2.2.2.Relational Database concept

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1 Foreign key:- is a field (or collection of fields) in one table that uniquely identifies a row of another table. In simpler words, the foreign key is defined in a second table, but it refers to the primary key in the first table Relational Database concept A relational database consists of a collection of tables, each of which assigned a unique name. The table consists of a number of rows. A row in a table represents a relation among a set of values. Figure2.6: explain the connection of two table Example : suppose there is a need to design a database to hold the history of each student of a school. The information of each student should have : - Student name - Student phone - Information about the student in each year in the school. This database should have at least two tables, the first one like the following schema: St Table-Schema=(st-name, st-phone) The second table contain the information about the history of the student : History-schema=(year, level, state) To link those two tables a primary key in the first table and a foreign key in the second table must be added to the tables as shown in figure

2 The new schemas will be : St Table-Schema=(S-ID, st-name, st-phone) History-schema =(S-ID, year, level, state) Figure2.7:relational between tables Relational database systems are expected to be equipped with a query language that can assist its users to query the database instances. There are two kinds of query languages: relational algebra_aql(algebra Query language), and relational calculus_ SQL(Structured Query language). Where SQL will study on the Practical lectures 18

3 2.2..Relational Algebra language (RAL) Relational algebra is a procedural query language, which takes instances of relations as input and yields instances of relations as output. It uses operators to perform queries. An operator can be either unary or binary. They accept relations as their input and yield relations as their output. Relational algebra is performed recursively on a relation and intermediate results are also considered relations. The fundamental operations of relational algebra are as follows: 1. Select 2. Project. Union 4. Intersection 5. Set different 6. Cartesian product 7. Rename We will discuss all these operations in the following sections. 1.Select Operation (σ) It selects tuples that satisfy the given predicate from a relation. Notation: σ p (r) Where σ stands for selection predicate and r stands for relation. p is prepositional logic formula which may use connectors like and, or, and not. These terms may use relational operators like: =,,, <, >,. 19

4 Example: - List all staff with a salary greater than 10,000. Output :- salary > (Staff) 20

5 2.Project Operation ( ) It projects column(s) that satisfy a given predicate. Notation: Π A1, A2, An (r) Where A1, A2, An are attribute names of relation r. Duplicate rows are automatically eliminated, as relation is a set. - Produce a list of salaries for all staff, showing only staffno, fname, lname, and salary details. Output:- staffno, fname, lname, salary(staff).union Operation ( ) It performs binary union between two given relations and is defined as: r s = { t t r or t s} Notion: r U s Where r and s are either database relations or relation result set (temporary relation). For a union operation to be valid, the following conditions must hold: 21

6 r and s must have the same number of attributes. Attribute domains must be compatible. Duplicate tuples are automatically eliminated. Example:- Table(1) Table(2) Output:- Table(1) Table(2) 4. Intersection operation - Defines a relation consisting of the set of all tuples that are in both R and S. - R and S must be union-compatible. - Expressed using basic operations: R S = R (R S) 22

7 Example:- Table(1) Table(2) Output:- Table(1) Table (2) 5.Set Difference ( ) The result of set difference operation is tuples, which are present in one relation but are not in the second relation. Notation: r s Finds all the tuples that are present in r but not in s. 2

8 Example:- Table(1) Table(2) Output:- Table(1) - Table(2) Table(2) - Table(1) 6.Cartesian Product (Χ) Combines information of two different relations into one. Notation: r Χ s 24

9 Where r and s are relations and their output will be defined as: r Χ s = { q t q r and t s} Example: - 7.Rename Operation (ρ) The results of relational algebra are also relations but without any name. The rename operation allows us to rename the output relation. rename operation is denoted with small Greek letter rho ρ. Notation: ρ x (E) Where the result of expression E is saved with name of x. Example: - consider the following invocation of ρ on an employee relation and the result of that invocation: 25

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