Tuesday, April 12, 2011

SQL*Plus and SQL - Part 1

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what is Table ?

A table is the basic unit of data storage in an ORACLE database. The tables of a database hold all of the user accessible data. Table data is stored in rows and columns.

What is a View ?

A view is a virtual table. Every view has a Query attached to it. (The Query is a SELECT statement that identifies the columns and rows of the table(s) the view uses.)

Do View contain Data ?

Views do not contain or store data.

Can a View based on another View ?


What are the advantages of Views ?

i. Provide an additional level of table security, by restricting access to a predetermined set of rows and columns of a table.
ii. Hide data complexity.
iii. Simplify commands for the user.
iv. Present the data in a different perpecetive from that of the base table.
v. Store complex queries.

Can a view be updated/inserted/deleted? If Yes under what conditions ?

A View can be updated/deleted/inserted if it has only one base table if the view is based on columns from one or more tables then insert, update and delete is not possible.

If a View on a single base table is manipulated will the changes be reflected on the base table ?

If changes are made to the tables which are base tables of a view will the changes be reference on the view.

What is a Sequence ?

A sequence generates a serial list of unique numbers for numerical columns of a database's tables.

How to see current user name
Sql> show user;

Change SQL prompt name

SQL> set sqlprompt ?Manimara > ?
Manimara >
Manimara >

Switch to DOS prompt
SQL> host

How do I eliminate the duplicate rows ?

SQL> delete from table_name where rowid not in (select max(rowid) from table group by duplicate_values_field_name);
SQL> delete duplicate_values_field_name dv from table_name ta where rowid <(select min(rowid) from table_name tb where ta.dv=tb.dv);
Table Emp
Empno Ename
101 Scott
102 Jiyo
103 Millor
104 Jiyo
105 Smith
delete ename from emp a where rowid < ( select min(rowid) from emp b where a.ename = b.ename);
The output like,
Empno Ename
101 Scott
102 Millor
103 Jiyo
104 Smith

How do I display row number with records?
To achive this use rownum pseudocolumn with query, like SQL> SQL> select rownum, ename from emp;
1 Scott
2 Millor
3 Jiyo
4 Smith

Display the records between two range
select rownum, empno, ename from emp where rowid in
(select rowid from emp where rownum <=&upto
select rowid from emp where rownum<&Start);
Enter value for upto: 10
Enter value for Start: 7
--------- --------- ----------
1 7782 CLARK
2 7788 SCOTT
3 7839 KING
4 7844 TURNER

I know the nvl function only allows the same data type(ie. number or char or date Nvl(comm, 0)), if commission is null then the text ?Not Applicable? want to display, instead of blank space. How do I write the query?
SQL> select nvl(to_char(comm.),'NA') from emp;
Output :

Oracle cursor : Implicit & Explicit cursors
Oracle uses work areas called private SQL areas to create SQL statements.
PL/SQL construct to identify each and every work are used, is called as Cursor.
For SQL queries returning a single row, PL/SQL declares all implicit cursors.
For queries that returning more than one row, the cursor needs to be explicitly declared.

Explicit Cursor attributes
There are four cursor attributes used in Oracle
cursor_name%Found, cursor_name%NOTFOUND, cursor_name%ROWCOUNT, cursor_name%ISOPEN

Implicit Cursor attributes
Same as explicit cursor but prefixed by the word SQL
Tips : 1. Here SQL%ISOPEN is false, because oracle automatically closed the implicit cursor after executing SQL statements.
: 2. All are Boolean attributes.

Find out nth highest salary from emp table
Enter value for n: 2

To view installed Oracle version information
SQL> select banner from v$version;

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