we put a Student tuple and a Courses tuple in the same relation? If not, how to prevent this? In other words, how to describe relations? CS2 Spring 2005 (LN6) 4

3 Describing relations relation is defined by a (relation) schema, which specifies domains of each field in the relation. Students (: string, : string, : real) Courses (cid: string, cname: string, credits: integer, instructor: string) Enroll (: string, cid: string, grade: string) (relation) schema is like a type, expressed in DDL (vs. type system). Therefore, it has many (possible) instances (values). relation (instance) is a table (a set of tuples). Example: Students table. (relational) database schema is a collection of (relation) schemas. relational database (instance) is a collection of tables, each has a distinct name. CS2 Spring 2005 (LN6) 5 questions re the following correct? relational database is a set of tuples. schema can be viewed as a type. Then, a relation of the student schema Students (: string, : string, : real) is a value of the type. CS2 Spring 2005 (LN6) 6

4 Collection types Sets: unordered, no duplicates. e.g., {1, 2, 3} = {3, 2, 1} = {1, 3, 2, 2} Bags: unordered, duplicates count. e.g., { 1, 2, 2, 3 } = { 2, 3, 1, 2 } { 1, 2, 3 } <> { 1, 2, 3, 3 } Lists: ordered, allowing duplicates. e.g.: [1, 2, 3] <> [2, 3, 1] [1, 2, 3, 3] <> [1, 2, 3] CS2 Spring 2005 (LN6) 7 Integrity constraints (ICs) IC: condition that must be true for any instance of the database. Example: a schema specifies domains of the fields in the relation, and is called domain constraints. ICs are specified when schema is defined. Note: this might not be true in Web databases. ICs are checked when relations are modified. legal instance of a relation is one that satisfies all specific ICs. CS2 Spring 2005 (LN6) 8

5 Integrity constraints (cont d) DBMS does not allow illegal instances. If the DBMS checks ICs, stored data is more faithful to realworld meaning, that is, for consistency and accuracy. When ICs are enforced, DBMS also avoids data entry errors. Remarks: For relational (and object-oriented) databases, domain constraints are a must! Contrast this with XML DTD There are many other common integrity constraints, which are given below. CS2 Spring 2005 (LN6) 9 key constraints set of fields is a (candidate) key for a relation if: no two distinct tuples can have same values in all key fields, any proper subset of the key does not satisfy this condition. Example: is a key for Students. Question: What about? What is a key of Enroll? cid CS2 grade B CS2 Spring 2005 (LN6) 10

6 Pri key constraints If a set of fields satisfies condition 1 but does not satisfy condition 2, then it is called a superkey for the relation. If there are more than one (candidate) keys for a relation, one is chosen to be the pri key. Example: is a pri key for Students. The set {, } is a superkey. cid grade B CS2 CS2 Spring 2005 (LN6) 11 More on pri keys There may be many candidate keys, but only one pri key. Given a relation schema, the pri key of the schema is underlined. Students (: string, : string, : real) Enroll (: string, cid: string, grade: string) Questions: re the following claims correct? Every relation has at least one key. Every relation has one and only one pri key. Every relation has one and only one superkey. CS2 Spring 2005 (LN6) 12

7 questions Recall that relation Courses is specified by the schema: Courses(cid: string, cname: string, credits: integer, instructor:string) What is the (reasonable) pri key of this relation? Can we insert a tuple (, john, ) into Students? CS2 Spring 2005 (LN6) 13 Foreign keys, referential integrity (1) Foreign key: set of fields in one relation that is used to refer to a tuple in another relation. This set must correspond to the pri key of the second relation. Example: is a foreign key in Enroll referring to Students. Students (: string, : string, : real) Enroll (: string, cid: string, grade: string) cid grade B CS2 CS2 Spring 2005 (LN6) 14

8 Foreign keys, referential integrity (2) Understanding: foreign keys vs. logical pointers. Foreign key constraints: Let S1 be a foreign key in relation R1 referring to R2. Then the values of S1 in R1 must match (be a subset of ) the values of S1 in R2. Question: does the following satisfy the foreign key constraint? cid grade B CS2 Spring 2005 (LN6) 005 CS2 15 Foreign keys, referential integrity (3) Referential integrity: the database does not include any invalid foreign key value. That is, all foreign key constraints are enforced. Questions: Why foreign key constraints? This is analogous to the dangling pointer problem in programming languages. Can you name a data model without referential integrity? Links in HTML. CS2 Spring 2005 (LN6) 16

9 Enforcing referential integrity Recall that in Enroll is a foreign key that references Students. What should be done if an Enroll tuple with a non-existent student is inserted? Reject it! What should we do if a Students tuple is deleted? lso delete all Enroll tuples that refer to it (CSCDE) Disallow deletion of a Students tuple that is referred to (NO CTION) Set in Enroll tuples that refer to it to a default (SET DEFULT). What should we do if pri key of Students tuple is updated? same as the deletion case CS2 Spring 2005 (LN6) 17 Example Recall that is the pri key of Students, and in Enroll is a foreign key that references Students. Question: what should we do when we insert (,, 3.8) into Students? insert (004, CS2, ) into Enroll? delete (,, ) from Students? modify to 004 in Students? cid CS2 grade B CS2 Spring 2005 (LN6) 18

10 Where do ICs come from? ICs are based upon the semantics of the real-world enterprises that is being described in the database relations. We can check a database instance to see if an IC is violated, but we can t infer if an IC is true by looking at an instance. n IC is a statement about all possible instances! Key and foreign key constraints are the most common ones; more general ICs are supported too. Why ICs? Database design, query optimization, updates, and much more! CS2 Spring 2005 (LN6) 19 Sum what you should remember! What is a relational database? Tuple? ttribute? Field? Domain? Schema? Database schema? Instance? What are integrity constraints? Why study them? How to decide whether an instance is legal? Where do ICs come from? What integrity constraints are a must for relational databases? Name two other common ICs. Pri key constraints. What? Why? Questions: re the following claims correct? o Every relation has at least one key. o Every relation has one and only one pri key. o Every relation has one and only one superkey. Foreign key constraints. What? Why? CS2 Spring 2005 (LN6) 20

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