Tuesday, April 6, 2010

Basics Sql-1 and interview questions

SQL is an English like language consisting of commands to store, retrieve, maintain & regulate access to your database.

SQL*Plus is an application that recognizes & executes SQL commands & specialized SQL*Plus commands that can customize reports, provide help & edit facility & maintain system variables.

NVL : Null value function converts a null value to a non-null value for the purpose of evaluating an expression. Numeric Functions accept numeric I/P & return numeric values. They are MOD, SQRT, ROUND, TRUNC & POWER.

Date Functions

Character Functions
Character Functions are INITCAP, UPPER, LOWER, SUBSTR & LENGTH. Additional functions are GREATEST & LEAST. Group Functions returns results based upon groups of rows rather than one result per row, use group functions. They are AVG, COUNT, MAX, MIN & SUM.

TTITLE & BTITLE are commands to control report headings & footers.

COLUMN command define column headings & format data values.

BREAK command clarify reports by suppressing repeated values, skipping lines & allowing for controlled break points.

command control computations on subsets created by the BREAK command.

SET command changes the system variables affecting the report environment.

SPOOL command creates a print file of the report.

JOIN is the form of SELECT command that combines info from two or more tables.
Types of Joins are Simple (Equijoin & Non-Equijoin), Outer & Self join.
Equijoin returns rows from two or more tables joined together based upon a equality condition in the WHERE clause.
Non-Equijoin returns rows from two or more tables based upon a relationship other than the equality condition in the WHERE clause.
Outer Join combines two or more tables returning those rows from one table that have no direct match in the other table.
Self Join joins a table to itself as though it were two separate tables.

Union is the product of two or more tables.

Intersect is the product of two tables listing only the matching rows.

Minus is the product of two tables listing only the non-matching rows.

Correlated Subquery
Correlated Subquery is a subquery that is evaluated once for each row processed by the parent statement. Parent statement can be Select, Update or Delete. Use CRSQ to answer multipart questions whose answer depends on the value in each row processed by parent statement.

Multiple columns
Multiple columns can be returned from a Nested Subquery.

Sequences are used for generating sequence numbers without any overhead of locking. Drawback is that after generating a sequence number if the transaction is rolled back, then that sequence number is lost.

Synonyms is the alias name for table, views, sequences & procedures and are created for reasons of Security and Convenience.
Two levels are Public - created by DBA & accessible to all the users. Private - Accessible to creator only. Advantages are referencing without specifying the owner and Flexibility to customize a more meaningful naming convention.

Indexes are optional structures associated with tables used to speed query execution and/or guarantee uniqueness. Create an index if there are frequent retrieval of fewer than 10-15% of the rows in a large table and columns are referenced frequently in the WHERE clause. Implied tradeoff is query speed vs. update speed. Oracle automatically update indexes. Concatenated index max. is 16 columns.

Data types
Max. columns in a table is 255. Max. Char size is 255, Long is 64K & Number is 38 digits.
Cannot Query on a long column.
Char, Varchar2 Max. size is 2000 & default is 1 byte.
Number(p,s) p is precision range 1 to 38, s is scale -84 to 127.
Long Character data of variable length upto 2GB.
Date Range from Jan 4712 BC to Dec 4712 AD.
Raw Stores Binary data (Graphics Image & Digitized Sound). Max. is 255 bytes.
Mslabel Binary format of an OS label. Used primarily with Trusted Oracle.

Order of SQL statement execution
Where clause, Group By clause, Having clause, Order By clause & Select.

Transaction is defined as all changes made to the database between successive commits.

Commit is an event that attempts to make data in the database identical to the data in the form. It involves writing or posting data to the database and committing data to the database. Forms check the validity of the data in fields and records during a commit. Validity check are uniqueness, consistency and db restrictions.

Posting is an event that writes Inserts, Updates & Deletes in the forms to the database but not committing these transactions to the database.

Rollback causes work in the current transaction to be undone.

Savepoint is a point within a particular transaction to which you may rollback without rolling back the entire transaction.

Set Transaction
Set Transaction is to establish properties for the current transaction.

Locking are mechanisms intended to prevent destructive interaction between users accessing data. Locks are used to achieve.

Consistency : Assures users that the data they are changing or viewing is not changed until the are thro' with it.

Assures database data and structures reflects all changes made to them in the correct sequence. Locks ensure data integrity and maximum concurrent access to data. Commit statement releases all locks. Types of locks are given below.
Data Locks protects data i.e. Table or Row lock.
Dictionary Locks protects the structure of database object i.e. ensures table's structure does not change for the duration of the transaction.
Internal Locks & Latches protects the internal database structures. They are automatic.
Exclusive Lock allows queries on locked table but no other activity is allowed.
Share Lock allows concurrent queries but prohibits updates to the locked tables.
Row Share allows concurrent access to the locked table but prohibits for a exclusive table lock.
Row Exclusive same as Row Share but prohibits locking in shared mode.
Shared Row Exclusive locks the whole table and allows users to look at rows in the table but prohibit others from locking the table in share or updating them.
Share Update are synonymous with Row Share.

Deadlock is a unique situation in a multi user system that causes two or more users to wait indefinitely for a locked resource. First user needs a resource locked by the second user and the second user needs a resource locked by the first user. To avoid dead locks, avoid using exclusive table lock and if using, use it in the same sequence and use Commit frequently to release locks.

Mutating Table
Mutating Table is a table that is currently being modified by an Insert, Update or Delete statement. Constraining Table is a table that a triggering statement might need to read either directly for a SQL statement or indirectly for a declarative Referential Integrity constraints. Pseudo Columns behaves like a column in a table but are not actually stored in the table. E.g. Currval, Nextval, Rowid, Rownum, Level etc.

SQL*Loader is a product for moving data in external files into tables in an Oracle database. To load data from external files into an Oracle database, two types of input must be provided to SQL*Loader : the data itself and the control file. The control file describes the data to be loaded. It describes the Names and format of the data files, Specifications for loading data and the Data to be loaded (optional). Invoking the loader sqlload username/password controlfilename .

1. The most important DDL statements in SQL are:

CREATE TABLE - creates a new database table

ALTER TABLE - alters (changes) a database table

DROP TABLE - deletes a database table 

CREATE INDEX - creates an index (search key) 

DROP INDEX - deletes an index 

2. Operators used in SELECT statements.

= Equal
<> or != Not equal
> Greater than
< Less than
>= Greater than or equal
<= Less than or equal
BETWEEN Between an inclusive range
LIKE Search for a pattern

3. SELECT statements:

SELECT column_name(s) FROM table_name
SELECT DISTINCT column_name(s) FROM table_name
SELECT column FROM table WHERE column operator value
SELECT column FROM table WHERE column LIKE pattern
SELECT column,SUM(column) FROM table GROUP BY column
SELECT column,SUM(column) FROM table GROUP BY column HAVING SUM(column) condition value
Note that single quotes around text values and numeric values should not be enclosed in quotes. Double quotes may be acceptable in some databases.

4. The SELECT INTO Statement is most often used to create backup copies of tables or for archiving records.

SELECT column_name(s) INTO newtable [IN externaldatabase] FROM source
SELECT column_name(s) INTO newtable [IN externaldatabase] FROM source WHERE column_name operator value

5. The INSERT INTO Statements:

INSERT INTO table_name VALUES (value1, value2,....)
INSERT INTO table_name (column1, column2,...) VALUES (value1, value2,....)

6. The Update Statement:

UPDATE table_name SET column_name = new_value WHERE column_name = some_value

7. The Delete Statements:

DELETE FROM table_name WHERE column_name = some_value
Delete All Rows:
DELETE FROM table_name or DELETE * FROM table_name

8. Sort the Rows:

SELECT column1, column2, ... FROM table_name ORDER BY columnX, columnY, ..
SELECT column1, column2, ... FROM table_name ORDER BY columnX DESC
SELECT column1, column2, ... FROM table_name ORDER BY columnX DESC, columnY ASC

9. The IN operator may be used if you know the exact value you want to return for at least one of the columns.

SELECT column_name FROM table_name WHERE column_name IN (value1,value2,..)


SELECT column_name FROM table_name WHERE column_name BETWEEN value1 AND value2 The values can be numbers, text, or dates.

11. What is the use of CASCADE CONSTRAINTS?

When this clause is used with the DROP command, a parent table can be dropped even when a child table exists.

12. Why does the following command give a compilation error?

DROP TABLE &TABLE_NAME; Variable names should start with an alphabet. Here the table name starts with an '&' symbol.

13. Which system tables contain information on privileges granted and privileges obtained?


14. Which system table contains information on constraints on all the tables created?obtained?


15. What is the difference between TRUNCATE and DELETE commands?


16. State true or false. !=, <>, ^= all denote the same operation?


17. State true or false. EXISTS, SOME, ANY are operators in SQL?


18. What will be the output of the following query?


19. What does the following query do?

This displays the total salary of all employees. The null values in the commission column will be replaced by 0 and added to salary.

20. What is the advantage of specifying WITH GRANT OPTION in the GRANT command?

The privilege receiver can further grant the privileges he/she has obtained from the owner to any other user.

21. Which command executes the contents of a specified file?

START or @.

22. What is the value of comm and sal after executing the following query if the initial value of ‘sal’ is 10000

sal = 11000, comm = 1000.

23. Which command displays the SQL command in the SQL buffer, and then executes it?


24. What command is used to get back the privileges offered by the GRANT command?


25. What will be the output of the following query? SELECT DECODE(TRANSLATE('A','1234567890','1111111111'), '1','YES', 'NO' );? NO.

Explanation : The query checks whether a given string is a numerical digit.

26. Which date function is used to find the difference between two dates?


27. What operator performs pattern matching?

LIKE operator.

28. What is the use of the DROP option in the ALTER TABLE command?

It is used to drop constraints specified on the table.

29. What operator tests column for the absence of data?

IS NULL operator.

30. What are the privileges that can be granted on a table by a user to others?

Insert, update, delete, select, references, index, execute, alter, all.

31. Which function is used to find the largest integer less than or equal to a specific value?


32. Which is the subset of SQL commands used to manipulate Oracle Database structures, including tables?

Data Definition Language (DDL).

33. What is the use of DESC in SQL?

DESC has two purposes. It is used to describe a schema as well as to retrieve rows from table in descending order.
Explanation :
The query SELECT * FROM EMP ORDER BY ENAME DESC will display the output sorted on ENAME in descending order.

34. What command is used to create a table by copying the structure of another table?

To copy only the structure, the WHERE clause of the SELECT command should contain a FALSE statement as in the following.
If the WHERE condition is true, then all the rows or rows satisfying the condition will be copied to the new table.

Will the outputs of the above two commands differ?

Both will result in deleting all the rows in the table EMP..

36. What is the output of the following query SELECT TRUNC(1234.5678,-2) FROM DUAL;?


37. What are the wildcards used for pattern matching.?

_ for single character substitution and % for multi-character substitution.

38. What is the parameter substitution symbol used with INSERT INTO command?


42. What is the sub-query?

Sub-query is a query whose return values are used in filtering conditions of the main query.

43. What is correlated sub-query?

Correlated sub-query is a sub-query, which has reference to the main query.


Retrieves rows in hierarchical order eg.
select empno, ename from emp where.

45. Difference between SUBSTR and INSTR?

INSTR (String1, String2 (n, (m)),
INSTR returns the position of the m-th occurrence of the string 2 in string1. The search begins from nth position of string1.
SUBSTR (String1 n, m)
SUBSTR returns a character string of size m in string1, starting from n-th position of string1.


INTERSECT - returns all distinct rows selected by both queries. MINUS - returns all distinct rows selected by the first query but not by the second. UNION - returns all distinct rows selected by either query UNION ALL - returns all rows selected by either query, including all duplicates.

47. What is ROWID?

ROWID is a pseudo column attached to each row of a table. It is 18 characters long, blockno, rownumber are the components of ROWID.

48. What is the fastest way of accessing a row in a table?

Using ROWID.

49. What is an integrity constraint?

Integrity constraint is a rule that restricts values to a column in a table.

50. What is referential integrity constraint?

Maintaining data integrity through a set of rules that restrict the values of one or more columns of the tables based on the values of primary key or unique key of the referenced table.

51. What is the usage of SAVEPOINTS?

SAVEPOINTS are used to subdivide a transaction into smaller parts. It enables rolling back part of a transaction. Maximum of five save points are allowed.


When ON DELETE CASCADE is specified Oracle maintains referential integrity by automatically removing dependent foreign key values if a referenced primary or unique key value is removed.

53. What are the data types allowed in a table?


54. What is difference between CHAR and VARCHAR2? What is the maximum SIZE allowed for each type?

CHAR pads blank spaces to the maximum length.
VARCHAR2 does not pad blank spaces.
For CHAR the maximum length is 255 and 2000 for VARCHAR2.

55. How many LONG columns are allowed in a table? Is it possible to use LONG columns in WHERE clause or ORDER BY?

Only one LONG column is allowed. It is not possible to use LONG column in WHERE or ORDER BY clause.

56. What are the pre-requisites to modify datatype of a column and to add a column with NOT NULL constraint?

- To modify the datatype of a column the column must be empty.
- To add a column with NOT NULL constrain, the table must be empty.

57. Where the integrity constraints are stored in data dictionary?

The integrity constraints are stored in USER_CONSTRAINTS.

58. How will you activate/deactivate integrity constraints?

The integrity constraints can be enabled or disabled by ALTER TABLE ENABLE CONSTRAINT / DISABLE CONSTRAINT.

59. If unique key constraint on DATE column is created, will it validate the rows that are inserted with SYSDATE?

It won't, Because SYSDATE format contains time attached with it.

60. What is a database link?

Database link is a named path through which a remote database can be accessed.

60. How to access the current value and next value from a sequence? Is it possible to access the current value in a session before accessing next value?

Sequence name CURRVAL, sequence name NEXTVAL. It is not possible. Only if you access next value in the session, current value can be accessed.

60.What is CYCLE/NO CYCLE in a Sequence?

CYCLE specifies that the sequence continue to generate values after reaching either maximum or minimum value. After pan-ascending sequence reaches its maximum value, it generates its minimum value. After a descending sequence reaches its minimum, it generates its maximum.
NO CYCLE specifies that the sequence cannot generate more values after reaching its maximum or minimum value.

61. What are the advantages of VIEW?

- To protect some of the columns of a table from other users.
- To hide complexity of a query.
- To hide complexity of calculations.

62. 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.

63. 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 and these tables are the base tables of a view, then the changes will be reference on the view.

64. Which of the following statements is true about implicit cursors?

1. Implicit cursors are used for SQL statements that are not named.
2. Developers should use implicit cursors with great care.
3. Implicit cursors are used in cursor for loops to handle data processing.
4. Implicit cursors are no longer a feature in Oracle.

65. Which of the following is not a feature of a cursor FOR loop?

1. Record type declaration.
2. Opening and parsing of SQL statements.
3. Fetches records from cursor.
4. Requires exit condition to be defined.

66. A developer would like to use referential datatype declaration on a variable. The variable name is EMPLOYEE_LASTNAME, and the corresponding table and column is EMPLOYEE, and LNAME, respectively. How would the developer define this variable using referential datatypes?

1. Use employee.lname%type.
2. Use employee.lname%rowtype.
3. Look up datatype for EMPLOYEE column on LASTNAME table and use that.
4. Declare it to be type LONG.

67. Which three of the following are implicit cursor attributes?

1. %found
2. %too_many_rows
3. %notfound
4. %rowcount
5. %rowtype

68. If left out, which of the following would cause an infinite loop to occur in a simple loop?


69. Which line in the following statement will produce an error?

1. cursor action_cursor is
2. select name, rate, action
3. into action_record
4. from action_table;
5. There are no errors in this statement.

70. The command used to open a CURSOR FOR loop is

1. open
2. fetch
3. parse
4. None, cursor for loops handle cursor opening implicitly.

71. What happens when rows are found using a FETCH statement

1. It causes the cursor to close
2. It causes the cursor to open
3. It loads the current row values into variables
4. It creates the variables to hold the current row values

72. Read the following code:
11. (v_movie_id {Argument Mode} NUMBER, v_cost_per_ticket {argument mode} NUMBER)
12. IS
14. IF v_cost_per_ticket > 8.5 THEN
15. SELECT cost_per_ticket
16. INTO v_cost_per_ticket
17. FROM gross_receipt
18. WHERE movie_id = v_movie_id;
19. END IF;
20. END;
Which mode should be used for V_COST_PER_TICKET?
1. IN
2. OUT

73. Read the following code:
22. CREATE OR REPLACE TRIGGER update_show_gross
23. {trigger information}
25. {additional code}
26. END;

The trigger code should only execute when the column, COST_PER_TICKET, is greater than $3. Which trigger information will you add?

1. WHEN (new.cost_per_ticket > 3.75)
2. WHEN (:new.cost_per_ticket > 3.75
3. WHERE (new.cost_per_ticket > 3.75)
4. WHERE (:new.cost_per_ticket > 3.75)

74. What is the maximum number of handlers processed before the PL/SQL block is exited when an exception occurs?

1. Only one
2. All that apply
3. All referenced
4. None

77. For which trigger timing can you reference the NEW and OLD qualifiers?

1. Statement and Row 2. Statement only 3. Row only 4. Oracle Forms trigger

78. Read the following code:
RETURN number IS

v_yearly_budget NUMBER;

SELECT yearly_budget
INTO v_yearly_budget
FROM studio
WHERE id = v_studio_id;

RETURN v_yearly_budget;
Which set of statements will successfully invoke this function within SQL*Plus?
1. VARIABLE g_yearly_budget NUMBER
EXECUTE g_yearly_budget := GET_BUDGET(11);
2. VARIABLE g_yearly_budget NUMBER
EXECUTE :g_yearly_budget := GET_BUDGET(11);
3. VARIABLE :g_yearly_budget NUMBER
EXECUTE :g_yearly_budget := GET_BUDGET(11);
4. VARIABLE g_yearly_budget NUMBER

32. (v_name IN VARCHAR v_theater_id IN NUMBER) IS
34. UPDATE theater
35. SET name = v_name
36. WHERE id = v_theater_id;
37. END update_theater;

79. When invoking this procedure, you encounter the error:
ORA-000: Unique constraint(SCOTT.THEATER_NAME_UK) violated.
How should you modify the function to handle this error?
1. An user defined exception must be declared and associated with the error code and handled in the EXCEPTION section.
2. Handle the error in EXCEPTION section by referencing the error code directly.
3. Handle the error in the EXCEPTION section by referencing the UNIQUE_ERROR predefined exception.
4. Check for success by checking the value of SQL%FOUND immediately after the UPDATE statement.

80. Read the following code:
41. v_budget studio.yearly_budget%TYPE;
43. v_budget := get_budget(11);
44. IF v_budget < 30000
45. THEN
46. set_budget(11,30000000);
47. END IF;
48. END;

You are about to add an argument to CALCULATE_BUDGET. What effect will this have?
1. The GET_BUDGET function will be marked invalid and must be recompiled before the next execution.
2. The SET_BUDGET function will be marked invalid and must be recompiled before the next execution.
3. Only the CALCULATE_BUDGET procedure needs to be recompiled.
4. All three procedures are marked invalid and must be recompiled.

81. Which procedure can be used to create a customized error message?

82. The CHECK_THEATER trigger of the THEATER table has been disabled. Which command can you issue to enable this trigger?
1. ALTER TRIGGER check_theater ENABLE;
2. ENABLE TRIGGER check_theater;
3. ALTER TABLE check_theater ENABLE check_theater;
4. ENABLE check_theater;

83. Examine this database trigger
52. CREATE OR REPLACE TRIGGER prevent_gross_modification
53. {additional trigger information}
55. IF TO_CHAR(sysdate, DY) = MON
56. THEN
57. RAISE_APPLICATION_ERROR(-20000,Gross receipts cannot be deleted on Monday);
58. END IF;
59. END;

This trigger must fire before each DELETE of the GROSS_RECEIPT table. It should fire only once for the entire DELETE statement. What additional information must you add?
1. BEFORE DELETE ON gross_receipt
2. AFTER DELETE ON gross_receipt
3. BEFORE (gross_receipt DELETE)
4. FOR EACH ROW DELETED FROM gross_receipt

84. Examine this function:
62. (v_studio_id IN NUMBER, v_new_budget IN NUMBER) IS
64. UPDATE studio
65. SET yearly_budget = v_new_budget
WHERE id = v_studio_id;



Which code must be added to successfully compile this function?
1. Add RETURN right before the IS keyword.
2. Add RETURN number right before the IS keyword.
3. Add RETURN boolean right after the IS keyword.
4. Add RETURN boolean right before the IS keyword.

85. Under which circumstance must you recompile the package body after recompiling the package specification?
1. Altering the argument list of one of the package constructs
2. Any change made to one of the package constructs
3. Any SQL statement change made to one of the package constructs
4. Removing a local variable from the DECLARE section of one of the package constructs

86. Procedure and Functions are explicitly executed. This is different from a database trigger. When is a database trigger executed?
1. When the transaction is committed
2. During the data manipulation statement
3. When an Oracle supplied package references the trigger
4. During a data manipulation statement and when the transaction is committed

87. Which Oracle supplied package can you use to output values and messages from database triggers, stored procedures and functions within SQL*Plus?

88. What occurs if a procedure or function terminates with failure without being handled?
1. Any DML statements issued by the construct are still pending and can be committed or rolled back.
2. Any DML statements issued by the construct are committed
3. Unless a GOTO statement is used to continue processing within the BEGIN section, the construct terminates.
4. The construct rolls back any DML statements issued and returns the unhandled exception to the calling environment.

89. Examine this code
72. theater_pck.v_total_seats_sold_overall := theater_pck.get_total_for_year;
73. END;

For this code to be successful, what must be true?
1. Both the V_TOTAL_SEATS_SOLD_OVERALL variable and the GET_TOTAL_FOR_YEAR function must exist only in the body of the THEATER_PCK package.
2. Only the GET_TOTAL_FOR_YEAR variable must exist in the specification of the THEATER_PCK package.
3. Only the V_TOTAL_SEATS_SOLD_OVERALL variable must exist in the specification of the THEATER_PCK package.
4. Both the V_TOTAL_SEATS_SOLD_OVERALL variable and the GET_TOTAL_FOR_YEAR function must exist in the specification of the THEATER_PCK package.

90 A stored function must return a value based on conditions that are determined at runtime. Therefore, the SELECT statement cannot be hard-coded and must be created dynamically when the function is executed. Which Oracle supplied package will enable this feature?


90 A stored function must return a value based on conditions that are determined at runtime. Therefore, the SELECT statement cannot be hard-coded and must be created dynamically when the function is executed. Which Oracle supplied package will enable this feature?


91 How to implement ISNUMERIC function in SQL *Plus ?

Method 1:

Select length (translate (trim (column_name),' +-.0123456789',' ')) from dual ;

Will give you a zero if it is a number or greater than zero if not numeric (actually gives the count of non numeric characters)

Method 2:

select instr(translate('wwww',
FROM dual;

It returns 0 if it is a number, 1 if it is not.

92 How to Select last N records from a Table?

select * from (select rownum a, CLASS_CODE,CLASS_DESC from clm)
where a > ( select (max(rownum)-10) from clm)

Here N = 10

The following query has a Problem of performance in the execution of the following query where the table ter.ter_master have 22231 records. So the results are obtained after hours.

Cursor rem_master(brepno VARCHAR2) IS
select a.* from ter.ter_master a
where NOT a.repno in (select repno from ermast) and
(brepno = 'ALL' or a.repno > brepno)
Order by a.repno

What are steps required tuning this query to improve its performance?

-Have an index on TER_MASTER.REPNO and one on ERMAST.REPNO

-Be sure to get familiar with EXPLAIN PLAN. This can help you determine the execution path that Oracle takes. If you are using Cost Based Optimizer mode, then be sure that your statistics on TER_MASTER are up-to-date. -Also, you can change your SQL to:

FROM ter.ter_master a
WHERE a.repno=b.repno) AND
(a.brepno = 'ALL' or a.repno > a.brepno)
ORDER BY a.repno;

93 What is the difference between Truncate and Delete interms of Referential Integrity?

DELETE removes one or more records in a table, checking referential Constraints (to see if there are dependent child records) and firing any DELETE triggers. In the order you are deleting (child first then parent) There will be no problems.
TRUNCATE removes ALL records in a table. It does not execute any triggers. Also, it only checks for the existence (and status) of another foreign key Pointing to the table. If one exists and is enabled, then you will get The following error. This is true even if you do the child tables first.
ORA-02266: unique/primary keys in table referenced by enabled foreign keys
You should disable the foreign key constraints in the child tables before issuing the TRUNCATE command, then re-enable them afterwards.


94. What does preemptive in preemptive multitasking mean ?

Preemptive refers to the fact that each task is alloted fixed time slots and at the end of that time slot the next task is started.

95. What does the OLTP stands for ?

OLTP stands for On Line Transaction Processing

96. What is the most important requirement for OLTP ?

OLTP requires real time response.

97. In a client server environment, what would be the major work that the client deals with ?

The client deals with the user interface part of the system.

98. Why is the most of the processing done at the sever ?

To reduce the network traffic and for application sharing and implementing business rules.

99. What does teh term upsizing refer to ?

Applications that have outgrown their environment are re-engineered to run in a larger environment. This is upsizing.

100. What does one do when one is rightsizing ?

With rightsizing, one would move applications to the most appropriate server platforms.

101. What does the term downsizing refer to ?

A host based application is re-engineered to run in smaller or LAN based environment.

102. What is event trigger ?

An event trigger, a segment of code which is associated with each event and is fired when the event occurs.

103. Why do stored procedures reduce network traffic ?

When a stored procedure is called, only the procedure call is sent to the server and not the statements that the procedure contains.

104. What are the types of processes that a server runs ?

Foreground process and Background process.

105. What is a event handler ?

An event handler is a routine that is written to respond to a particular event.

106. What is an integrity constraint ?

An integrity constraint allows the definition of certain restrictions, at the table level, on the data that is entered into a table.

107. What are the various uses of database triggers ?

Database triggers can be used to enforce business rules, to maintain derived values and perform value-based auditing.

108. What is a transaction ?

A transaction is a set of operations that begin when the first DML is issued and end when a commit or rollback is issued. BEGIN COMMIT/ROLLBACK are the boundries of a transaction.

109. Why are the integrity constraints preferred to database triggers ?

Because it is easier to define an integrity constraint than a database trigger.

110. Why is it better to use an integrity constraint to validate data in a table than to use a stored procedure ?

Because an integrity constraint is automatically checked while data is inserted into a table. A stored has to be specifically invoked.

111. What are the three components of a client server model ?

A Client,
A Server and
A Network/Communication software.

112. What are the advantages of client/server model ?

Flexibility of the system, scalability, cost saving, centralised control and implementation of business rules, increase of developers productivity, portability, improved network and resource utilization.

113. What are the disadvantages of the client/server model ?

Heterogeneity of the system results in reduced reliablity. May not be suitable for all applications. Managing and tuning networks becomes difficult.

114. What are the different topologies available for network ?


115. What is the first work of Client process ?

A client process at first establishes connection with the Server.

115. What are the responsibilities of a Server ?

1. Manage resources optimally across multiple clients.
2. Controlling database access and security.
3. Protecting the databse and recovering it from crashes.
4. Enforcing integrity rules globally.

116. In a Client/Server context, what does API (Application Programming Interface) refer to ?

An API, in a Client/Server context, is a specification of a set of functions for communication between the client and the server.

117. Give some examples of standard API??s ?

Open Database Connectivity (ODBC),
Integrated Database Application Programming Interface (IDAPI),

118. What is the main advantage of developing an application using an API ?

The application can be connected to any back end server that is supported by the API.

119. What is the main disadvantage of developing an application using an API ?

The application cannot use any special features of the backend server.

120. Why is an event driven program referred to a passive program ?

Because an event driven program is always waiting for something to happen before processing.

120. What are the four types of events ?

1. System Events.
2. Control Events
3. User Events
4. Other Events.

121. What is the difference between file server and a database server ?

A file server just transfers all the data requested by all its client and the client processes the data while a database server runs the query and sends only the query output.

122. What is inheritance ?

Inheritance is a method by which properties and methods of an existing object are automatically passed to any object derived from it.

123. What are the two components of ODBC ?

1. An ODBC manager/administrator and
2. ODBC driver.

124. What is the function of a ODBC manager ?

The ODBC Manager manages all the data sources that exists in the system.

125. What is the function of a ODBC Driver ?

The ODBC Driver allows the developer to talk to the back end database.

126. What description of a data source is required for ODBC ?

The name of the DBMS, the location of the source and the database dependent information.

127. How is a connection establised by ODBC ?

ODBC uses the description of the datasource available in the ODBC.INI file to load the required drivers to access that particular back end database.

PL/SQL Questions:

1. Describe the difference between a procedure, function and anonymous pl/sql block.
Level: Low
Expected answer : Candidate should mention use of DECLARE statement, a function must return a value while a procedure doesn?t have to.

2. What is a mutating table error and how can you get around it?
Level: Intermediate
Expected answer: This happens with triggers. It occurs because the trigger is trying to update a row it is currently using. The usual fix involves either use of views or temporary tables so the database is selecting from one while updating the other.

3. Describe the use of %ROWTYPE and %TYPE in PL/SQL
Level: Low
Expected answer: %ROWTYPE allows you to associate a variable with an entire table row. The %TYPE associates a variable with a single column type.

4. What packages (if any) has Oracle provided for use by developers?
Level: Intermediate to high
Expected answer: Oracle provides the DBMS_ series of packages. There are many which developers should be aware of such as DBMS_SQL, DBMS_PIPE, DBMS_TRANSACTION, DBMS_LOCK, DBMS_ALERT, DBMS_OUTPUT, DBMS_JOB, DBMS_UTILITY, DBMS_DDL, UTL_FILE. If they can mention a few of these and describe how they used them, even better. If they include the SQL routines provided by Oracle, great, but not really what was asked.

5. Describe the use of PL/SQL tables
Level: Intermediate
Expected answer: PL/SQL tables are scalar arrays that can be referenced by a binary integer. They can be used to hold values for use in later queries or calculations. In Oracle 8 they will be able to be of the %ROWTYPE designation, or RECORD.

6. When is a declare statement needed ?
Level: Low
The DECLARE statement is used in PL/SQL anonymous blocks such as with stand alone, non-stored PL/SQL procedures. It must come first in a PL/SQL stand alone file if it is used.

7. In what order should a open/fetch/loop set of commands in a PL/SQL block be implemented if you use the %NOTFOUND cursor variable in the exit when statement? Why?
Level: Intermediate
Expected answer: OPEN then FETCH then LOOP followed by the exit when. If not specified in this order will result in the final return being done twice because of the way the %NOTFOUND is handled by PL/SQL.

8. What are SQLCODE and SQLERRM and why are they important for PL/SQL developers?
Level: Intermediate
Expected answer: SQLCODE returns the value of the error number for the last error encountered. The SQLERRM returns the actual error message for the last error encountered. They can be used in exception handling to report, or, store in an error log table, the error that occurred in the code. These are especially useful for the WHEN OTHERS exception.

9. How can you find within a PL/SQL block, if a cursor is open?
Level: Low
Expected answer: Use the %ISOPEN cursor status variable.

10. How can you generate debugging output from PL/SQL?
Level:Intermediate to high
Expected answer: Use the DBMS_OUTPUT package. Another possible method is to just use the SHOW ERROR command, but this only shows errors. The DBMS_OUTPUT package can be used to show intermediate results from loops and the status of variables as the procedure is executed. The new package UTL_FILE can also be used.

11. What are the types of triggers?
Level:Intermediate to high
Expected Answer: There are 12 types of triggers in PL/SQL that consist of combinations of the BEFORE, AFTER, ROW, TABLE, INSERT, UPDATE, DELETE and ALL key words:


1. How can variables be passed to a SQL routine?
Level: Low
Expected answer: By use of the & symbol. For passing in variables the numbers 1-8 can be used (&1, &2,...,&8) to pass the values after the command into the SQLPLUS session. To be prompted for a specific variable, place the ampersanded variable in the code itself:
"select * from dba_tables where owner=&owner_name;" . Use of double ampersands tells SQLPLUS to resubstitute the value for each subsequent use of the variable, a single ampersand will cause a reprompt for the value unless an ACCEPT statement is used to get the value from the user.

2. You want to include a carriage return/linefeed in your output from a SQL script, how can you do this?
Level: Intermediate to high
Expected answer: The best method is to use the CHR() function (CHR(10) is a return/linefeed) and the concatenation function "||". Another method, although it is hard to document and isn?t always portable is to use the return/linefeed as a part of a quoted string.

3. How can you call a PL/SQL procedure from SQL?
Level: Intermediate
Expected answer: By use of the EXECUTE (short form EXEC) command.

4. How do you execute a host operating system command from within SQL?
Level: Low
Expected answer: By use of the exclamation point "!" (in UNIX and some other OS) or the HOST (HO) command.

5. You want to use SQL to build SQL, what is this called and give an example
Level: Intermediate to high
Expected answer: This is called dynamic SQL. An example would be:
set lines 90 pages 0 termout off feedback off verify off
spool drop_all.sql
select ?drop user ?||username||? cascade;? from dba_users
where username not in ("SYS?,?SYSTEM?);
spool off
Essentially you are looking to see that they know to include a command (in this case DROP USER...CASCADE;) and that you need to concatenate using the ?||? the values selected from the database.

6. What SQLPlus command is used to format output from a select?
Level: low
Expected answer: This is best done with the COLUMN command.

7. You want to group the following set of select returns, what can you group on?
Max(sum_of_cost), min(sum_of_cost), count(item_no), item_no
Level: Intermediate
Expected answer: The only column that can be grouped on is the "item_no" column, the rest have aggregate functions associated with them.

8. What special Oracle feature allows you to specify how the cost based system treats a SQL statement?
Level: Intermediate to high
Expected answer: The COST based system allows the use of HINTs to control the optimizer path selection. If they can give some example hints such as FIRST ROWS, ALL ROWS, USING INDEX, STAR, even better.

9. You want to determine the location of identical rows in a table before attempting to place a unique index on the table, how can this be done?
Level: High
Expected answer: Oracle tables always have one guaranteed unique column, the rowid column. If you use a min/max function against your rowid and then select against the proposed primary key you can squeeze out the rowids of the duplicate rows pretty quick. For example:
select rowid from emp e
where e.rowid > (select min(x.rowid)
from emp x
where x.emp_no = e.emp_no);
In the situation where multiple columns make up the proposed key, they must all be used in the where clause.

10. What is a Cartesian product?
Level: Low
Expected answer: A Cartesian product is the result of an unrestricted join of two or more tables. The result set of a three table Cartesian product will have x * y * z number of rows where x, y, z correspond to the number of rows in each table involved in the join.

11. You are joining a local and a remote table, the network manager complains about the traffic involved, how can you reduce the network traffic?
Level: High
Expected answer: Push the processing of the remote data to the remote instance by using a view to pre-select the information for the join. This will result in only the data required for the join being sent across.

12. What is the default ordering of an ORDER BY clause in a SELECT statement?
Level: Low
Expected answer: Ascending

13. What is tkprof and how is it used?
Level: Intermediate to high
Expected answer: The tkprof tool is a tuning tool used to determine cpu and execution times for SQL statements. You use it by first setting timed_statistics to true in the initialization file and then turning on tracing for either the entire database via the sql_trace parameter or for the session using the ALTER SESSION command. Once the trace file is generated you run the tkprof tool against the trace file and then look at the output from the tkprof tool. This can also be used to generate explain plan output.

14. What is explain plan and how is it used?
Level: Intermediate to high
Expected answer: The EXPLAIN PLAN command is a tool to tune SQL statements. To use it you must have an explain_table generated in the user you are running the explain plan for. This is created using the utlxplan.sql script. Once the explain plan table exists you run the explain plan command giving as its argument the SQL statement to be explained. The explain_plan table is then queried to see the execution plan of the statement. Explain plans can also be run using tkprof.

15. How do you set the number of lines on a page of output? The width?
Level: Low
Expected answer: The SET command in SQLPLUS is used to control the number of lines generated per page and the width of those lines, for example SET PAGESIZE 60 LINESIZE 80 will generate reports that are 60 lines long with a line width of 80 characters. The PAGESIZE and LINESIZE options can be shortened to PAGES and LINES.

16. How do you prevent output from coming to the screen?
Level: Low
Expected answer: The SET option TERMOUT controls output to the screen. Setting TERMOUT OFF turns off screen output. This option can be shortened to TERM.

17. How do you prevent Oracle from giving you informational messages during and after a SQL statement execution?
Level: Low
Expected answer: The SET options FEEDBACK and VERIFY can be set to OFF.

18. How do you generate file output from SQL?
Level: Low
Expected answer: By use of the SPOOL command

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