Indexes in dbms

1. Bordoloi and Bock
Chapter 2 :TABLES AND INDEXES

2. Bordoloi and Bock
• One of the first steps in creating a
database is to create the tables that will
store organization’s data.
• In order to create a table , four pieces of
information must be determined:
1. The table name
2. The column (field) names
3. Column data types and
4. Column sizes
Table Creation

3. Bordoloi and Bock
• Table and Column names should be meaningful
and reflect the nature of the data that is to be
stored.
• If the data stored is about the products that a
firm sells , then the table should probably be
named product!
• If products are identified by a string of eight
characters, then the column that stores the
product information data should be named
product_number, or product_code.
Naming Tables and Columns

4. Bordoloi and Bock
Picking a Data type
• The data type chosen for a column determines
the nature of the data that can be stored in the
column.
• This is termed the Domain of valid column
values.
• Oracle provides 14 pre-determined data types as
well as the ability to declare user defined data
types.

5. Bordoloi and Bock
Data Types.
• CHAR - Used to store fixed-length, character
data.
• VARCHAR2 - Used to store variable-length,
character data that is up to 2000 characters per
column entry.
• LOB – Used to store large object data. A LOB
column can store either character or binary data
that is up to four gigabytes in size.
• LONG – Used to store variable-length,
character data that is up to two gigabytes in
size.
contd.

6. Bordoloi and Bock
Data Types
• NUMBER – Used to store numeric data values
of a specified precision and scale.
• DATE – Used to store valid dates.

7. Bordoloi and Bock
CREATING A TABLE
• Creating a simple table that stores five items of
information about employees for an
organization.
• The table is named “employee” and stores
information about each employee’s social
security number, last name, first name, date
hired, and annual salary.
Contd.

8. Bordoloi and Bock
Table Creation
CREATE TABLE employee (
emp_ssn CHAR(9),
emp_last_name VARCHAR2(25),
emp_first_name VARCHAR2(25),
emp_date_of_birth DATE,
emp_salary NUMBER(7,2)
);
• The table name “employee”, is specified
along with five data columns.
• Each column has a name that is unique within
the table and is specified to store a specific
type of data.

9. Bordoloi and Bock
Data Integrity and Table Constraints
• The term data integrity simply means that the
data stored in the table is valid.
• There are different types of data integrity, often
referred to as constraints.
• The specifications of different data types aids in
maintaining certain aspects of the data stored for
employees.

10. Bordoloi and Bock
NOT NULL Constraint
• A NOT NULL constraint means that a data row
must have a value for the column specified as
NOT NULL.
• A fairly standard practice is to assign each
constraint a unique constraint name.
• If constraints are not named, then Oracle assigns
meaningless system-generated names to each
constraint.

11. Bordoloi and Bock
Example
emp_last_name VARCHAR2(25)
CONSTRAINT nn_emp_last_name NOT NULL,
emp_first_name VARCHAR2(25)
CONSTRAINT nn_emp_first_name NOT NULL,
PRIMARY KEY Constraint
• Each table must normally contain a column or
set of columns that uniquely identifies rows of
data that are stored in the table. This column or
set of columns is referred to as the primary key.

12. Bordoloi and Bock
PRIMARY KEY Constraint
• If a table requires two or more columns in order
to identify each row , the primary key is termed
a composite primary key.
emp_ssn CHAR(9)
CONSTRAINT pk_employee
PRIMARY KEY,

13. Bordoloi and Bock
CHECK Constraint
• Sometimes the data values stored in a specific
column must fall within some acceptable range
of values.
• A CHECK constraint is used to enforce this data
limit.
emp_salary NUMBER(7,2)
CONSTRAINT ck_emp_salary
CHECK (emp_salary <= 85000),

14. Bordoloi and Bock
UNIQUE Constraint
• Sometimes it is necessary to enforce uniqueness
for a column value that is not primary key
column.
• The UNIQUE constraint can be used to enforce
this rule and Oracle rejects any rows that violate
the constraint.
• Assume that each parking space for the
organization is numbered and that no two
employees can be assigned same parking space.
emp_parking_space NUMBER(4)
CONSTRAINT un_emp_parking_space
UNIQUE,

15. Bordoloi and Bock
Commands to Manage, Drop and Alter Tables
Viewing a Table Description
• The SQL*PLUS DESCRIBE (DESC) command
can display the column names and data types for
any table.
• This command can be used when exact data
types and column sizes for a table are unknown.

16. Bordoloi and Bock
DESCRIBE Command
DESC employee;
Name Null? Type
------------------- -------- ------------
EMP_SSN NOT NULL CHAR(9)
EMP_LAST_NAME NOT NULL VARCHAR2(25)
EMP_FIRST_NAME NOT NULL VARCHAR2(25)
EMP_DATE_OF_BIRTH DATE
EMP_SALARY NOT NULL NUMBER(7,2)
EMP_PARKING_SPACE NUMBER(4)
• Note that while emp_ssn column was specified to have a
PRIMARY KEY constraint, the Null? Column displayed
in the table description indicates whether or not a
column is constrained as NOT NULL.

17. Bordoloi and Bock
Dropping a Table
• Employee table can be deleted with the DROP
TABLE command.
• This command deletes both the table structure,
its data, related constraints, and indexes.
DROP TABLE employee;

18. Bordoloi and Bock
Renaming a Table
• A table can be renamed with the RENAME
command.
• This command does not affect table structure or
data; it simply gives the current table a new
name.
RENAME employee TO worker;

19. Bordoloi and Bock
Adding and Altering a column for an
existing Table
• Modifying existing tables to either add
new columns or alter existing columns can
be accomplished with the ALTER TABLE
MODIFY and ALTER TABLE ADD
commands.
• The current data type of the
emp_parking_space column is
NUMBER(4). A very large organization
may have in excess of 9,999 employees.

20. Bordoloi and Bock
ALTER TABLE Command
• The ALTER TABLE command can be used to
modify the emp_parking_space column to
enable the allocation of upto 99,999 parking
spaces.
ALTER TABLE employee MODIFY
(emp_parking_space NUMBER(5));
• If a column modification will result in a column
that is smaller than was originally specified,
Oracle will return an error message if data rows
exist such that their data will not fit into the new
specified column size.

21. Bordoloi and Bock
ALTER TABLE Command
• The ALTER TABLE command can be used to
add a new column to the employee table.
• Suppose that an organization recognizes the
need to track the gender of employees in order to
meet a governmental reporting requirement, an
emp_gender column can be added to the
employee table.
ALTER TABLE employee ADD
(emp_gender CHAR(1));

22. Bordoloi and Bock
Relating Tables – Identifying Foreign Keys
• Normally, data rows in one table are related to
data rows in other tables.
• The department table will store information
about departments within the organization.
• Each department has a unique, 2-digit
department number, department name, location,
and primary telephone number for contacting the
department manager.
Contd.

23. Bordoloi and Bock
Identifying Foreign Keys
CREATE TABLE department (
dpt_no NUMBER(2)
CONSTRAINT pk_department PRIMARY KEY,
dpt_name VARCHAR2(20)
CONSTRAINT nn_dpt_name NOT NULL
);
• Employees are generally assigned to work in
departments. In an organization, at a given time,
an employee is assigned to a single department.
• In order to link rows in the employee table to
rows in department table we need to introduce a
new type of constraint, the FOREIGN KEY
constraint.

24. Bordoloi and Bock
Identifying Foreign Keys
• Foreign keys (FKs) are columns in one table that
reference primary key (PK) values in another or
in the same table.
• Lets relate employee rows to the PK column
named dpt_no in the department table.
• The value stored to the emp_dpt_number
column for any given employee row must match
a value stored in the dpt_no column in the
department table.

25. Bordoloi and Bock
Identifying Foreign Keys
CREATE TABLE employee (
emp_ssn CHAR(9)
CONSTRAINT pk_employee PRIMARY KEY,
emp_last_name VARCHAR2(25)
CONSTRAINT nn_emp_last_name NOT NULL,
emp_first_name VARCHAR2(25)
CONSTRAINT nn_emp_first_name NOT NULL,
emp_date_of_birth DATE,
emp_salary NUMBER(7,2)
CONSTRAINT ck_emp_salary
CHECK (emp_salary <= 85000),
emp_parking_space NUMBER(4)
CONSTRAINT un_emp_parking_space UNIQUE,
emp_gender CHAR(1),
emp_dpt_number NUMBER(2),
CONSTRAINT fk_emp_dpt FOREIGN KEY (emp_dpt_number)
REFERENCES department ON DELETE SET NULL
);

26. Bordoloi and Bock
MAINTAINING REFERRENTIAL INTEGRITY
• FOREIGN KEY constraints are also referred to
as referential integrity constraints, and assist in
maintaining database integrity.
• Referential integrity stipulates that values of a
foreign key must correspond to values of a
primary key in the table that it references. But
what happens if the primary key values change
or the row that is referenced is deleted?

27. Bordoloi and Bock
MAINTAINING REFERRENTIAL INTEGRITY
• Several methods exist to ensure that referential
integrity is maintained.
• ON DELETE SET NULL clause can be used to
specify that the value of emp_dpt_number
should be set to null if the referenced
department row is deleted.
• There are additional referential integrity
constraints that can be used to enforce database
integrity.

28. Bordoloi and Bock
MAINTAINING REFERRENTIAL INTEGRITY
On Update (Delete)
Restrict
Any update/delete made to the
department table that would delete or
change a primary key value will be
rejected unless no foreign key
references that value in the employee
table. This is the default constraint in
Oracle.
On Update (Delete)
Cascade
Any update/delete made to the
department table should be cascaded
through to the employee table.
On Update (Delete)
Set Null
Any values that are updated/deleted in
the department table cause affected
columns in the employee table to be set
to null.

29. Bordoloi and Bock
THE INSERT Command
• The INSERT command is used to store data in
tables.
• The INSERT command is often embedded in
higher-level programming language applications
as an embedded SQL command.
• There are two different forms of the INSERT
command.
• The first form is used if a new row will have a
value inserted into each column of the row.

30. Bordoloi and Bock
THE INSERT Command
• The general form of the INSERT command is
INSERT INTO table
VALUES (column1 value, column2
value, …);
• The second form of the INSERT command is
used to insert rows where some of the column
data is unknown (NULL).

31. Bordoloi and Bock
THE INSERT Command
• This form of the INSERT command requires
that you specify the names of the columns for
which data are being stored.
INSERT INTO employee (emp_ssn,
emp_last_name, emp_first_name)
VALUES ('999111111', 'Bock',
'Douglas');

32. Bordoloi and Bock
The DELETE Command
• The DELETE command is perhaps the
simplest of the SQL statements.
• It removes one or more rows from a table.
Multiple table delete operations are not
allowed in SQL.
• The syntax of the DELETE command is:
DELETE FROM table_name
[WHERE condition];

33. Bordoloi and Bock
The DELETE Command
• Since the WHERE clause is optional, you can
easily delete all rows from a table by omitting
a WHERE clause since the WHERE clause
limits the scope of the DELETE operation.
• For example, the DELETE FROM command
shown here removes all rows in the
assignment table.
DELETE FROM assignment;

34. Bordoloi and Bock
The UPDATE Command
• Values stored in individual columns of
selected rows can be modified (updated) with
the UPDATE command.
• Updating columns is different from altering
columns.
• The ALTER command changes the table
structure, but leaves the table data unaffected.
• The UPDATE command changes data in the
table, not the table structure.

35. Bordoloi and Bock
The UPDATE Command
• The general syntax of the UPDATE
command is:
UPDATE table
SET column = expression [,column =
expression]...
[WHERE condition];

36. Bordoloi and Bock
The COMMIT Command
• INSERT, UPDATE, and DELETE commands
are not committed to the database until the
COMMIT statement is executed.
• COMMIT is a transaction managing
command that confirms operations to the
database on the server (closing Oracle also
acts as a confirmation of the commands
entered).

37. Bordoloi and Bock
The ROLLBACK Command
• SQL command ROLLBACK (ROLL) can be
issued immediately to cancel any database
operations since the most recent COMMIT.
• Like COMMIT, ROLLBACK is also a
transaction managing command; however, it
cancels operations instead of confirming
them.

38. Bordoloi and Bock
INDEXES
• Indexes are optional structures associated
with tables.
• There are different types of indexes including
those used to enforce primary key constraints,
unique indexes, non-unique indexes,
concatenated indexes, and others.

39. Bordoloi and Bock
PRIMARY KEY indexes
• When a PRIMARY KEY constraint is
specified, Oracle will automatically create a
unique index to support rapid data retrieval
for the specified table.
• Without an index, a command that retrieves
data will cause Oracle to completely scan a
table for rows that satisfy the retrieval
condition.
• For example, consider the following SELECT
statement that will display a list of employees
assigned to a specific department.

40. Bordoloi and Bock
PRIMARY KEY indexes
SELECT emp_last_name,emp_first_name,
emp_dpt_number FROM employee
WHERE emp_dpt_number = 7;
• If the employee table is not indexed on the
emp_dpt_number column, Oracle will have to
scan the entire table in order to satisfy the
query.

41. Bordoloi and Bock
Creating an Index
• The general form of the CREATE INDEX
command is:
CREATE INDEX <index name>
ON <table name> (column1,
column2…);

42. Bordoloi and Bock
Creating a UNIQUE Index
• The general form of the CREATE UNIQUE
INDEX command is:
CREATE UNIQUE INDEX <index name>
ON <table name> (column1,
column2…);

43. Bordoloi and Bock
DROPPING Indexes
• An index should not be retained unless it
improves system processing in some fashion.
• All indexes on a table must be updated
whenever row data is changed that is
referenced by an index.
• Useless indexes burden the system by adding
unnecessary maintenance and by needlessly
occupying disk space. These indexes should
be dropped.

44. Bordoloi and Bock
DROPPING Indexes
• The syntax of the DROP INDEX command is
simple:
DROP INDEX index_name;