Oracle Interview Questions and Answers

Oracle Interview Questions and Answers
1. To see current user name
Sql> show user;
2. Change SQL prompt name
SQL> set sqlprompt “Manimara > “
Manimara >
Manimara >
3. Switch to DOS prompt
SQL> host
4. 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);
or
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);
Example.
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);
or
delete from t1 a where rowid not in (select max(rowid) from ti b where a.no=b.n0)
The output like,
Empno Ename
101 Scott
102 Millor
103 Jiyo
104 Smith
5. How do I display row number with records?
To achive this use rownum pseudocolumn with query, like SQL> SQL> select rownum, ename from emp;
Output:
1 Scott
2 Millor
3 Jiyo
4 Smith
6. Display the records between two range
select rownum, empno, ename from emp where rowid in
(select rowid from emp where rownum <=&upto
minus
select rowid from emp where rownum<&Start);
Enter value for upto: 10
Enter value for Start: 7
ROWNUM EMPNO ENAME
--------- --------- ----------
1 7782 CLARK
2 7788 SCOTT
3 7839 KING
4 7844 TURNER
7. 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;
Page 11 of 259
Output :
NVL(TO_CHAR(COMM),'NA')
-----------------------
NA
300
500
NA
1400
NA
NA
8. 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.
9. Explicit Cursor attributes
There are four cursor attributes used in Oracle
cursor_name%Found, cursor_name%NOTFOUND, cursor_name%ROWCOUNT, cursor_name%ISOPEN
10. Implicit Cursor attributes
Same as explicit cursor but prefixed by the word SQL
SQL%Found, SQL%NOTFOUND, SQL%ROWCOUNT, SQL%ISOPEN
Tips : 1. Here SQL%ISOPEN is false, because oracle automatically closed the implicit cursor after executing
SQL statements.
: 2. All are Boolean attributes.
11. Find out nth highest salary from emp table
SELECT DISTINCT (a.sal) FROM EMP A WHERE &N = (SELECT COUNT (DISTINCT (b.sal)) FROM EMP
B WHERE a.sal<=b.sal);
or
select min(sal) from(select distinct sal from emp order by sal desc where rownum<=&n)
Enter value for n: 2
SAL
---------
3700
12. To view installed Oracle version information
SQL> select banner from v$version;
13. Display the number value in Words
SQL> select sal, (to_char(to_date(sal,'j'), 'jsp')) from emp;
the output like,
SAL (TO_CHAR(TO_DATE(SAL,'J'),'JSP'))
--------- -----------------------------------------------------
800 eight hundred
1600 one thousand six hundred
1250 one thousand two hundred fifty
If you want to add some text like,
Rs. Three Thousand only.
SQL> select sal "Salary ",(' Rs. ' (to_char(to_date(sal,'j'), 'Jsp')) ' only.'))"Sal in Words" from emp
/
Salary Sal in Words
------- ------------------------------------------------------
800 Rs. Eight Hundred only.
1600 Rs. One Thousand Six Hundred only.
1250 Rs. One Thousand Two Hundred Fifty only.
14. Display Odd/ Even number of records
Odd number of records:
select * from emp where (rowid,1) in (select rowid, mod(rownum,2) from emp);
1
3
5
Even number of records:
select * from emp where (rowid,0) in (select rowid, mod(rownum,2) from emp)
2
Page 12 of 259
4
6
15. Which date function returns number value?
months_between
16. Any three PL/SQL Exceptions?
Too_many_rows, No_Data_Found, Value_Error, Zero_Error, Others
17. What are PL/SQL Cursor Exceptions?
Cursor_Already_Open, Invalid_Cursor
18. Other way to replace query result null value with a text
SQL> Set NULL ‘N/A’
to reset SQL> Set NULL ‘’
19. What are the more common pseudo-columns?
SYSDATE, USER , UID, CURVAL, NEXTVAL, ROWID, ROWNUM
20. What is the output of SIGN function?
1 for positive value,
0 for Zero,
-1 for Negative value.
21. What is the maximum number of triggers, can apply to a single table?
12 triggers.

PL/SQL
Basics of PL/SQL
1. What is PL/SQL ?
PL/SQL is a procedural language that has both interactive SQL and procedural programming language
constructs such as iteration, conditional branching.
2. What is the basic structure of PL/SQL ?
PL/SQL uses block structure as its basic structure. Anonymous blocks or nested blocks can be used in PL/SQL.
3. What are the components of a PL/SQL block ?
A set of related declarations and procedural statements is called block.
4. What are the components of a PL/SQL Block ?
Declarative part, Executable part and Execption part.
5. What are the datatypes a available in PL/SQL ?
Some scalar data types such as NUMBER, VARCHAR2, DATE, CHAR, LONG, BOOLEAN.
Some composite data types such as RECORD & TABLE.
6. What are % TYPE and % ROWTYPE ? What are the advantages of using these over datatypes?
% TYPE provides the data type of a variable or a database column to that variable.
% ROWTYPE provides the record type that represents a entire row of a table or view or columns selected in
the cursor.
The advantages are :
I. Need not know about variable's data type
ii. If the database definition of a column in a table changes, the data type of a variable changes accordingly.
7. What is difference between % ROWTYPE and TYPE RECORD ?
% ROWTYPE is to be used whenever query returns a entire row of a table or view.
TYPE rec RECORD is to be used whenever query returns columns of different
table or views and variables.
E.g. TYPE r_emp is RECORD (eno emp.empno% type,ename emp ename %type);
e_rec emp% ROWTYPE
cursor c1 is select empno,deptno from emp;
e_rec c1 %ROWTYPE.
8. What is PL/SQL table ?
Objects of type TABLE are called "PL/SQL tables", which are modelled as (but not the same as) database tables, PL/SQL tables use a primary PL/SQL tables can have one column and a primary key.
9. What is a cursor ? Why Cursor is required ?
Cursor is a named private SQL area from where information can be accessed. Cursors are required to process rows individually for queries returning multiple rows.
10. Explain the two type of Cursors ?
There are two types of cursors, Implict Cursor and Explicit Cursor.
PL/SQL uses Implict Cursors for queries.
User defined cursors are called Explicit Cursors. They can be declared and used.
11. What are the PL/SQL Statements used in cursor processing ?
DECLARE CURSOR cursor name, OPEN cursor name, FETCH cursor name INTO or Record types,
CLOSE cursor name.
12. What are the cursor attributes used in PL/SQL ?
%ISOPEN - to check whether cursor is open or not
% ROWCOUNT - number of rows fetched/updated/deleted.
% FOUND - to check whether cursor has fetched any row. True if rows are fetched.
% NOT FOUND - to check whether cursor has fetched any row. True if no rows are fetched.
These attributes are proceded with SQL for Implict Cursors and with Cursor name for Explict Cursors.
13. What is a cursor for loop ?
Cursor for loop implicitly declares %ROWTYPE as loop index,opens a cursor, fetches rows of values from
active set into fields in the record and closes
when all the records have been processed.
eg. FOR emp_rec IN C1 LOOP
salary_total := salary_total +emp_rec sal;
END LOOP;
14. What will happen after commit statement ?
Cursor C1 is
Select empno,
ename from emp;
Begin
open C1; loop
Fetch C1 into
eno.ename;
Exit When
Page 14 of 259
C1 %notfound;-----
commit;
end loop;
end;
The cursor having query as SELECT .... FOR UPDATE gets closed after COMMIT/ROLLBACK.
The cursor having query as SELECT.... does not get closed even after COMMIT/ROLLBACK.
15. Explain the usage of WHERE CURRENT OF clause in cursors ?
WHERE CURRENT OF clause in an UPDATE,DELETE statement refers to the latest row fetched from a
cursor.
Database Triggers
16. What is a database trigger ? Name some usages of database trigger ?
Database trigger is stored PL/SQL program unit associated with a specific database table. Usages are Audit
data modificateions, Log events transparently, Enforce complex business rules Derive column values
automatically, Implement complex security authorizations. Maintain replicate tables.
17. How many types of database triggers can be specified on a table ? What are they ?
Insert Update Delete
Before Row o.k. o.k. o.k.
After Row o.k. o.k. o.k.
Before Statement o.k. o.k. o.k.
After Statement o.k. o.k. o.k.
If FOR EACH ROW clause is specified, then the trigger for each Row affected by the statement.
If WHEN clause is specified, the trigger fires according to the retruned boolean value.
18. Is it possible to use Transaction control Statements such a ROLLBACK or COMMIT in Database Trigger ? Why ?
It is not possible. As triggers are defined for each table, if you use COMMIT of ROLLBACK in a trigger, it affects logical transaction processing.
19. What are two virtual tables available during database trigger execution ?
The table columns are referred as OLD.column_name and NEW.column_name.
For triggers related to INSERT only NEW.column_name values only available.
For triggers related to UPDATE only OLD.column_name NEW.column_name values only available. For triggers related to DELETE only OLD.column_name values only available.
20. What happens if a procedure that updates a column of table X is called in a database trigger of the same table ?
Mutation of table occurs.
21. Write the order of precedence for validation of a column in a table ?
I. done using Database triggers.
ii. done using Integarity Constraints.
22. What is an Exception ? What are types of Exception ?
Exception is the error handling part of PL/SQL block. The types are Predefined and user_defined. Some of
Predefined execptions are.
CURSOR_ALREADY_OPEN
DUP_VAL_ON_INDEX
NO_DATA_FOUND
TOO_MANY_ROWS
INVALID_CURSOR
INVALID_NUMBER
LOGON_DENIED
NOT_LOGGED_ON
PROGRAM-ERROR
STORAGE_ERROR
TIMEOUT_ON_RESOURCE
VALUE_ERROR
ZERO_DIVIDE
OTHERS.
23. What is Pragma EXECPTION_INIT ? Explain the usage ?
The PRAGMA EXECPTION_INIT tells the complier to associate an exception with an oracle error. To get an error message of a specific oracle error.
e.g. PRAGMA EXCEPTION_INIT (exception name, oracle error number)
24. What is Raise_application_error ?
Raise_application_error is a procedure of package DBMS_STANDARD which allows to issue an
user_defined error messages from stored sub-program or database trigger.
25. What are the return values of functions SQLCODE and SQLERRM ?
SQLCODE returns the latest code of the error that has occured.
SQLERRM returns the relevant error message of the SQLCODE.
26. Where the Pre_defined_exceptions are stored ?
In the standard package.
Procedures, Functions & Packages ;
27. What is a stored procedure ?
A stored procedure is a sequence of statements that perform specific function.
28. What is difference between a PROCEDURE & FUNCTION ?
A FUNCTION is alway returns a value using the return statement.
A PROCEDURE may return one or more values through parameters or may not return at all.
29. What are advantages fo Stored Procedures /
Extensibility,Modularity, Reusability, Maintainability and one time compilation.
30. What are the modes of parameters that can be passed to a procedure ?
IN,OUT,IN-OUT parameters.
31. What are the two parts of a procedure ?
Procedure Specification and Procedure Body.
32. Give the structure of the procedure ?
PROCEDURE name (parameter list.....)
is
local variable declarations
BEGIN
Executable statements.
Exception.
exception handlers
end;
33. Give the structure of the function ?
FUNCTION name (argument list .....) Return datatype is
local variable declarations
Begin
executable statements
Exception
execution handlers
End;
34. Explain how procedures and functions are called in a PL/SQL block ?
Function is called as part of an expression.
sal := calculate_sal ('a822');
procedure is called as a PL/SQL statement
calculate_bonus ('A822');
35. What is Overloading of procedures ?
The Same procedure name is repeated with parameters of different datatypes and parameters in different positions, varying number of parameters is called overloading of procedures.
e.g. DBMS_OUTPUT put_line
36. What is a package ? What are the advantages of packages ?
Package is a database object that groups logically related procedures.
The advantages of packages are Modularity, Easier Applicaton Design, Information. Hiding,. reusability and
Better Performance.
37.What are two parts of package ?
The two parts of package are PACKAGE SPECIFICATION & PACKAGE BODY.
Package Specification contains declarations that are global to the packages and local to the schema.
Package Body contains actual procedures and local declaration of the procedures and cursor declarations.
38. What is difference between a Cursor declared in a procedure and Cursor declared in a package specification ?
A cursor declared in a package specification is global and can be accessed by other procedures or procedures in a package.
A cursor declared in a procedure is local to the procedure that can not be accessed by other procedures.
39. How packaged procedures and functions are called from the following?
a. Stored procedure or anonymous block
b. an application program such a PRC *C, PRO* COBOL
c. SQL *PLUS
a. PACKAGE NAME.PROCEDURE NAME (parameters);
variable := PACKAGE NAME.FUNCTION NAME (arguments);
EXEC SQL EXECUTE
b.
BEGIN
PACKAGE NAME.PROCEDURE NAME (parameters)
variable := PACKAGE NAME.FUNCTION NAME (arguments);
END;
END EXEC;
c. EXECUTE PACKAGE NAME.PROCEDURE if the procedures does not have any
out/in-out parameters. A function can not be called.
40. Name the tables where characteristics of Package, procedure and functions are stored ?
User_objects, User_Source and User_error.