Transcript Creating And Modifying Database Tables

Creating and Modifying
Database Tables
Chapter 2
A Guide to Oracle9i
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Objectives
• Become acquainted with Structured Query
Language (SQL)
• Learn about Oracle9i user schemas
• Learn how to define Oracle9i database tables
• Create database tables using SQL*Plus
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Objectives
• Learn how to debug Oracle9i SQL commands
and use Oracle Corporation online help
resources
• Learn how to view information about your
database tables using Oracle9i data
dictionary views
• Modify and delete database tables using
SQL*Plus
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Introduction to SQL
• Structured Query Language (SQL): the
standard query language for relational
databases
• Data definition language (DDL)
– Create new database objects
– Modify or delete existing objects.
• Data manipulation language (DML)
– Insert, update, delete, and view database data.
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Oracle9i User Accounts
• User account - identified by a unique
username and password
• User schema - all of the objects that the user
creates and stores in the database
• Object owner has privileges to perform all
possible actions on an object
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Defining Oracle9i Database Tables
• To create a new table specify:
– Table name
– Name of each data field
– Data type and size of each data field
• Constraints: restrictions on the data values that a field can store
• Oracle Naming Standard:
– Series of rules Oracle Corporation established for naming all
database objects
– Objects must be from 1 to 30 characters long
– Can contain letters, numbers, and the special symbols $, _, and #
– Must begin with a character
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Creating a Table
CREATE TABLE tablename
(fieldname1data_type,
(fieldname2 data_type,
…)
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Oracle9i Data Types
• Data type: specifies the kind of data that a field
stores
• Assigning a data type provides a means for error
checking
• Data types enable the DBMS to use storage space
more efficiently by internally storing different types of
data in different ways
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Character Data Types
• VARCHAR2
– Stores variable-length character data up to a
maximum of 4,000 characters
– Values in different records can have a different
number of characters
– fieldname VARCHAR2(maximum_size)
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Character Data Types
• CHAR
– Fixed-length character data up to a maximum size
of 2,000 characters
– Data values for different records all have the same
number of characters
– DBMS adds trailing blank spaces to the end of the
entry to make the entry fill the maximum_size
value
– Data longer than maximum_size causes an error
– fieldname CHAR[(maximum_size)]
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Character Data Types
• NVARCHAR2 and NCHAR
– Analogous to VARCHAR2 and CHAR but use
Unicode rather than ASCII
– Used to hold character data in languages other
than English
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Number Data Types
• Stores negative, positive, fixed, and floating point
numbers between 10–130 and 10125, with precision
up to 38 decimal places
• General Syntax: fieldname NUMBER [([precision,]
[scale])]
• Integer: fieldname NUMBER(precision)
• Fixed point: fieldname NUMBER[([precision],[scale])]
• Floating point: fieldname NUMBER
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Date and Time Data Types
• DATE
– Dates from December 31, 4712 BC to December 31, 4712
AD
– Default format DD-MON-YY
– Default time format HH:MI:SS A.M.
– fieldname DATE
• TIMESTAMP
– Similar to DATE but stores fractional seconds
– fieldname TIMESTAMP (fractional_seconds_precision)
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Date and Time Data Types
• INTERVAL YEAR TO MONTH
– Time interval, expressed in years and months
– +02-11 specifies a positive time interval of 2 years and 11 months
– fieldname INTERVAL YEAR[(year_precision)] TO MONTH
• INTERVAL DAY TO SECOND
– Time interval, expressed in days, hours, minutes, and seconds
– -04 03:20:32.00: 4 days, 3 hours, 20 minutes, and 32 seconds
– fieldname INTERVAL DAY[(leading_precision)] TO
SECOND[(fractional_seconds_precision)]
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Large Object (LOB) Data Types
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Constraints
•
Rules that restrict the data values that you can enter into a field in a
database table
•
Integrity constraints: define primary and foreign keys
•
Value constraints: define specific data values or data ranges that
must be inserted into columns and whether values must be unique or
not NULL
•
Table constraint: restricts the data value with respect to all other
values in the table
•
Field constraint: limits the value that can be placed in a specific field,
irrespective of values that exist in other table records
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Oracle Constraint Naming
Convention
• tablename_ fieldname_constraintID
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Integrity Constraints
• Define primary key fields
• Specify foreign keys and their corresponding
table and column references
• Specify composite keys
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Value Constraints
• Check conditions: field value must be a specific value or fall
within a range of values
• NOT NULL constraints: specify whether a field value can be
NULL
• Default constraints: specify that a field has a default value that
the DBMS automatically inserts for every record, unless the user
specifies an alternate value
• Unique constraints: specify that a field must have a unique value
for every table record
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Creating Database Tables Using
SQL*Plus
• Type SQL commands at the SQL prompt
• End each command with a semicolon (;)
• Not case sensitive
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Log On to SQL*Plus
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SQL*Plus Program Window
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Create a Table
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Table Creation Sequence
• When creating tables with foreign key references,
create referenced tables first
• Always create tables without foreign keys before
those with foreign keys
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Using Notepad
• Useful to use Notepad to edit sql commands
– Commands can be edited without retyping
– Commands can be saved
– Saving multiple sql commands in a file creates a
script
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Errors
• When an error occurs error information is displayed:
–
–
–
–
Line number
Position on line
Error code
Description of error
• Error codes
– 3 letter prefix (I.e. ORA)
– 5 digit code
– More information on errors can be found at
http://otn.oracle.com
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Exiting SQL*Plus
• Three ways to exit SQL*Plus:
– Type exit at the SQL prompt
–Click File on the menu bar, and then click Exit
– Click the Close button on the program window title
bar
• Database session ends when SQL*Plus exits
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Create Table with Foreign Key
Constraint
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Viewing Information About Tables
• describe tablename: displays column names and
data types
• Data dictionary: tables that contain information
about the structure of the database.
– USER: shows the objects in the current user’s schema
– ALL: shows both objects in the current user’s schema and
objects that the user has privileges to manipulate
– DBA: allows users who are database administrators to view
information about all database objects
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Viewing Tables in the Database
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Viewing Constraints on One Table
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Modifying and Deleting Database
Tables
• Modify existing database tables by:
–
–
–
–
Changing the name of a table
Adding new columns
Deleting columns that are no longer needed
Changing the data type or maximum size of an existing
column
• Unrestricted action: some specifications can always
be modified
• Restricted action: specifications modified only in
certain situations
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Unrestricted Action
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Restricted Actions
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Deleting and Renaming Tables
• To delete:
– Drop table [tablename]
– Use with caution
– To delete foreign key constraints, add “cascade constraints”
• To rename:
– Rename old_tablename to new_tablename
– DBMS automatically transfers to the new table integrity
constraints, indexes, and privileges that referenced the old
table.
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Adding Fields to Existing Tables
• To add a field:
– ALTER TABLE tablename ADD(fieldname
data_declaration constraints);
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Modifying Existing Field Data
Definitions
• Can only change datatype to compatible data
type (i.e. varchar2 to char)
• ALTER tablename MODIFY(fieldname
new_data_declaration);
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Deleting a Field
• ALTER TABLE tablename DROP COLUMN
fieldname;
• Can be used to rename a field – first drop,
then add
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Adding and Deleting Constraints
• Add a constraint: ALTER TABLE tablename
ADD CONSTRAINT constraint_name
constraint_definition;
• Remove a constraint: ALTER TABLE
tablename DROP CONSTRAINT
constraint_name;
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Enabling and Disabling Constraints
• When modifying a database it can be useful to
disable constraints
• Constraints are enabled by default
• To disable a constraint: ALTER TABLE tablename
DISABLE CONSTRAINT constraint_name;
• To enable a constraint: ALTER TABLE tablename
ENABLE CONSTRAINT constraint_name;
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Summary
• SQL commands include:
– Data description language (DDL) commands: create, modify,
Deleted database objects
– Data manipulation language (DML) commands: insert,
update, delete, view database data
• To create a table:
– specify the table name, the name of each data field, and the
data type and size of each data field
• Data types ensure correct data values
• Constraints restrict values of database fields
• SQL*Plus commands are not case sensitive
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Summary
• Errors include line number, position, error
code
• Use DESCRIBE command to display a table’s
fieldnames and data types
• Tables can be modified or deleted but some
changes are restricted
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