java barcode reader free download DATA DEFINITION, PART I in Font

Generation DataMatrix in Font DATA DEFINITION, PART I

CHAPTER 3 DATA DEFINITION, PART I
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Listing 3-3. The SALGRADES Table SQL> 2 3 4 5 create table ( grade , lowerlimit , upperlimit , bonus SALGRADES number(2) number(6,2) number(6,2) number(6,2) not not not not null null null null );
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Listing 3-4. The COURSES Table SQL> 2 3 4 5 create table ( code , description , category , duration COURSES varchar2(6) varchar2(30) char(3) number(2) not not not not null null null null );
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Listing 3-5. The OFFERINGS Table SQL> 2 3 4 5 create table ( course , begindate , trainer , location OFFERINGS varchar2(6) date number(4) varchar2(8) not null not null );
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Listing 3-6. The REGISTRATIONS Table SQL> 2 3 4 5 create table ( attendee , course , begindate , evaluation REGISTRATIONS number(4) not null varchar2(6) not null date not null number(1) );
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Listing 3-7. The HISTORY Table SQL> 2 3 4 5 6 7 8 create table ( empno , beginyear , begindate , enddate , deptno , msal , comments HISTORY number(4) number(4) date date number(2) number(6,2) varchar2(60) not null not null not null not null not null );
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CHAPTER 3 DATA DEFINITION, PART I
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3.5 The Data Dictionary
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If you are interested in knowing which tables are present in your database, which columns they have, whether or not those columns are indexed, which privileges are granted to you, and similar information, you should query the data dictionary. Another common term for data dictionary is catalog. By the way, we already queried the data dictionary implicitly before, in 2, when using the SQL*Plus DESCRIBE command; this command queries the data dictionary under the hood. The data dictionary is more or less the internal housekeeping administration of Oracle. The data dictionary stores information about the data, also referred to as metadata. The data dictionary is automatically maintained by Oracle; therefore, the data dictionary is always up-to-date. DBMSs like Oracle store data dictionary data in precisely the same way as they store regular data: in tables. This is in compliance with Ted Codd s rule 4 (see 1). The big advantage of this approach is that you can use the SQL language to query data dictionary data in the same way that you query ordinary data. In other words, if you master the SQL language, you need to know only the names of the data dictionary tables and the names of their columns. Data dictionary access is a potential security risk. That s why the Oracle DBMS offers system privileges and roles to regulate and protect access to the data dictionary. For example, there is a role SELECT_CATALOG_ROLE, which contains all privileges that you need to be able to access the data dictionary data. Listing 3-8 demonstrates how Oracle controls data dictionary access. Listing 3-8. Needing the SELECT_CATALOG_ROLE Role SQL> describe dba_sys_privs ERROR: ORA-04043: object "SYS"."DBA_SYS_PRIVS" does not exist SQL> connect / as sysdba Connected. SQL> grant select_catalog_role to book; Grant succeeded. SQL> connect book/book Connected. SQL> desc dba_sys_privs Name ----------------------------GRANTEE PRIVILEGE ADMIN_OPTION SQL>
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Null -------NOT NULL NOT NULL
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Type --------------VARCHAR2(30) VARCHAR2(40) VARCHAR2(3)
CHAPTER 3 DATA DEFINITION, PART I
Although the information is stored in data dictionary tables, most of the time, you access data dictionary views instead. On the other hand, views are tables anyway. See 10 for details about views. You can refer to Oracle Server Reference in the Oracle documentation to get a complete overview of the Oracle data dictionary. Fortunately, the Oracle data dictionary contains a view that lists all Oracle data dictionary views, with a short description of their contents. This view is called DICTIONARY; DICT is a shorter synonym for the same view. Listing 3-9 shows an abbreviated version of the query results. It s abbreviated for a practical reason: the DICT view contains more than 600 rows! Listing 3-9. Using the DICT View SQL> col COLUMN_NAME format a30 SQL> col COMMENTS format a40 word SQL> SQL> select * from dict order by table_name; TABLE_NAME COMMENTS -------------------- ---------------------------------------ALL_ALL_TABLES Description of all object and relational tables accessible to the user ALL_APPLY Details about each apply process that dequeues from the queue visible to the current user ... USER_COL_COMMENTS Comments on columns of user's tables and views USER_COL_PRIVS Grants on columns for which the user is the owner, grantor or grantee ... V$TIMEZONE_NAMES Synonym for V_$TIMEZONE_NAMES V$VERSION Synonym for V_$VERSION 610 rows selected. SQL> Data dictionary view names typically have prefixes that suggest the existence of four main categories. In Listing 3-9, you can see the ALL, USER, and V$ prefixes. The fourth common prefix is DBA. The idea behind this is that, most of the time, you are interested in information about a certain subcategory of database objects. By using the appropriate views, you automatically suppress information that is not of interest to you. Also, depending on your database privileges, you will not be allowed to use certain categories of data dictionary views. Table 3-4 lists the most common data dictionary view name prefixes. (Note that not all data dictionary views have one of these prefixes.)
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