Structured Query Language

CHAPTER : STRUCTURED QUERY LANGUAGE

Chapter Objectives
At the end of this chapter, you should be able to:
 understand the general concepts of the relational model;

 explain the common integrity rules.

Essential Reading
An introduction to Database System Volume I (5th Edition), C.J.Date (1990), Addison
Wesley. [Chapter 11 - 12]
Modem Database Management (4th Edition), Fred McFadden & Jeffrey A. Hoffer (1994),
Benjamin/Cummings. [Chapter 8, page 283-316]

Useful Websites to learn Database and Programming:

http://erwinglobio.wix.com/ittraining
http://ittrainingsolutions.webs.com/
http://erwinglobio.sulit.com.ph/
http://erwinglobio.multiply.com/

Prof. Erwin M. Globio, MSIT 7-1

DB212 CHAPTER 7: STRUCTURED QUERY LANGUAGE

7-2 Prof. Erwin M. Globio, MSIT


7.1 Relational Data Structures and Concepts
Two-dimensional tables whose attributes values are atomic. This is to say that at every
row-and-column position within the table, there always exists precisely one value, never a list
of values. (Recalls: Relations do not contain repeating groups after INF.)
There are no duplicate rows. (Recall: There is always a primary key in a relation.)
Rows (tuples) are unordered.

S# SNAME STATUS CITY
S1 Smith 21 London
S2 Jones 10 Paris
S3 Mary 30 Paris
S4 Clark 20 Athens
<----------------------------------- Degree ------------------------------------->
Degree of a relation is the number of attributes in a relation.
Cardinality of a relation is the number of tuples in a relation.
Domain is a pool of values, from which one of more attributes draw their actual values. For
example, a domain of supplier numbers is the set of all possible supplier numbers and the
domain.

7.2 Integrity Rules
 The entity integrity rules No component of the primary key of a relation is allowed to
accept nulls. Bu "null", we mean that information is missing for some reason. Suppose in the
supplier relation, there is a tuple for which the supplier-no is null. This would be like saying
that there was a supplier in the real world that had no identity. This is absurd.
 The referential integrity rule The database must not contain any unmatched foreign key
values. By the term "unmatched foreign key value", we mean a non-null foreign key value for
which there does not exist a matching value of the primary key in the relevant target relation.
 The referential integrity rule says that if B references A, then A must exist.
Note: Foreign key and referential integrity concepts are defined in terms of one
another. Thus, "support for referential integrity" and "support for foreign keys" mean
exactly the same thing.)
The basic idea is as follows:
For each foreign key, there are three questions that need to be answered:
 Can that foreign key accept nulls? For example, does it make sense for a shipment to
exist for which the supplier is unknown?



 What should happen on an attempt to delete the target of a foreign key references?
For example, an attempt to delete a supplier for which there exists at least at one
matching shipment? Let us consider more definitely the following cases:
Restricted The delete operation is "restricted" to the case where there are
no such matching shipments (it is rejected otherwise)
Cascades The delete operation "cascades" to delete those matching
shipments also
Nullifies The foreign key is set to null in all such matching shipments
and the supplier is then deleted (of course, this case could not
apply if the foreign key cannot accept nulls in the first place)

 What should happen on an attempt to update the primary key of a foreign key
reference? For example, an attempt to update the supplier number for a supplier for
which there exists at least one matching shipment? Let us consider the following
cases:
Restricted The update operation is "restricted" to the case where there are
no such matching shipments (it is rejected otherwise)
Cascades The update operation "cascades" to updates the foreign key in
those matching shipments also
Nullifies The foreign key is set to null in all such matching shipments
and the supplier is then updated (of course, this case could not
apply if the foreign cannot accept nulls in the first place)

Note: Restricted-Cascaded-Nullifies options for the foreign key delete and update rules do
not exhaust the possibilities. They are just common practices.

7.3 SQL and Relational Model
SQL (Structured Query Language) is a relational language, which provides an interface
between the user and the RDBMS

7.3.1 A Brief Decomposition
SQL's capabilities are divided between the data description language (DDL) and the data
manipulation language (DML).

DDL
Create tables Query table data
Modify tables (ALTER) Modify table data
Create indexes - Insert
Drop tables - Delete
Drop indexes - Update
Authorization (GRANT) Aggregate functions
Deauthorisation (REVOKE)



The purpose of the DDL is to create entries in the catalogue (sometimes called the data
dictionary or, in some large systems, the repository). The catalogue is managed by the
RDBMS and it is a requirement of the Relational Model that the catalogue itself takes the
form of normalized tables. The update of these tables is quite transparent to the user.

7.4 Relational Data Definition Insql (DDL)

Data Types
DECIMAL (m,n) Signed numbers, where m is the total number of
digits to the right of the decimal point.
INTEGER Large (up to 11 digits) positive or negative
whole numbers
SMALL Small (5 or 6 digits, depending on the DBMS)
positive or negative whole numbers. By
specifying this data type, less storage space
is required
FLOAT (m,n) Whole and fractional numbers represented in
scientific notation, where m is the total number
of digits (including sign), and n is the number
of digits to the right of the decimal point.
CHAR (n) Alphanumeric (character) data, where n is the
maximum length for the character string; n
character positions are allocated to each
instance of a CHAR column.
DATE Calendar dates. A system variable can be used
to set the format for the date data type.
LOGICAL True or false values

DDL Commands Description
CREATE TABLE Define the structure of a new table
INSERT Places a new row in a table based on values
supplied in the statement, copies one or more
rows derived from other database into a table,
extracts data from one table and inserts them
into another.
UPDATE Changes values in one or more specified rows
of a table by replacing current values with
constant or the results of calculation.
DELETE Removes one of more rows from a table.
CREATE VIEW Names the view and optionally specifies
column names to be used in place of those of
the base table.
CREATE INDEX Defines an index on one column that enables
rapid access to the rows of a table in a sequence
or randomly by the key value.
ALTER TABLE Adds columns to an existing.
DROP TABLE Deletes table.
DROP INDEXES Deletes index.
GRANT Granting user access.
REVOKE Removing privileges given to a user.
(DBA privileges)



Illustrations: The following is the suppliers-parts-projects database

SUPPLIER PROJECTS

S# SNAME STATUS CITY J# JNAME CITY
S1 SMITH 20 LONDON J1 SORTER PARIS
S2 JONES 10 PARIS J2 PUNCH ROME
S3 BLAKE 30 PARIS J3 READER ATHENS
S4 CLARK 20 LONDON J4 CONSOLE ATHENS
S5 ADAMS 30 ATHENS J5 COLLATOR LONDON
J6 TERMINAL OSLO
J7 TAPE LONDON

PART
P# PNAME COLOR WEIGHT CITY
P1 NUT RED 12 LONDON
P2 BOLT GREEN 17 PARIS
P3 SCREW BLUE 17 ROME
P4 SCREW RED 14 LONDON
P5 CAM BLUE 14 PARIS
P6 COQ RED 19 LONDON

S# P# J# QTY S# P# QTY
S1 P1 J1 200 S1 P1 300
S1 P1 J4 700 S1 P2 200
S2 P3 J1 400 S1 P3 400
S2 P3 J2 200 S1 P4 200
S2 P3 J3 200 S1 P5 100
S2 P3 J4 500 S1 P6 100
S2 P3 J5 600 S2 P1 300
S2 P3 J6 400 S2 P2 400
S2 P3 J7 800 S3 P2 200
S2 P5 J2 100 S4 P2 200
S3 P3 J1 200 S4 P4 300
S3 P4 J2 500 S4 P5 400
S4 P6 J3 300
S5 P6 J7 300
S5 P6 J2 200
S5 P1 J4 100
S5 P3 J4 200
S5 P4 J4 800
S5 P5 J4 400
S5 P6 J4 500



Example:
 Adding data to a table
 Adding of one new row to a table
INSERT INTO SUPPLIE
VALUES ( "S6", "MARY" , 30, "NEW YORK" ) ;

 Transferring rows from external file to a database table
INSERT INTO SUPPLIER
SELECT *
FROM NEW-SUPPLIER
WHERE STATUS >20 ;

 Deleting database contents
Rows can be deleted individually or in groups.
 Deleting a row from a table
DELETE FROM PART
WHERE WEIGHT = 12 ;

 Deleting all rows of a table DELETE FROM PART ;
Note: Deletion must be done with care when rows from several relations are involved.
For example, if we deleted a SUPPLIER row or a PROJECT row, or a PART
row, we will have a referential integrity violation in the SPJ or SP tables.)

 Changing database contents
To update data in SQL, we must inform the DBMS what relation, columns and rows are
involved.
UPDATE SUPPLIER
SET CITY = ROME
WHERE SUPPLIER-NO = "S3" ;

 Constructing a View for more than one table
A view is also called a virtual table because it does not actually contain stored data,
combines data that is selected from one or more base tables of the other views. If the data
in the base tables changes, so does the information displayed using the view.
We can use a view to let someone access just the rows and columns that the person needs
from base tables. We can omit columns containing irrelevant or sensitive information.
For instance,
CREATE VIEW SUPPLIER-PART
(SUPPLIER-NO, PART-NO,QUANTITY)
AS SELECT S#, P#, QTY
FROM SP
WHERE QTY > = 200;



This will create an external presentation to the user on the table SP.
CREATE VIEW DEALS
(SUPPLIER-NAME, CITY, PROJRCT)
AS SELECT SNAME, SUPPLIER. CITY, JNAME
FROM SUPPLIER, PROJECT
WHERE SUPPLIER, CITY = PROJECT. CITY;
This view links suppliers with the project. The two tables are related using their common
CITY column.

 Creating indexes
You can create indexes in most RDBMSs to provide rapid random and sequential access
to base-table data.
For example, to create an index on the supplier table for the S#:
CREATE INDEX S-NO
ON SUPPLIER (S#);

 Changing table definitions
We may change table definitions in many SQL RDBMSs by ALTERING column
specifications.
For example, to add a new column ADDRESS in the SUPPLIER table:
ALTER TABLE SUPPLIER
ADD (ADDRESS CHAR (30) ) ;

 Deleting tables
Once a table is deleted, all the data will be deleted permanently and you cannot get it
back.
DROP VIEW tablename;

 Deleting views
Here, the TABLES(s) on which the VIEW is based will not be changed.
DROP INDEX indexname;



7.5 Basic SQL Construct (DML)
SQL's DML is divided into two parts, retrieval commands and updates commands. The
following are the eight operators used by the RM (Relational Model) for retrieval of data from
normalized relations:
Restrict Project
a X a x
b y b y
c c x
a y
b x
c y

Union Intersection Difference

These represent the eight original operators used by the relational model for retrieval model
for retrieval of data from normalization relations. Product, union, insertion and difference are
operators in a branch of mathematics called set theory. Restrict, project, join and divide are
operators in a branch of mathematics called relational theory.



SELECT Columns to be projected out eventually.
SELECT * To display all data in a table.
SELECT DISTINCT To display only one of the number of identical
rows.
SELCT <column name. .> To display data from specific rows.
WHERE Search condition for the rows.

SQL Logical Operations
NOT Select rows that do not meet the search condition.
AND Both conditions must be true to qualify the row.
OR One or both of the conditions must be true.

Predicates used:
BETWEEN Select column values that fall within a range.
In Selects column values that match one of a
number of values.
ANY Select rows whose values match any of the values
returned by the inner query.
ALL Select rows whose values have some relation to
all of the values returned by the inner query.
EXISTS The EXISTS predicate determines whether the
outer WHERE condition is true or false. If the
inner query returns any value, the WHERE
condition is true. Otherwise, the condition is
false. Thus a simple subquery that uses EXISTS
returns all rows if the outer WHERE condition is
true.
LIKE Selects column values that contain a character
string
Underscore (-) - represent any single
character.
Percent symbol (%) - represent no
characters or a number of characters.

GROUP BY Rearranges the rows extracted into partitions or
groups, such that within any one group, all rows
have the same value for the GROUP BY field.

HAVING Conditions applied to each group from previous
GROUP BY clause.

ORDER BY Sort required columns either in ascending or
descending order.

Aggregate functions: Number of values in the column
COUNT Sum of the values in the column
SUM Average of the values in the column
AVG Largest value in the column
MAX Smallest value in the column

7 - 10 Prof. Erwin M. Globio, MSIT


 Restrict
Restriction retrieves rows from a relation based upon selection criteria. Thus,
SELECT * (* means all columns)
FROM S (target table)
WHERE SCITY= `LONDON' ; (selection criterion)
will retrieve the following:

S# SNAME STATUS CITY
S1 SMITH 20 LONDON
S2 CLARK 20 LONDON

 Project
Projection retrieves columns form a relation based upon selection criteria. For example,
SELECT S#, SNAME (named columns)
FROM SUPPLIER
WHERE SUPPLIER.CITY = 1LONDON' ;
will retrieve the following:
S# SNAME
S1 SMITH
S2 CLARK

What would the following achieve?
SELECT STATUS
FROM SUPPLIER
WHERE SUPPLIER.CITY = `LONDON' :
The answer is
STATUS
20
20

Is this a proper relation? (Hint: it contains duplicate rows)
This can be solved easily by using the command called DISTINCT, i.e.
SELECT DISTINCT STATUS
FROM SUPPLIER
WHERE SUPPLIER.CITY = `LONDON' ;
which eliminates duplicates from the resultant relation.

Prof. Erwin M. Globio, MSIT 7 - 11


 Product
This is an expensive operation. For example, Let S is a relation with 5 rows; P is a
relation with 6 rows. Then the product S x P will contain 30 rows, i.e. [R1 (S)
R1(P), R1(S) R1(P), . . . R5(S) R6(P) ]
The importance of product is that it provides the basis for the relational operator join.
The following will result in a product of S, P
SELECT * (all columns)
FROM SUPLIER, PART; (from both tables SUPPLIER & PART)
Note: Such a product is called the Cartesian product and is specifically discouraged by
the Relational Model. For example, if 2 tables, each has a million entries, so if
combined, it would tie up computing resources.
Union
Union can be described as a collection of columns that exist in one relation, or both. Each
column must be of the same type and size and be drawn from the same domain. For example,
if we look at SUPPLIER and PART tables, only one column qualifies (CITY).
SELECT #
FROM SUPLIER
WHERE SUPPLIER.CITY = `LONDON'
UNION SELECT S#
FROM SP
WHERE P# = `P2' ;
This will extract S# for those suppliers who supply the part P2 or who are located to
LONDON, or both. The answer is
S#
S1
S2
S3
Duplicates are automatically eliminated. It should be noted that UNION has an associative
property inbuilt, i.e. A UNION B and B UNION A are identical. Similarly, (A UNION B) UNION
C is equal to (A UNION C) UNION B.
 Intersection
Two (or more) tables which share one (or more) union-compatible attributes may be
intersected. However, SQL requires special construct to achieve this, each involving what
is called sub-query.
For example, we went to list what suppliers have supplied (some) parts.
SELCT S#
FROM SUPPLIER
WHERE S# IN
(SELECT S#
FROM SP) ;
This will find all S# in SUPPLIER which match some S# in SP.



Is this operation equivalent to equijoin operation?
Note: The outer SELCT list is not restricted. Thus,
SELECT *
FROM SUPPLIER
WHERE S# IN
(SELECT S#
FROM SP) ;
is perfectly fine, since only S# in SUPPLIER and S# in SP are union compatible.
The Existential Quantifier ÈXISTS".
The above example can also be written as
SELECT S#
FROM S
WHERE EXISTS
(SELECT *
FROM SP
WHERE S.S# = SP.S#);
Note: The `SELECT *' in the subquery does not have the meaning `select all columns'
as you have been as far led to believe: it mean's `scan all rows'!)
The method of execution is as follows: `For each S# in S, scan all rows in SP until the
expression S.S# is satisfied the search is exhausted, which ever occurs first. Return `YES' if a
match is made, `NO' otherwise. If `YES', then the current S# in S is selected.
In other words, ÈXISTS' returns a binary valued response whereas ÌN' returns a relation.
ÈXISTS" can always be used in place of ÌN', but not vice versa.

 Difference
Difference is the negation of two relation. For example,
SELECT S#
FROM SUPPLIER
WHERE NOT S# IN
(SELECT S#
FROM SP) ;
(Think about: Is negation associative?)
 Divide
For example, SP divide by PART will give S1 only.
SP divide by P
S1 P1 P1
S1 P2 P2
S1 P3 P3
S1 P4 P4
S1 P5 P5
Since this is not commonly represented in SQL.



 Join
The purpose of a join is to create a new relation from two or more existing relations. In
SQL, this is achieved implicitly. This is no JOIN command in SQL. There are some rules
involved:
 In order for two tables to be joined, they must share an attribute whose values are
taken from the same domain.
 The result table must be a relation. Thus, if it has no primary key, the select list must
include `DISTINCT' to remove duplicate rows.
 Where the result table may contain duplicate columns (SQL does not eliminate
them), the SELECT * construct is therefore discouraged.
 The most meaningful join is one which joins a primary key to its foreign key(s).
(Recall: Normalization helps to decompose a complex model and JOIN is used to reconstruct
the original model.)
For example, we want to know for all suppliers, what parts and how many they have supplied.
SELECT SUPPLIER.S#, SNAME, P#, QTY
FROM SUPPLIER, SP (Tables to be joined)
WHERE SUPPLIER.S# = SP.S#;
If we want to extract the name of the part supplied, then
SELECT SUPPLIER.S#, SNAME, SP.P#,PNAME,QTY
FROM SUPPLIER, SP, PART
WHERE SUPPLIER.S# = SP.S#
AND SP.P# = PART.P# ;
We can also use the greater sign `>', or smaller sign `<', or even the not equal sign `<>' in the
WHERE condition.



7.6 Review Questions

1. What are the differences between equijoin and natural join. Illustrate with examples.
[4 marks]

2. a. Answer the following questions using the below table structures.

Student Course
Student-ID Numeric 5 Course-ID Character 5
Name Character 15 Title Character 20
Major Character 3 Instructor Character 10

Registration
Student-ID Numeric 5 Instructor
Name Character 15 Instructor Character 10
Major Character 3 Location Character 4

i. Write a full database description using the SQL data definition
language for the above table Student. [2 marks]
ii. Write a SQL command to add a new student "John" with student
number 12221 and majoring in "IT". [1 mark]
iii. Define the following view by SQL definition. [2 marks]

Student-ID Name Major Course-ID Grade

iv. Write a SQL command to change the grade for student number
38214 from 51 to 57 [2 marks]
v. Write a SQL command to display the average grade for each course.
[3 marks]
vi. Write a SQL command to count the number of students taught
by each instructor. [3 marks]
vii. Write a SQL command to display an ordered list of courses and the
respective instructors, of which all student's grades in those courses
are above 50. [3 marks]
b. Use SQL commands to illustrate:
i. Natural join [2 marks] ii.
Search condition where exact matches are necessary [2 marks]



Useful Websites to learn Database and Programming:

http://erwinglobio.wix.com/ittraining
http://ittrainingsolutions.webs.com/
http://erwinglobio.sulit.com.ph/
http://erwinglobio.multiply.com/


Structured Query Language

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