Oracle Important Questions

1. Difference between group functions and single row functions.

Group Function Single Row Function

A group function operates A single row function

on many rows returns one and result for one row.

returns single result.

Not allowed in Pl/sql procedural Allowed in Pl/Sql

Procedural statements

statements.

eg SUM(),AVG,MIN,MAX etc eg UPPER,LOWER,CHR...

2. Difference between DECODE and TRANSLATE



DECODE is value by value TRANSLATE is character by

character replacement. replacement.

Ex SELECT DECODE('ABC','A',1,'B',2,'ABC',3) eg SELECT

from dual; o/p 3 TRANSLATE('ABCGH',

'ABCDEFGHIJ', 1234567899)

FROM DUAL; o/p 12378



(DECODE command is used to bring IF,THEN,ELSE logic to SQL.It tests for the IF values(s) and then aplies THEN value(s) when true, the ELSE value(s) if not.)

3. Difference between TRUNCATE and DELETE



TRUNCATE deletes much faster than DELETE



Truncate Delete

It is a DDL statement It is a DML statement

It is a one way trip,cannot One can Rollback

ROLLBACK

Doesn't have selective features (where clause) Has

Doesn't fire database triggers Does

It requires disabling of referential Does not require

constraints.

4. What is a CO-RELATED SUBQUERY

A CO-RELATED SUBQUERY is one that has a correlation

name as table or view designator in the FROM clause of the outer

query and the same correlation name as a qualifier of a search

condition in the WHERE clause of the subquery.



eg

SELECT field1 from table1 X

WHERE field2>(select avg(field2) from table1 Y

where

field1=X.field1);



(The subquery in a correlated subquery is revaluated

for every row of the table or view named in the outer query.)

5. What are various joins used while writing SUBQUERIES



Self join-Its a join foreign key of a table references the same table.



Outer Join--Its a join condition used where One can query all the rows of one of the

tables in the join condition even though they don't satisfy the join condition.



Equi-join--Its a join condition that retrieves rows from one or more tables in which one

or more columns in one table are equal to one or more columns in the second table.





6. What are various constraints used in SQL



NULL

NOT NULL

CHECK

DEFAULT



7. What are different Oracle database objects



TABLES

VIEWS

INDEXES

SYNONYMS

SEQUENCES

TABLESPACES etc

8. What is difference between Rename and Alias

Rename is a permanent name given to a table or column whereas Alias is a temporary

name given to a table or column which do not exist once the SQL statement is

executed.



9. What is a view

A view is stored procedure based on one or more tables, it’s a virtual table.



What are various privileges that a user can grant to another user



SELECT

CONNECT

RESOURCES

10. What is difference between UNIQUE and PRIMARY KEY constraints



A table can have only one PRIMARY KEY whereas there can be any number of UNIQUE keys.

The columns that compose PK are automatically define NOT NULL, whereas a column that compose a UNIQUE is not automatically defined to be mandatory must also specify the column is NOT NULL.



11. Can a primary key contain more than one columns



Yes



12. How you will avoid duplicating records in a query



By using DISTINCT



13. What is difference between SQL and SQL*PLUS



SQL*PLUS is a command line tool where as SQL and PL/SQL language interface and

reporting tool. Its a command line tool that allows user to type SQL commands to be

executed directly against an Oracle database. SQL is a language used to query the

relational database(DML,DCL,DDL). SQL*PLUS commands are used to format query

result, Set options, Edit SQL commands and PL/SQL.



14. Which datatype is used for storing graphics and images



LONG RAW data type is used for storing BLOB's (binary large objects).

15. How will you delete duplicating rows from a base table

DELETE

FROM table_name A

WHERE rowid>(SELECT min(rowid) from table_name B where

B.table_no=A.table_no);



CREATE TABLE new_table AS SELECT DISTINCT * FROM old_table;



DROP old_table

RENAME new_table TO old_table

DELETE FROM table_name A

WHERE rowid NOT IN (SELECT MAX(ROWID) FROM table_name

GROUP BY column_name)

16. What is difference between SUBSTR and INSTR



SUBSTR returns a specified portion of a string

eg SUBSTR('BCDEF',4) output BCDE

INSTR provides character position in which a pattern

is found in a string.



eg INSTR('ABC-DC-F','-',2) output 7 (2nd occurence of '-')



17. There is a string '120000 12 0 .125' ,how you will find the

position of the decimal place



INSTR('120000 12 0 .125',1,'.')

output 13



18. There is a '%' sign in one field of a column. What will be

the query to find it.



'\' Should be used before '%'.



19. When you use WHERE clause and when you use HAVING clause



HAVING clause is used when you want to specify a condition for a group function and it

is written after GROUP BY clause



The WHERE clause is used when you want to specify a condition for columns, single

row functions except group functions and it is written before GROUP BY clause if it is

used.



20. Which is more faster - IN or EXISTS



EXISTS is more faster than IN because EXISTS returns

a Boolean value whereas IN returns a value.



21. What is a OUTER JOIN



Outer Join--Its a join condition used where you can query all the rows of one of the

tables in the join condition even though they don’t satisfy the join condition.



22. How you will avoid your query from using indexes



SELECT * FROM emp

Where emp_no+' '=12345;



i.e you have to concatenate the column name with

space within codes in the where condition.



SELECT /*+ FULL(a) */ ename, emp_no from emp

where emp_no=1234;

i.e using HINTS



23. What is a pseudo column. Give some examples



It is a column that is not an actual column in the

table.



eg USER, UID, SYSDATE, ROWNUM, ROWID, NULL, AND LEVEL.



24. Suppose customer table is there having different columns

like customer no, payments.What will be the query to select top three max payments.



SELECT customer_no, payments from customer C1

WHERE 3<=(SELECT COUNT(*) from customer C2

WHERE C1.payment <= C2.payment)

25. What is the purpose of a cluster.

Oracle does not allow a user to specifically locate tables, since that is a part of the

function of the RDBMS. However, for the purpose of increasing performance, oracle allows a developer to create a CLUSTER. A CLUSTER provides a means for storing

data from different tables together for faster retrieval than if the table placement were

left to the RDBMS.

26. What is a cursor.



Oracle uses work area to execute SQL statements and store processing information

PL/SQL construct called a cursor lets you name a work area and access its stored

information A cursor is a mechanism used to fetch more than one row in a Pl/SQl

block.



27. Difference between an implicit & an explicit cursor.



PL/SQL declares a cursor implicitly for all SQL data manipulation statements, including

quries that return only one row. However,queries that return more than one row you

must declare an explicit cursor or use a cursor FOR loop.



Explicit cursor is a cursor in which the cursor name is explicitly assigned to a SELECT

statement via the CURSOR...IS statement. An implicit cursor is used for all SQL statements Declare, Open, Fetch, Close. An explicit cursors are used to process multirow SELECT statements An implicit cursor is used to process INSERT, UPDATE,

DELETE and single row SELECT. .INTO statements.





28. What are cursor attributes



%ROWCOUNT

%NOTFOUND

%FOUND

%ISOPEN



29. What is a cursor for loop.



Cursor For Loop is a loop where oracle implicitly declares a loop variable, the loop index that of the same record type as the cursor's record.



30. Difference between NO DATA FOUND and %NOTFOUND



NO DATA FOUND is an exception raised only for the SELECT....INTO statements

when the where clause of the querydoes not match any rows. When the where clause

of the explicit cursor does not match any rows the %NOTFOUND attribute is set to

TRUE instead.



31. What a SELECT FOR UPDATE cursor represent.



SELECT......FROM......FOR......UPDATE[OF column-reference][NOWAIT] The

processing done in a fetch loop modifies the rows that have been retrieved by the

cursor.



A convenient way of modifying the rows is done by a method with two parts: the FOR

UPDATE clause in the cursor declaration, WHERE CURRENT OF CLAUSE in an

UPDATE or declaration statement.



32. What 'WHERE CURRENT OF ' clause does in a cursor.



LOOP

SELECT num_credits INTO v_numcredits FROM classes

WHERE dept=123 and course=101;

UPDATE students

SET current_credits=current_credits+v_numcredits

WHERE CURRENT OF X;

END LOOP

COMMIT;

END;



33. What is use of a cursor variable? How it is defined.



A cursor variable is associated with different statements at run time, which can hold

different values at run time. Static cursors can only be associated with one run time

query. A cursor variable is reference type(like a pointer in C).



Declaring a cursor variable:

TYPE type_name IS REF CURSOR RETURN return_type type_name is the name of

the reference type,return_type is a record type indicating the types of the select list

that will eventually be returned by the cursor variable.



34. What should be the return type for a cursor variable.Can we use a scalar data type as return type.

The return type for a cursor must be a record type.It can be declared explicitly as a

user-defined or %ROWTYPE can be used. eg TYPE t_studentsref IS REF

CURSOR RETURN students%ROWTYPE



35. How you open and close a cursor variable.Why it is required.



OPEN cursor variable FOR SELECT...Statement

CLOSE cursor variable In order to associate a cursor variable with a particular

SELECT statement OPEN syntax is used.In order to free the resources used

for the query CLOSE

statement is used.



36. How you were passing cursor variables in PL/SQL 2.2.

In PL/SQL 2.2 cursor variables cannot be declared in a package.This is because the storage for a cursor variable has to be allocated using Pro*C or OCI with version 2.2,the only means of passing a cursor variable to a PL/SQL block is via bind variable or a procedure parameter.



37. Can cursor variables be stored in PL/SQL tables.If yes how.If not why.



No, a cursor variable points a row which cannot be stored in a two-dimensional PL/SQL table.



38. Difference between procedure and function.



Functions are named PL/SQL blocks that return a value and can be called with arguments procedure a named block that can be called with parameter. A procedure all is a PL/SQL statement by itself, while a Function call is called as part of an expression.



39. What are different modes of parameters used in functions and procedures.



IN

OUT

INOUT



40. What is difference between a formal and an actual parameter



The variables declared in the procedure and which are passed, as arguments are called

actual, the parameters in the procedure declaration. Actual parameters contain the

values that are passed to a procedure and receive results. Formal parameters are the

placeholders for the values of actual parameters



41. Can the default values be assigned to actual parameters.



Yes



42. Can a function take OUT parameters.If not why.



No.A function has to return a value,an OUT parameter cannot return a value.



43. What is syntax for dropping a procedure and a function .Are these operations possible.



Drop Procedure procedure_name

Drop Function function_name





44. What are ORACLE PRECOMPILERS.



Using ORACLE PRECOMPILERS ,SQL statements and PL/SQL blocks can be contained

inside 3GL programs written in C,C++,COBOL,PASCAL, FORTRAN,PL/1 AND ADA.



The Precompilers are known as Pro*C,Pro*Cobol,...

This form of PL/SQL is known as embedded pl/sql,the language in which pl/sql is

embedded is known as the host language.



The prcompiler translates the embedded SQL and pl/sql ststements into calls to the

precompiler runtime library.The output must be compiled and linked with this library to

creater an executable.







45. What is OCI. What are its uses.



Oracle Call Interface is a method of accesing database

from a 3GL program. Uses--No precompiler is required,PL/SQL blocks are executed like

other DML

statements.

The OCI library provides

-functions to parse SQL statemets

-bind input variables

-bind output variables

-execute statements

-fetch the results



46. Difference between database triggers and form triggers.



a) Data base trigger(DBT) fires when a DML operation is performed on a data base table.Form trigger(FT) Fires when user presses a key or navigates between fields on the screen



b) Can be row level or statement level No distinction between row level and statement level.



c) Can manipulate data stored in Oracle tables via SQL Can manipulate data in Oracle tables as well as

variables in forms.



d) Can be fired from any session executing the triggering DML statements. Can be fired only from the form that define the trigger.



e) Can cause other database triggers to fire.Can cause other database triggers to fire,but not other form triggers.



47. What is an UTL_FILE.What are different procedures and functions associated with it.



UTL_FILE is a package that adds the ability to read and write to operating system files

Procedures associated with it are FCLOSE, FCLOSE_ALL and 5 procedures to output

data to a file PUT, PUT_LINE, NEW_LINE, PUTF, FFLUSH.PUT, FFLUSH.PUT_LINE,FFLUSH.NEW_LINE. Functions associated with it are FOPEN, ISOPEN.



48. Can you use a commit statement within a database trigger.

No

49. What is the maximum buffer size that can be specified using the DBMS_OUTPUT.ENABLE function?

1,000,000