STRUCTURED QUERY LANGUAGE

Module Coordinator

Tan Szu TakSchool of Information and Communication Technology, Politeknik Brunei

Semester 3 2014

Contents

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History Data Definition Language Domain Types in SQL Create Table Construct Integrity Constraints in Create Table Creating Table Exercise Drop Table Constructs

*Words with underline can be clicked on

Contents

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Alter Table Constructs Data Manipulating Language The select Clause The where Clause The from Clause Cartesian Product The join Clause

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Natural Join The Rename Operation String Operations Ordering the Display of Tuples Where Clause Predicates Null Values Null Values and Three Valued Logic

*Words with underline can be clicked on

What are the fours types of Data Model covered in Lecture 2? Hierarchical Network Relational Object Oriented (OO)

In which data model introduced a powerful and flexible query language?

Recap

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In lecture 3 – Relational Data Model, we covered EIGHT relational algebra operators.

What are they? SELECT PROJECT UNION INTERSECT

Cont. Recap

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DIFFERENCE PRODUCT JOIN DIVIDE

SELECT Yields values for all rows found in a table that satisfy a

given condition.

PROJECT Yields values for all columns for selected attributes.

UNION Combines all rows from two tables, excluding duplicate

rows.

Cont. Recap

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JOIN (Natural Join) Links tables by selecting rows with common values in

common attributes.

DIVIDE Uses one single-column tables as the divisor and one

2-column table as the divident.

Cont. Recap

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IBM Sequel language was developed as part of the System R project at the IBM San Jose Research Laboratory.

Later renamed as Structured Query Language (SQL).

ANSI and ISO standard SQL (version):SQL-86, SQL-89, SQL-92, SQL:1999, SQL:2003, SQL:2008

Commercial systems offer most of SQL-92 features plus varying features sets from later standards.

History

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The SQL data-definition language (DDL) allows the specification of information about relations, including:

The schema for each relation.

The domain of values associated with each attribute.

Integrity constraints The set of indices to be maintained for each relations.

Security and authorization information for each relations.

The physical storage structure of each relation on disk.

Data Definition Language

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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).

Domain Types in SQL

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numeric(p,d) – Fixed point number, with user-specified precision of p digits, with d digits to the right of decimal point.

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.

Cont. Domain Types in SQL

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A SQL relation is defined using the create table command:

create table r (A1 D1, A2 D2, ..., An Dn ,(integrity-constraint1),...,(integrity-constraintk) );

Where: r is the name of the relation each A1 is an attribute name in the schema of relation r. D1 is the data type of values in the domain of attribute A1.

Create Table Construct

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Example:create table instructor (

ID char(5),name varchar(20) not null,dept_name varchar(20),salary numeric(8, 2));

insert into instructor values (‘10211’, ’Smith’, ’Biology’, 66000);insert into instructor values (‘10211’, null, ’Biology’, 66000);

Cont. Create Table Construct

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not null ◦attribute cannot be unknown/empty

primary key (A1, ..., An )◦primary key declaration on an attribute

automatically ensures not null

foreign key (Am, ..., An ) references r

Integrity Constraints in Create Table

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Example: Declare ID as the primary key for instructor.

create table instructor (ID char(5),name varchar(20) not null,dept_name varchar(20),salary numeric(8, 2),primary key (ID),foreign key (dept_name) references department);

Cont. Integrity Constraints in Create Table

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How to create a table in SQL with the following criteria?

dept_name refers to dept_name in department table.

Creating Table Exercise

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coursecourse_id (primary key) varchar - 8title varchar - 50dept_name (foreign key) varchar - 20credits numeric – 2 significant, 0

decimal point

create table course ( course_id varchar(8) primary key, title varchar(50), dept_name varchar(20), credits numeric(2,0), foreign key (dept_name) references department );

Primary key declaration can be combined with attribute declaration as shown above

Cont. Creating Table Exercise

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drop table student◦Deletes the table and its contents. (rollback

disable)

delete from student◦Deletes all contents of table, but retains table

(rollback enable)

truncate table student◦Deletes all contents of table, but retains table

(rollback disable)

Drop Table Constructs

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alter table◦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 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

Alter Table Constructs

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The SQL data-manipulation language (DML) provide the ability to: Query information Insert tuples Delete tuples Update tuples.

Data Manipulation Language

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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.

Cont. Data Definition Language

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The select clause list the attributes desired in the result of a query

◦corresponds to the projection operation of the relational algebra

Example: find the names of all instructors:select namefrom instructor

The select Clause

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NOTE: SQL names are case insensitive (i.e., you may use upper- or lower-case letters.)

◦E.g. Name ≡ NAME ≡ name◦Some people use upper case wherever we use bold

font.

SQL allows duplicates in relations as well as in query results.

To force the elimination of duplicates, insert the keyword distinct after select.

Cont. The select Clause

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E.g. Find the names of all departments with instructor, and remove duplicates

select distinct dept_namefrom instructor

The keyword all specifies that duplicates not be removed.

select all dept_namefrom instructor

Cont. The select Clause

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An asterisk in the select clause denotes “all attributes”

select *from instructor

The select clause can contain arithmetic expressions involving the operation, +, –, *, and /, and operating on constants or attributes of tuples.

Cont. The select Clause

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The query: select ID, name, salary/12 from instructor

would return a relation that is the same as the instructor relation, except that the value of the attribute salary is divided by 12.

Cont. The select Clause

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The where clause specifies conditions that the result must satisfy

◦Corresponds to the selection predicate of the relational algebra.

E.g. To find all instructors in Comp. Sci. dept with salary > 80000

select namefrom instructorwhere dept_name = ‘Comp. Sci.' and salary > 80000

The where Clause

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Comparison results can be combined using the logical connectives and, or, and not.

where dept_name = ‘Comp. Sci.' or salary > 80000

Comparisons can be applied to results of arithmetic expressions.

where salary/12 > 500

Cont. The where Clause

29Visit this link for more operators

The from clause lists the relations involved in the query◦Corresponds to the Cartesian product operation of

the relational algebra.

Find the Cartesian product instructor X teaches

select * from instructor, teaches◦generates every possible instructor – teaches pair,

with all attributes from both relations

Cartesian product not very useful directly, but useful combined with where-clause condition

The from Clause

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Cartesian Product: instructor X teaches

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instructor teaches

Join clause is used to combine rows from two or more tables, based on a common field between them.

Example: For all instructors who have taught some course, find their names and the course ID of the courses they taught.

select name, course_id from instructor, teaches where instructor.ID = teaches.ID

The join Clause

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Example: Find the course ID, semester, year and title of each course offered by the Comp. Sci. department

select section.course_id, semester, year, title from section, course where section.course_id = course.course_id and dept_name = ‘Comp. Sci.'

Cont. The join Clause

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Natural join matches tuples with the same values for all common attributes, and retains only one copy of each common column

select * from instructor natural join teaches;

Natural Join

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List the names of instructors along with the course ID of the courses that they taught.

select name, course_idfrom instructor, teacheswhere instructor.ID = teaches.ID;

select name, course_idfrom instructor natural join teaches;

Cont. Natural Join

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Tables below will be used as an example to describe natural join.

Cont. Natural Join

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1. PRODUCT

Cont. Natural Join

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2. SELECT

3. PROJECT

Cont. Natural Join

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Danger in natural join: beware of unrelated attributes with same name which get equated incorrectly.

List the names of instructors along with the titles of courses that they teach

Incorrect version (makes course.dept_name = instructor.dept_name)

select name, titlefrom instructor natural join teaches natural join course;

Cont. Natural Join

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Correct versionselect name, titlefrom instructor natural join teaches,

coursewhere teaches.course_id =

course.course_id;

Another correct versionselect name, titlefrom (instructor natural join teaches)

join course using(course_id);

Cont. Natural Join

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The SQL allows renaming relations and attributes using the as clause:

old-name as new-name

E.g. select ID, name, salary/12 as monthly_salaryfrom instructor

The Rename Operation

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Find the names of all instructors who have a higher salary than some instructor in ‘Comp. Sci’.

select distinct T. name from instructor as T, employee as S where T.salary > S.salary and S.dept_name = ‘Comp. Sci.’

Keyword as is optional and may be omitted instructor as T ≡ instructor TKeyword as must be omitted in Oracle

Cont. The Rename Operation

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SQL includes a string-matching operator for comparisons on character strings. The operator “like” uses patterns that are described using two special characters:

◦percent (%) - The % character matches any substring.

◦underscore (_) - The _ character matches any character.

String Operations

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Example: Find the names of all instructors whose name includes the substring “dar”.

select namefrom instructorwhere name like '%dar%'

Match the string “100 %”like ‘100 \%' escape '\‘

\ is used the escape character.

Cont. String Operations

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Patterns are case sensitive.

Pattern matching examples: ‘Intro%’ matches any string beginning with “Intro”.

‘%Comp%’ matches any string containing “Comp” as a substring.

‘_ _ _’ matches any string of exactly three characters.

‘_ _ _ %’ matches any string of at least three characters.

Cont. String Operations

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SQL supports a variety of string operations such as converting from upper to lower case (and vice

versa)

E.g. SELECT UCASE(CustomerName) AS Customer, City FROM Customers;

finding string length, extracting substrings, etc.

Cont. String Operations

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List in alphabetic order the names of all instructors select distinct name

from instructororder by name

We may specify desc for descending order or asc for ascending order, for each attribute; ascending order is the default. Example: order by name desc

Can sort on multiple attributesExample: order by dept_name, name

Ordering the Display of Tuples

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SQL includes a between comparison operator

Example: Find the names of all instructors with salary between $90,000 and $100,000 (that is, > $90,000 and < $100,000)

select name from instructor where salary between 90000 and 100000

Where Clause Predicates

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It is possible for tuples to have a null value, denoted by null, for some of their attributes

null signifies an unknown value or that a value does not exist.

The result of any arithmetic expression involving null is nullExample: 5 + null returns null

Null Values

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The predicate is null can be used to check for null values.

Example: Find all instructors whose salary is null.select namefrom instructorwhere salary is null

Any comparison with null returns unknownExample: 5 < null or null <> null or null = null

Cont. Null Values

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Three-valued logic using the truth value unknown:

OR: (unknown or true) = true, (unknown or false) = unknown (unknown or unknown) = unknown

AND: (true and unknown) = unknown, (false and unknown) = false, (unknown and unknown) = unknown

Null Values and Three Valued Logic

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NOT: (not unknown) = unknown◦“P is unknown” evaluates to true if predicate P

evaluates to unknown

Result of where clause predicate is treated as false if it evaluates to unknown

Cont. Null Values and Three Valued Logic

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Find out what is the difference between char VS varchar and varchar VS varchar2.

Find out does double precision mean.

Will cover: Structured Query Language IIAggregate Functions such as: Avg, Min, Max, Sum, Count

Group by, Having, Not Exists

Insert, Delete, Update tuples

Next Lecture

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Reference: Database System Concepts – 6th EditionLecture Slides by Tan Szu Tak, SICT, Politeknik Brunei