SQL

Department of Information Technology 1Data base Technologies (ITB4201)
Introduction to
Structured Query Language (SQL)
Dr. C.V. Suresh Babu
Professor
Department of IT
Hindustan Institute of Science & Technology

Discussion Topics
• SQL Introduction
• Simple Queries in SQL
• Queries with more than one relation

SQL Introduction
Standard language for querying and manipulating data
Structured Query Language
Many standards out there:
• ANSI SQL
• SQL92 (a.k.a. SQL2)
• SQL99 (a.k.a. SQL3)
• Vendors support various subsets of these
• What we discuss is common to all of them

SQL
• Data Definition Language (DDL)
– Create/alter/delete tables and their attributes
• Data Manipulation Language (DML)
– Query one or more tables – discussed next !
– Insert/delete/modify tuples in tables
• Transact-SQL
– Idea: package a sequence of SQL statements  server

Data Definition Language
• The schema for each relation.
• The domain of values associated with each attribute.
• Integrity constraints
• And as we will see later, also other information such as
– The set of indices to be maintained for each relations.
– Security and authorization information for each relation.
– The physical storage structure of each relation on disk.
The SQL data-definition language (DDL) allows the specification of information about
relations, including:

Domain Types in SQL
• char(n). Fixed length character string, with user-specified length n.
• varchar(n). Variable length character strings, with user-specified
maximum length n.
• int. Integer (a finite subset of the integers that is machine-dependent).
• smallint. Small integer (a machine-dependent subset of the integer
domain type).
• numeric(p,d). Fixed point number, with user-specified precision of p
digits, with d digits to the right of decimal point. (ex., numeric(3,1),
allows 44.5 to be stores exactly, but not 444.5 or 0.32)
• real, double precision. Floating point and double-precision floating
point numbers, with machine-dependent precision.
• float(n). Floating point number, with user-specified precision of at
least n digits.

Tables in SQL
PName Price Category Manufacturer
Gizmo $19.99 Gadgets GizmoWorks
Powergizmo $29.99 Gadgets GizmoWorks
SingleTouch $149.99 Photography Canon
MultiTouch $203.99 Household Hitachi
Product
Attribute namesTable name
Tuples or rows

Tables Explained
• A tuple = a record
– Restriction: all attributes are of atomic type
• A table = a set of tuples
– Like a list…
– …but it is unordered: no first(), no next(), no last().

Create Table Construct
• An SQL relation is defined using the create table command:
create table r (A1 D1, A2 D2, ..., An Dn,
(integrity-constraint1),
...,
(integrity-constraintk))
– r is the name of the relation
– each Ai is an attribute name in the schema of relation r
– Di is the data type of values in the domain of attribute Ai
• Example:
create table instructor (
ID char(5),
name varchar(20),
dept_name varchar(20),
salary numeric(8,2))

Integrity Constraints in Create Table
• not null
• primary key (A1, ..., An )
• foreign key (Am, ..., An ) references r
Example:
create table instructor (
ID char(5),
name varchar(20) not null,
salary numeric(8,2),
primary key (ID),
foreign key (dept_name) references department);
primary key declaration on an attribute automatically ensures not null

And a Few More Relation Definitions
• create table student (
ID varchar(5),
name varchar(20) not null,
tot_cred numeric(3,0),
primary key (ID),
• create table takes (
ID varchar(5),
course_id varchar(8),
sec_id varchar(8),
semester varchar(6),
year numeric(4,0),
grade varchar(2),
primary key (ID, course_id, sec_id, semester, year) ,
foreign key (ID) references student,
foreign key (course_id, sec_id, semester, year) references
section);
– Note: sec_id can be dropped from primary key above, to ensure a
student cannot be registered for two sections of the same course in
the same semester

And more still
• create table course (
course_id varchar(8),
title varchar(50),
credits numeric(2,0),
primary key (course_id),

Updates to tables
• Insert
– insert into instructor values (‘10211’, ’Smith’, ’Biology’, 66000);
• Delete
– Remove all tuples from the student relation
• delete from student
• Drop Table
– drop table r
• Alter
– alter table r add A D
• where A is the name of the attribute to be added to relation r and D is the domain of A.
• All exiting tuples in the relation are assigned null as the value for the new attribute.
– alter table r drop A
• where A is the name of an attribute of relation r
• Dropping of attributes not supported by many databases.

Basic Query Structure
• A typical SQL query has the form:
select A1, A2, ..., An
from r1, r2, ..., rm
where P
– Ai represents an attribute
– Ri represents a relation
– P is a predicate.
• The result of an SQL query is a relation.

Tables Explained
• The schema of a table is the table name and
its attributes:
Product(PName, Price, Category, Manfacturer)
• A key is an attribute whose values are
unique;
we underline a key

SQL Query
Basic form: (plus many many more bells and whistles)
SELECT attributes
FROM relations (possibly multiple)
WHERE conditions (selections)

Simple SQL Query
SELECT *
FROM Product
WHERE category=‘Gadgets’
Product
Powergizmo $29.99 Gadgets GizmoWorks“selection”

Simple SQL Query
SELECT PName, Price, Manufacturer
FROM Product
WHERE Price > 100
Product
PName Price Manufacturer
SingleTouch $149.99 Canon
MultiTouch $203.99 Hitachi
“selection” and
“projection”

A Notation for SQL Queries
SELECT PName, Price, Manufacturer
FROM Product
WHERE Price > 100
Answer(PName, Price, Manfacturer)
Input Schema
Output Schema

Selections
What goes in the WHERE clause:
• x = y, x < y, x <= y, etc
– For number, they have the usual meanings
– For CHAR and VARCHAR: lexicographic ordering
• Expected conversion between CHAR and VARCHAR
– For dates and times, what you expect...
• Pattern matching on strings...

The LIKE operator
• s LIKE p: pattern matching on strings
• p may contain two special symbols:
– % = any sequence of characters
– _ = any single character
Product(PName, Price, Category, Manufacturer)
Find all products whose name mentions ‘gizmo’:
SELECT *
FROM Products
WHERE PName LIKE ‘%gizmo%’

Eliminating Duplicates
SELECT DISTINCT category
FROM Product
Compare to:
SELECT category
FROM Product
Category
Gadgets
Gadgets
Photography
Household
Category
Gadgets
Photography
Household

Ordering the Results
SELECT pname, price, manufacturer
FROM Product
WHERE category=‘gizmo’ AND price > 50
ORDER BY price, pname
Ordering is ascending, unless you specify the DESC keyword.
Ties are broken by the second attribute on the ORDER BY list, etc.

SELECT category
FROM Product
ORDER BY pname
?

SELECT DISTINCT category
FROM Product
ORDER BY category
Compare to:
Category
Gadgets
Household
Photography
SELECT category
FROM Product
ORDER BY pname ?

Joins in SQL
• Connect two or more tables:
Product
Company Cname StockPrice Country
GizmoWorks 25 USA
Canon 65 Japan
Hitachi 15 Japan
What is
the connection
between
them ?

Joins
Product (pname, price, category, manufacturer)
Company (cname, stockPrice, country)
Find all products under $200 manufactured in Japan;
return their names and prices.
SELECT pname, price
FROM Product, Company
WHERE manufacturer=cname AND country=‘Japan’
AND price <= 200
Join
between Product
and Company

Joins in SQL
Product
Company
Cname StockPrice Country
GizmoWorks 25 USA
Canon 65 Japan
Hitachi 15 Japan
PName Price
SingleTouch $149.99
SELECT pname, price
WHERE manufacturer=cname AND country=‘Japan’
AND price <= 200

Joins
Company (cname, stockPrice, country)
Find all countries that manufacture some product in the ‘Gadgets’
category.
SELECT country
WHERE manufacturer=cname AND category=‘Gadgets’

Joins in SQL
Name Price Category Manufacturer
Product
Company
Cname StockPrice Country
GizmoWorks 25 USA
Canon 65 Japan
Hitachi 15 Japan
SELECT country
WHERE manufacturer=cname AND category=‘Gadgets’
Country
??
??
What is
the problem ?
What’s the
solution ?

Joins
Purchase (buyer, seller, store, product)
Person(persname, phoneNumber, city)
Find names of people living in Chennai that bought some product in the
‘Gadgets’ category, and the names of the stores they bought such product
from
SELECT DISTINCT persname, store
FROM Person, Purchase, Product
WHERE persname=buyer AND product = pname AND
city=‘Chennai’ AND category=‘Gadgets’

When are two tables related?
• You guess they are
• Foreign keys are a method for schema designers to tell you so
– A foreign key states that a column is a reference to the key of another table
ex: Product.manufacturer is foreign key of Company
– Gives information and enforces constraint

Disambiguating Attributes
• Sometimes two relations have the same attr:
Person(pname, address, worksfor)
Company(cname, address)
SELECT DISTINCT pname, address
FROM Person, Company
WHERE worksfor = cname
SELECT DISTINCT Person.pname, Company.address
FROM Person, Company
WHERE Person.worksfor = Company.cname
Which
address ?

Tuple Variables
SELECT DISTINCT x.store
FROM Purchase AS x, Purchase AS y
WHERE x.product = y.product AND y.store = ‘BestBuy’
Find all stores that sold at least one product that the store
‘BestBuy’ also sold:
Answer (store)
Person(persname, phoneNumber, city)

Tuple VariablesGeneral rule:
tuple variables introduced automatically by the system:
Product (name, price, category, manufacturer)
Becomes:
Doesn’t work when Product occurs more than once:
In that case the user needs to define variables explicitly.
SELECT name
FROM Product
WHERE price > 100
SELECT Product.name
FROM Product AS Product
WHERE Product.price > 100

Meaning (Semantics) of SQL Queries
SELECT a1, a2, …, ak
FROM R1 AS x1, R2 AS x2, …, Rn AS xn
WHERE Conditions
1. Nested loops:
Answer = {}
for x1 in R1 do
for x2 in R2 do
…..
for xn in Rn do
if Conditions
then Answer = Answer  {(a1,…,ak)}
return Answer

Meaning (Semantics) of SQL Queries
SELECT a1, a2, …, ak
FROM R1 AS x1, R2 AS x2, …, Rn AS xn
WHERE Conditions
2. Parallel assignment
Doesn’t impose any order !
Answer = {}
for all assignments x1 in R1, …, xn in Rn do
if Conditions then Answer = Answer  {(a1,…,ak)}
return Answer

First Unintuitive SQLism
SELECT R.A
FROM R, S, T
WHERE R.A=S.A OR R.A=T.A
Looking for R (S T)
But what happens if T is empty? 

Exercises
Company (cname, stock price, country)
Person(per-name, phone number, city)
Ex #1: Find people who bought telephony products.
Ex #2: Find names of people who bought Chinese products
Ex #3: Find names of people who bought Chinese products and they
live in Chenai.
Ex #4: Find people who have both bought and sold something.
Ex #5: Find people who bought stuff from Joe or bought products
from a company whose stock prices is more than 5000.

SQL

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