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Object Types This query lists in a neatly formatted output, the count by object types for the objects that happen to exist in this particular database: SQL> select object_type,countobjec

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17 þ B If AUDIT_TRAIL is set to NONE, there will be no standard database

auditing

ý A, C, and D A is wrong because auditing the SYS user is in addition to standard database auditing C is wrong because standard database auditing

will record access to the object, regardless of whether any rows were retrieved

D is wrong because audits of parameter changes require an instance restart,

not audits of commands

PART II

SQL

■ Chapter 7 DDL and Schema Objects

■ Chapter 8 DML and Concurrency

■ Chapter 9 Retrieving, Restricting, and Sorting Data Using SQL

■ Chapter 10 Single-Row and Conversion Functions

■ Chapter 11 Group Functions

■ Chapter 12 SQL Joins

■ Chapter 13 Subqueries and Set Operators

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CHAPTER 7

DDL and Schema Objects

Exam Objectives

In this chapter you will learn to

• 051.10.1 Categorize the Main Database Objects

• 051.10.2 Review the Table Structure

• 051.10.3 List the Data Types That Are Available for Columns

• 051.10.4 Create a Simple Table

• 051.10.5 Explain How Constraints Are Created at the Time of Table Creation

• 051.10.6 Describe How Schema Objects Work

• 052.8.1 Create and Modify Tables

• 052.8.2 Manage Constraints

• 052.8.3 Create Indexes

• 052.8.4 Create and Use Temporary Tables

• 051.11.1 Create Simple and Complex Views

• 051.11.2 Retrieve Data from Views

• 051.11.3 Create, Maintain, and Use Sequences

• 051.11.4 Create and Maintain Indexes

• 051.11.5 Create Private and Public Synonyms

259

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In terms of the sheer number of exam objectives covered in this chapter, it looks horrific Do not worry: there is some duplication in the objectives, and many of the objectives are revisited in other chapters as well

Understanding the primitive data types and the standard heap-organized table structure is the first topic Then the chapter moves on to defining the object types that are dependent on tables (indexes, constraints, and views), and then sequences and synonyms Objects of all these types will be used throughout the remainder of this book, sometimes with more detail provided

Categorize the Main Database Objects

There are various object types that can exist within a database, many more with the current release than with earlier versions All objects have a name and a type, and

each object is owned by a schema Various common object types and the rules to

which they must conform will be discussed

Object Types

This query lists (in a neatly formatted output), the count by object types for the objects that happen to exist in this particular database:

SQL> select object_type,count(object_type) from dba_objects

group by object_type order by object_type;

OBJECT_TYPE COUNT(OBJECT_TYPE) OBJECT_TYPE COUNT(OBJECT_TYPE)

CLUSTER 10 PACKAGE 1240

CONSUMER GROUP 12 PACKAGE BODY 1178

CONTEXT 6 PROCEDURE 118

DIMENSION 5 PROGRAM 17

DIRECTORY 9 QUEUE 37

EDITION 1 RESOURCE PLAN 7

EVALUATION CONTEXT 13 RULE 1

FUNCTION 286 RULE SET 21

INDEX 3023 SCHEDULE 2

INDEX PARTITION 342 SEQUENCE 204

INDEXTYPE 12 SYNONYM 26493

JAVA CLASS 22018 TABLE 2464

JAVA DATA 322 TABLE PARTITION 199

JAVA RESOURCE 820 TRIGGER 413

JOB 11 TYPE 2630

JOB CLASS 11 TYPE BODY 231

LIBRARY 177 UNDEFINED 6

LOB 769 VIEW 4669

LOB PARTITION 7 WINDOW 9

MATERIALIZED VIEW 3 WINDOW GROUP 4

OPERATOR 60 XML SCHEMA 93

42 rows selected.

This query addresses the view DBA_OBJECTS, which has one row for every object

in the database The numbers are low, because the database is a very small one used only for teaching A database used for a business application might have hundreds of

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thousands of objects You may not be able to see the view DBA_OBJECTS, depending

on what permissions your account has Alternate views are USER_OBJECTS, which

will show all the objects owned by you, and ALL_OBJECTS, which will show all the

objects to which you have been granted access (including your own) All users have

access to these views

The objects of greatest interest to a SQL programmer are those that contain, or

give access to, data These include: Tables, Views, Synonyms, Indexes, and Sequences

Tables basically store data in rows segmented by columns A view is a stored

SELECT statement that can be referenced as though it were a table It is nothing more

than a query, but rather than running the statement itself, the user issues a SELECT

statement against the view instead In effect, the user is selecting from the result of

another selection A synonym is an alias for a table (or a view) Users can execute SQL

statements against the synonym, and the database will map them into statements

against the object to which the synonym points Indexes are a means of improving

access times to rows in tables If a query requires only one row, then rather than

scanning the entire table to find the row, an index can provide a pointer to the row’s

exact location Of course, the index itself must be searched, but this is often faster

than scanning the table A sequence is a construct that generates unique numbers

There are many cases where unique numbers are needed Sequences issue numbers in

order, on demand: it is absolutely impossible for the same number to be issued twice

The remaining object types are less commonly relevant to a SQL programmer

Their use falls more within the realm of PL/SQL programmers and database

administrators

Naming Schema Objects

A schema object is owned by a user and must conform to certain rules:

• The name may be between 1 to 30 characters long (with the exception of

database link names that may be up to 128 characters long)

• Reserved words (such as SELECT) cannot be used as object names

• All names must begin with a letter of the alphabet

• Object names can only include letters, numbers, the underscore (_), the dollar

sign ($), or the hash symbol (#)

• Lowercase letters will be automatically converted to uppercase

By enclosing the name within double quotes, all these rules (with the exception

of the length) can be broken, but to get to the object subsequently, it must always be

specified with double quotes, as in the examples in Figure 7-1 Note that the same

restrictions also apply to column names

EXAM TIP Object names must be no more than 30 characters The

characters can be letters, digits, underscore, dollar, or hash

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Although tools such as SQL*Plus and SQL Developer will automatically convert lowercase letters to uppercase unless the name is enclosed within double quotes, remember that object names are always case sensitive In this example, the two tables are completely different:

SQL> create table lower(c1 date);

Table created.

SQL> create table "lower"(col1 varchar2(2));

Table created.

SQL> select table_name from dba_tables where lower(table_name) = 'lower'; TABLE_NAME

-lower

LOWER

TIP While it is possible to use lowercase names and nonstandard characters

(even spaces), it is considered bad practice because of the confusion it can cause

Object Namespaces

It is often said that the unique identifier for an object is the object name, prefixed with the schema name While this is generally true, for a full understanding of naming

it is necessary to introduce the concept of a namespace A namespace defines a group

of object types, within which all names must be uniquely identified—by schema and name Objects in different namespaces can share the same name

These object types all share the same namespace:

Private synonyms Stand-alone procedures Stand-alone stored functions

Figure 7-1 Using double quotes to use nonstandard names

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Thus it is impossible to create a view with the same name as a table—at least, it is

impossible if they are in the same schema And once created, SQL statements can address

a view as though it were a table The fact that tables, views, and private synonyms share

the same namespace means that you can set up several layers of abstraction between

what the users see and the actual tables, which can be invaluable for both security and

for simplifying application development

These object types each have their own namespace:

Thus it is possible (though perhaps not a very good idea) for an index to have the

same name as a table, even within the same schema

EXAM TIP Within a schema, tables, views, and synonyms cannot have the

same names

Exercise 7-1: Determine What Objects Are Accessible to Your

Session In this exercise, query various data dictionary views as user HR to determine

what objects are in the HR schema and what objects in other schemas HR has access to

1 Connect to the database with SQL*Plus or SQL Developer as user HR

2 Determine how many objects of each type are in the HR schema:

select object_type,count(*) from user_objects group by object_type;

The USER_OBJECTS view lists all objects owned by the schema to which the

current session is connected, in this case HR

3 Determine how many objects in total HR has permissions on:

select object_type,count(*) from all_objects group by object_type;

The ALL_OBJECTS view lists all objects to which the user has some sort of

access

4 Determine who owns the objects HR can see:

select distinct owner from all_objects;

List the Data Types That Are Available

for Columns

When creating tables, each column must be assigned a data type, which determines

the nature of the values that can be inserted into the column These data types are also

used to specify the nature of the arguments for PL/SQL procedures and functions

When selecting a data type, you must consider the data that you need to store and the

operations you will want to perform upon it Space is also a consideration: some data

types are fixed length, taking up the same number of bytes no matter what data is

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actually in it; others are variable If a column is not populated, then Oracle will not give it any space at all If you later update the row to populate the column, then the row will get bigger, no matter whether the data type is fixed length or variable

The following are the data types for alphanumeric data:

VARCHAR2 Variable-length character data, from 1 byte to 4KB The data is stored in

the database character set.

NVARCHAR2 Like VARCHAR2, but the data is stored in the alternative national language

character set, one of the permitted Unicode character sets.

CHAR Fixed-length character data, from 1 byte to 2KB, in the database character set

If the data is not the length of the column, then it will be padded with spaces.

TIP For ISO/ANSI compliance, you can specify a VARCHAR data type, but any

columns of this type will be automatically converted to VARCHAR2

The following are the data types for numeric data, all variable length:

NUMBER Numeric data, for which you can specify precision and scale The precision

can range from 1 to 38, the scale can range from –84 to 127.

FLOAT This is an ANSI data type, floating-point number with precision of

126 binary (or 38 decimal) Oracle also provides BINARY_FLOAT and BINARY_DOUBLE as alternatives.

INTEGER Equivalent to NUMBER, with scale zero.

The following are the data types for date and time data, all fixed length:

DATE This is either length zero, if the column is empty, or 7 bytes All DATE

data includes century, year, month, day, hour, minute, and second The valid range is from January 1, 4712 BC to December 31, 9999 AD.

TIMESTAMP This is length zero if the column is empty, or up to 11 bytes, depending

on the precision specified Similar to DATE, but with precision of up to

9 decimal places for the seconds, 6 places by default.

TIMESTAMP WITH

TIMEZONE

Like TIMESTAMP, but the data is stored with a record kept of the time zone to which it refers The length may be up to 13 bytes, depending

on precision This data type lets Oracle determine the difference between two times by normalizing them to UTC, even if the times are for different time zones.

TIMESTAMP

WITH LOCAL

TIMEZONE

Like TIMESTAMP, but the data is normalized to the database time zone

on saving When retrieved, it is normalized to the time zone of the user process selecting it.

INTERVAL YEAR

TO MONTH

Used for recording a period in years and months between two DATEs

or TIMESTAMPs.

INTERVAL DAY TO

SECOND

Used for recording a period in days and seconds between two DATEs

or TIMESTAMPs.

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The following are the large object data types:

CLOB Character data stored in the database character set, size effectively unlimited:

4GB multiplied by the database block size.

NCLOB Like CLOB, but the data is stored in the alternative national language

character set, one of the permitted Unicode character sets.

BLOB Like CLOB, but binary data that will not undergo character set conversion by

Oracle Net.

BFILE A locator pointing to a file stored on the operating system of the database

server The size of the files is limited to 4GB.

LONG Character data in the database character set, up to 2GB All the functionality

of LONG (and more) is provided by CLOB; LONGs should not be used in a modern database, and if your database has any columns of this type, they should

be converted to CLOB There can only be one LONG column in a table.

LONG RAW Like LONG, but binary data that will not be converted by Oracle Net Any

LONG RAW columns should be converted to BLOBs.

The following are RAW and ROWID data types:

RAW Variable-length binary data, from 1 byte to 4KB Unlike the CHAR and

VARCHAR2 data types, RAW data is not converted by Oracle Net from the database’s character set to the user process’s character set on SELECT or the other way on INSERT.

ROWID A value coded in base 64 that is the pointer to the location of a row in a table

Within it is the exact physical address ROWID is an Oracle proprietary data type, not visible unless specifically selected.

EXAM TIP All examinees will be expected to know about these data types:

VARCHAR2, CHAR, NUMBER, DATE, TIMESTAMP, INTERVAL, RAW, LONG,

LONG RAW, CLOB, BLOB, BFILE, and ROWID Detailed knowledge will also

be needed for VARCHAR2, NUMBER, and DATE

The VARCHAR2 data type must be qualified with a number indicating the maximum

length of the column If a value is inserted into the column that is less than this, it is

not a problem: the value will only take up as much space as it needs If the value is

longer than this maximum, the INSERT will fail with an error If the value is updated

to a longer or shorter value, the length of the column (and therefore the row itself)

will change accordingly If is not entered at all or is updated to NULL, then it will take

up no space at all

The NUMBER data type may optionally be qualified with a precision and a scale

The precision sets the maximum number of digits in the number, and the scale is how

many of those digits are to the right of the decimal point If the scale is negative, this has

the effect of replacing the last digits of any number inserted with zeros, which do not

count toward the number of digits specified for the precision If the number of digits

exceeds the precision, there will be an error; if it is within the precision but outside the

scale, the number will be rounded (up or down) to the nearest value within the scale