OCA /OCP Oracle Database 11g A ll-in-One Exam Guide- P31 pps

OCA/OCP Oracle Database 11g All-in-One Exam Guide 256 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 This page intentionally left blank 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 OCA/OCP Oracle Database 11g All-in-One Exam Guide 260 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 Chapter 7: DDL and Schema Objects 261 PART II 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. OCA/OCP Oracle Database 11g All-in-One Exam Guide 262 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: Tables Views Sequences Private synonyms Stand-alone procedures Stand-alone stored functions Packages Materialized views User-defined types Figure 7-1 Using double quotes to use nonstandard names Chapter 7: DDL and Schema Objects 263 PART II 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: Indexes Constraints Clusters Database triggers Private database links Dimensions 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 OCA/OCP Oracle Database 11g All-in-One Exam Guide 264 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. Chapter 7: DDL and Schema Objects 265 PART II 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. . OCA/ OCP Oracle Database 11g All-in-One Exam Guide 256 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. 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 OCA/ OCP Oracle Database 11g All-in-One Exam. column names. EXAM TIP Object names must be no more than 30 characters. The characters can be letters, digits, underscore, dollar, or hash. OCA/ OCP Oracle Database 11g All-in-One Exam Guide 262 Although