1.       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.

2.           eg

3.                SELECT  field1 from table1 X

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

5.                                                  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.)

6.       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.

7.       What are various constraints used in SQL
NULL
NOT NULL
CHECK
DEFAULT

8.       What are different Oracle database objects
TABLES
VIEWS
INDEXES
SYNONYMS
SEQUENCES
TABLESPACES etc

9.       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.

10.    What is a view
A view is stored procedure based on one or more tables, its a virtual table.

11.    What are various privileges that a user can grant to another user
SELECT
CONNECT
RESOURCE

12.    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.

13.    Can a primary key contain more than one columns
Yes

14.    How you will avoid duplicating records in a query
By using DISTINCT

15.    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.

16.    Which datatype is used for storing graphics and images
LONG RAW data type is used for storing BLOB's (binary large objects).

17.    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)

18.    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 '-')

19.    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

20.    There is a '%' sign in one field of a column. What will be the query to find it.
'\' Should be used before '%'.

21.    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.

22.    Which is more faster - IN or EXISTS
EXISTS is more faster than IN because EXISTS returns a Boolean value whereas IN returns a value.

23.    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 dont satisfy the join condition.

24.    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

25.    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.

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)

26.    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.

27.    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.

28.    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.

29.    What are cursor attributes
%ROWCOUNT
%NOTFOUND
%FOUND
%ISOPEN

30.    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.

31.    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.

32.    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.

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

34.     LOOP

35.                               SELECT  num_credits  INTO  v_numcredits  FROM classes

36.                               WHERE  dept=123 and course=101;

37.                               UPDATE  students

38.                               SET current_credits=current_credits+v_numcredits

39.                               WHERE  CURRENT OF  X;

40.                               END  LOOP

41.                               COMMIT;

                          END;

42.    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.

43.    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

44.    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.

45.    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.

46.    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.

47.    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.

48.    What are different modes of parameters used in functions and procedures.
IN
OUT
INOUT

49.    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

50.    Can the default values be assigned to actual parameters.
Yes

51.    Can a function take OUT parameters.If not why.
No.A function has to return a value,an OUT parameter cannot return a value.

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

53. 

54.               Drop Procedure procedure_name

55.               Drop Function function_name

56.    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.

57.    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.

58.                      The OCI library provides

59.                     -functions to parse SQL statemets

60.                     -bind input variables

61.                     -bind output variables

62.                     -execute statements

63.                     -fetch the results

64.    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.

65.    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.

66.    Can you use a commit statement within a database trigger.
No

67.    What is the maximum buffer size that can be specified using the DBMS_OUTPUT.ENABLE function?
1,000,000

Oracle Interview Questions

1.       How would you determine the time zone under which a database was operating?

2.       Explain the use of setting GLOBAL_NAMES equal to TRUE.

3.       What command would you use to encrypt a PL/SQL application?

4.       Explain the difference between a FUNCTION, PROCEDURE and PACKAGE.

5.       Explain the use of table functions.

6.       Name three advisory statistics you can collect.

7.       Where in the Oracle directory tree structure are audit traces placed?

8.       Explain materialized views and how they are used.

9.       When a user process fails, what background process cleans up after it?

10.    What background process refreshes materialized views?

11.    How would you determine what sessions are connected and what resources they are waiting for?

12.    Describe what redo logs are.

13.    How would you force a log switch?

14.    Give two methods you could use to determine what DDL changes have been made.

15.    What does coalescing a tablespace do?

16.    What is the difference between a TEMPORARY tablespace and a PERMANENT tablespace?

17.    Name a tablespace automatically created when you create a database.

18.    When creating a user, what permissions must you grant to allow them to connect to the database?

19.    How do you add a data file to a tablespace?

20.    How do you resize a data file?

21.    What view would you use to look at the size of a data file?

22.    What view would you use to determine free space in a tablespace?

23.    How would you determine who has added a row to a table?

24.    How can you rebuild an index?

25.    Explain what partitioning is and what its benefit is.

26.    You have just compiled a PL/SQL package but got errors, how would you view the errors?

27.    How can you gather statistics on a table?

28.    How can you enable a trace for a session?

29.    What is the difference between the SQL*Loader and IMPORT utilities?

30.    Name two files used for network connection to a database.