Difference between Unique key and Primary key(other than normal difference)

Hello,
1).Can any one tell me any other difference between Unique key and Primary key other than it having NULLs.
2). What is the difference the words 'DISTINCT' and 'UNIQUE' in a sql query.
Thanks in advance.

Hi
If you don't believe me than see the documentation in
OTN.
Ott Karesz
http://www.trendo-kft.hu
SQL> create table scott.tbl_clob
2 (sss CLOB)
3 /
Tabelle wurde angelegt.
SQL> insert into scott.tbl_clob values('wrwrwrw')
2 /
1 Zeile wurde erstellt.
SQL> insert into scott.tbl_clob values('wrwrwrw')
2 /
1 Zeile wurde erstellt.
SQL> select distinct sss from scott.tbl_clob
2 /
select distinct sss from scott.tbl_clob
FEHLER in Zeile 1:
ORA-00932: nicht übereinstimmende Datentypen
SQL> select unique sss from scott.tbl_clob
2 /
select unique sss from scott.tbl_clob
FEHLER in Zeile 1:
ORA-00932: nicht übereinstimmende Datentypen
SQL> select distinct to_char(sss) from scott.tbl_clob
2 /
TO_CHAR(SSS)
wrwrwrw
SQL> select unique to_char(sss) from scott.tbl_clob
2 /
TO_CHAR(SSS)
wrwrwrw
SQL>

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Difference between unique constraint and unique index

1. What is the difference between unique constraint and unique index when unique constraint is always indexed ? Which one is better in this case for better performance ?
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or having independent/ seperate indexes on 3 columns x,y,z is better for better performance ?
3. It has been very confusing for me to decide which columns to index, I have indexed most foreignkey columns, is it a good idea ? We do lot of selects and DMLS on most of our tables. Is there any query that I can run and find out if indexes are really being used and if they are improving any performance. I have analyzed and computed my indexes using ANALYZE index index_name validate structure and COMPUTE STATISTICS;
null

1. Unique index is part of unique constraint. Of course you can create standalone unique index. But is is no point to skip the logical view of business if you spend same effort to achive.
You create unique const. Oracle create the unique index for you. You may specify index characteristic in unique constraint.
2. Depends. You can't utilize the composite index if the searching condition is not whole or front part of the indexing key. You can't utilize your index if you query the table for y=2. That is.
3. As old words in database arena, Index may be good or bad for a table depending on the size of table, number of columns in the table... etc. It is very environmental dependent. In fact, It is part of database nomalization. Statistic is a way oracle use to determine the execution plan.
Steve
null

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Hi rajesh,
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Hi Friends,
I am confused about primary keys.
What is the purpose of this key again? I know it is used for unique constraints.
Supposing I have a table with two (2) columns which are each indexed as unique.
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A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. This constraint does not apply to NULL values except for the BDB storage engine. For other engines, a UNIQUE index allows multiple NULL values for columns that can contain NULL
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2. There can be only one Primary Key defined on the table where as you can have many unique indexes defined on the table (if needed).
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What is the diffrence between Row id and primary key ?

dear all
my question is about creating materialized views parameters (With Rowid and
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my master table contains a primary key
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Hi again,
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(You're example shows that) are not possible.
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for 10.1
http://download-west.oracle.com/docs/cd/B14117_01/server.101/b10759/statements_6002.htm#sthref5054
for 10.2
http://download-uk.oracle.com/docs/cd/B19306_01/server.102/b14200/statements_6002.htm#sthref6873
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This is not documented clearly though (documentation bug ?!)
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GROUP BY or CONNECT BY clauses
Subqueries
Joins
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Message was edited by:
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Narender wrote:
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Edited by: Nilesh Hole,Pune, India on Aug 22, 2009 10:33 AM

Nilesh Hole,Pune, India wrote:
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