13
Other Database Objects
Objectivesj i
After completing this lesson, you should be able to do the following:
•• Describeri some databaset objectsj ts and theirt ir uses
•• Create,r t , maintain,i t i , and use sequences
•• Creater te and maintaini t in indexesi
•• Creater te privateri te and publiclic synonyms
13-2
Database Objectsj
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Description |
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Table |
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Basic unit of storage; composed of rows |
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and columns |
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View |
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Logically represents subsets of data from |
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one or more tables |
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Sequence |
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Generates primary key values |
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Index |
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Improves the performance of some queries |
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Synonym |
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Alternative name for an object |
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13-3
What IsIs a Sequence?
•• Automaticallyt ti lly generatesr t uniquei numbersrs
•• IsIs a sharabler le objectj t
•• IsIs typicallyt i lly used toto creater te a primaryri ry key valuel
•• Replacesl applicationli ti code
•• Speeds up thet efficiencyffi i off accessingi sequence valuesl when cached inin memoryry
13-4
The CREATE SEQUENCE
Statement
Define a sequence to generate sequential numbers automatically
CREATE SEQUENCE sequence [INCREMENT BY n] [START WITH n]
[{MAXVALUE n | NOMAXVALUE}]
[{MINVALUE n | NOMINVALUE}] [{CYCLE | NOCYCLE}]
[{CACHE n | NOCACHE}];
13-5
Creatingi a Sequence
••CreateCreateaasequencesequencenamednamedDEPTDEPT__DEPTNODEPTNOtotobebeusedusedforforthetheprimaryprimarykeykeyofofthethe
DEPTDEPTtabletable..
••DoDonotnotuseusethetheCYCLECYCLEoptionoption..
SQL> CREATE SEQUENCE dept_deptno
2 INCREMENT BY 1
3 START WITH 91
4MAXVALUE 100
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NOCACHE |
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NOCYCLE; |
Sequence created.
13-6
Confirmingi i Sequences
•• Verifyrify yourr sequence valuesl inin thet
USER_SEQUENCES datata dictionaryi ti ry tablet le..
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SQL> |
SELECT |
sequence_name, min_value, max_value, |
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increment_by, last_number |
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FROM |
user_sequences; |
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•• The LAST_NUMBER columnl displaysi l thet nextt availableil le sequence numberr..
13-7
NEXTVAL and CURRVAL Pseudocolumnsl
•• NEXTVAL returnsr t r thet nextt availableil le sequence valuel ..
ItIt returnsr t r a uniquei valuel everyry timeti itit isis referenced,r f r , even forf r differentiff r t usersrs..
•• CURRVAL obtainst i thet currentrr t sequence valuel ..
NEXTVAL mustt be issuedi forf r thatt t sequence beforef re CURRVAL containst i a valuel ..
13-8
Usingi a Sequence
•• InsertI rt a new departmentrt t named
“MARKETING”I inin San Diegoi ..
SQL> |
INSERT |
INTO |
dept(deptno, dname, loc) |
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VALUES |
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(dept_deptno.NEXTVAL, |
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'MARKETING', 'SAN DIEGO'); |
1 row created.
•• Viewi thet currentrr t valuel forf r thet
DEPT_DEPTNO sequence..
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SQL> |
SELECT |
dept_deptno.CURRVAL |
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2 |
FROM |
dual; |
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13-10
Usingi a Sequence
•• Cachingi sequence valuesl inin memoryry allowsll fasterf t r access toto thoset valuesl ..
•• Gaps inin sequence valuesl can occurr when::
–A rollbackr ll occursrs
–The systemt crashesr
–A sequence isis used inin anothert r tablet le
•• Viewi thet nextt availableil le sequence,, ifif itit was createdr t withith NOCACHE,, by queryingr i thet USER_SEQUENCES tablet le..
13-11
Modifyingi i a Sequence
Change the increment value, maximum value, minimum value, cycle option, or cache option.
SQL> ALTER SEQUENCE dept_deptno
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INCREMENT BY 1 |
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MAXVALUE 999999 |
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NOCACHE |
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NOCYCLE; |
Sequence altered.
13-12