INTRODUCTION QUERY LANGUAGE
Lecture 9
Query Language
Relational Query Language is used by the user to communicate with the database
through it user requests information from the database.
Types of Relational Query Language
There are two types of relational query language:
 Procedural Query Language
 Non-Procedural Language
PROCEDURAL
The program code is written as a sequence of instructions. User has to specify “what
to do” and also “how to do” (step by step procedure).
 Relational Algebra is a Procedural Query Language
NON-PROCEDURAL LANGUAGE
The user has to specify only “what to do” and not “how to do”.
 It only gives a single Query on one or more tables to get.
SQL
 Lets you access or modify databases.
 SQL can execute queries,
o retrieve data
o insert records
o update records
o delete records
o create a new database, new tables, views, and set permissions on tables,
procedures, or views.
RELATIONAL QUERY LANGUAGE IN DBMS
 SQL has its own querying methods to interact with the database. These queries
work similarly to Relational Algebra and Relational Calculus.
 Relational Database systems are expected to have a query language that help
users to query the database.
RELATIONAL ALGEBRA
 It is a mathematical theory that uses algebraic structures to model data and
define queries. It's a set of mathematical operators that can compose, modify,
and combine tuples within different relations.
 Its operations work on one or more table to define another table without
changing the original table.
 Both operands and results are table, so output from one operation can
become input to another operation.
 Allows expressions to be nested, just as in arithmetic. This property is called
closure.
 Five basic operations in relational algebra: Selection, Projection, Cartesian
product, Union, and Set Difference.
 These perform most of the data retrieval operations needed.
 Also have Join, Intersection, and Division operations, which can be expressed
in terms of 5 basic operations.
SELECTION (OR RESTRICTION)

σpredicate(R)
o Works on a single relation(table) R and defines a relation that contains only
those rows of R that satisfy the specified condition (predicate).
 Notation − σp(r)
o σ stands for selection predicate
o r stands for relation
o p is prepositional logic (may use connectors like and, or, and not)
o May use relational operators like − =, ≠, ≥, < , >, ≤.
 List all staff with a salary greater than £10,000.
o σsalary > 1000(Staff)
PROJECTION
 ∏col1,…,coln(R)
o Works on a single relation R and defines a relation that contains a specific
columns of R, and eliminating duplicates.
o It projects column(s) that satisfy a given predicate.
 Notation − ∏A1, A2, An (r)
o Where A1, A2 , An are attribute names of relation r.
 Produce a list of salaries for all staff, showing only staffNo, fName, lName, and
salary details.
o ∏staffNo, fName, lName, salary(Staff)
UNION OPERATION
 Denoted by: r U s
 Defined as: r U s = {t | t ∈ r or t ∈ s}
 The result of r U s will include all tuples which are either in r or in s or in both.
 For r U s to be valid r and s must be union compatible
 Union operation is:
o Commutative: r U s = s U r
o Associative: r U (s U w) = (r U s) U w
 E.g. to find all the names of faculty and students in the FACULTY and STUDENT
tables:
o ∏name (FACULTY) (STUDENT) U ∏name
When two or more tables share the same number of columns, they are said union compatible
SET DIFFERENCE OPERATION
 Is denoted by: r – s
 Is defined as: r – s
= {t | t ∈ r and t ∈ s}
 The result of r – s will include all the tuples that are in r but not in s.
 r and s must be union compatible
 Neither Commutative nor Associative.
SET-INTERSECTION OPERATION
 Is denoted by: r ∩ s
 Is defined as: r ∩ s = { t | t ∈ r and t ∈ s }
 The result of r ∩ s will include all the tuples that are in both r and s.
 r and s must be union compatible.
 Intersection is:
o Commutative: r ∩ s = s ∩ r
o Associative: r ∩ (s ∩ w) = (r ∩ s) ∩ w
 Note: r ∩ s = r - (r - s)
CARTESIAN-PRODUCT OPERATION
 Is denoted by: r x s
 Is defined as: r x s = { t | t ∈ r and t ∈ s }
 The result of r x s will combine tuples from both r and s in a combinatorial fashion.
 Assume that attributes of r(A) and s(B) are disjoint. (That is, A ∩ B = Ø).
 If attributes of r(A) and s(B) are not disjoint, then renaming must be used.
CHARACTERISTICS OF CARTESIAN-PRODUCT OPERATION
 Degree r X s = degree(r) + degree(s)
 Cardinality of r X s = cardinality(r) * cardinality(s)
 Generally the result of CARTESIAN PRODUCT is meaningless unless is followed by
SELECT, and is called JOIN.
EXAMPLE QUERIES
The following Relations are used for the coming Examples.
 branch (branch-name, branch-city, assets)
 customer (customer-name, customer-street, customer-only)
 account (account-number, branch-name, balance)
 loan (loan-number, branch-name, amount)
 depositor (customer-name, account-number)
 borrower (customer-name, loan-number)
1. Find all loans of over $1200
T = Samount > 1200 (loan)
2. Find the loan number for each loan of an amount greater than $1200
T = ∏loan-number (Samount> 1200(loan))
3. Find the names of all customers who have a loan, an account, or both, from the
bank
T = ∏customer-name (borrower) U ∏customer-name (depositor)
4. Find the names of all customers who have a loan and an account at bank
T = ∏customer-name (borrower) ∩ ∏customer-name (depositor)