SQL Interview Questions and Answers
Freshers / Beginner level questions & answers
Ques 1. What is SQL?
SQL-Structured Query Language is a language consisting of commands to store, retrieve, maintain & regulate access to your database.
Ques 2. SQL*Plus
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.
Ques 3. NVL
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.
Ques 4. Date Functions
Date Functions are ADD_MONTHS, LAST_DAY, NEXT_DAY, MONTHS_BETWEEN & SYSDATE.
Ques 5. 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.
Ques 6. TTITLE & BTITLE
TTITLE & BTITLE are commands to control report headings & footers.
Ques 7. COLUMN
COLUMN command define column headings & format data values.
Ques 8. BREAK
BREAK command clarify reports by suppressing repeated values, skipping lines & allowing for controlled break points.
Ques 9. COMPUTE
command control computations on subsets created by the BREAK command.
Ques 10. SET
SET command changes the system variables affecting the report environment.
Ques 11. SPOOL
SPOOL command creates a print file of the report.
Ques 12. JOIN
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.
Ques 13. Union
Union is the product of two or more tables.
Ques 14. Intersect
Intersect is the product of two tables listing only the matching rows.
Ques 15. Minus
Minus is the product of two tables listing only the non-matching rows.
Ques 16. 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.
Ques 17. Multiple columns
Multiple columns can be returned from a Nested Subquery.
Ques 18. Sequences
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.
Ques 19. Synonyms
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.
Ques 20. Indexes
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.
Ques 21. 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.
Ques 22. Order of SQL statement execution
Where clause, Group By clause, Having clause, Order By clause & Select.
Ques 23. Transaction
Transaction is defined as all changes made to the database between successive commits.
Ques 24. Commit
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.
Ques 25. Posting
Posting is an event that writes Inserts, Updates & Deletes in the forms to the database but not committing these transactions to the database.
Ques 26. Rollback
Rollback causes work in the current transaction to be undone.
Ques 27. Savepoint
Savepoint is a point within a particular transaction to which you may rollback without rolling back the entire transaction.
Ques 28. Set Transaction
Set Transaction is to establish properties for the current transaction.
Ques 29. Locking
Locking are mechanisms intended to prevent destructive interaction between users accessing data. Locks are used to achieve.
Ques 30. Consistency
Consistency : Assures users that the data they are changing or viewing is not changed until the are throw with it.
Ques 31. Integrity
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.
Ques 32. Deadlock
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.
Ques 33. 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.
Ques 34. SQL*Loader
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
Ques 35. 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
Ques 36. Operators used in SELECT statements.
= Equal
or != Not equal
> Greater than
< Less than
>= Greater than or equal
<= Less than or equal
BETWEEN an inclusive range
LIKE Search for a pattern
Ques 37. SELECT statements:
SELECT * FROM WITHOUTBOOK;
Ques 38. The INSERT INTO Statements:
INSERT INTO WITHOUTBOOK(NAME,ADDRESS) VALUES('JOHN','GURGAON, INDIA');
Ques 39. The Update Statement:
UPDATE WITHOUTBOOK SET ADDRESS='KOLKATA, INDIA' WHERE NAME='JOHN';
Ques 40. The Delete Statements:
DELETE FROM WITHOUTBOOK WHERE NAME='JOHN';
Ques 41. Sort the Rows:
SELECT * FROM WITHOUTBOOK ORDER BY NAME ASC;
SELECT * FROM WITHOUTBOOK ORDER BY NAME DESC;
Ques 42. The IN operator may be used if you know the exact value you want to return for at least one of the columns.
SELECT * FROM WITHOUTBOOK WHERE NAME IN(SELECT NAME FROM WITHOUTBOOK_BACK WHERE ADDRESS='KOLKATA, INDIA');
Ques 43. BETWEEN ... AND
SELECT column_name FROM table_name WHERE column_name BETWEEN value1 AND value2 The values can be numbers, text, or dates.
Ques 44. 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.
Ques 45. 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.
Ques 46. Which system tables contain information on privileges granted and privileges obtained?
USER_TAB_PRIVS_MADE, USER_TAB_PRIVS_RECD
Ques 47. Which system table contains information on constraints on all the tables created?
USER_CONSTRAINTS.
Ques 48. What is the difference between TRUNCATE and DELETE commands?
TRUNCATE is a DDL command and cannot be rolled back. All of the memory space is released back to the server.
DELETE is a DML command and can be rolled back.
Both commands accomplish identical tasks (removing all data from a table), but TRUNCATE is much faster.
Ques 49. State true or false. !=, <>, ^= all denote the same operation?
True.
Ques 50. State true or false. EXISTS, SOME, ANY are operators in SQL?
True.
Ques 51. What will be the output of the following query?
SELECT REPLACE(TRANSLATE(LTRIM(RTRIM('!! ATHEN !!','!'), '!'), 'AN', '**'),'*','TROUBLE') FROM DUAL;?
TROUBLETHETROUBLE.
Ques 52. What does the following query do? SELECT SAL + NVL(COMM,0) FROM EMP;
This displays the total salary of all employees. The null values in the commission column will be replaced by 0 and added to salary.
Ques 53. 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.
Ques 54. Which command executes the contents of a specified file?
START or @.
Ques 55. What is the value of comm and sal after executing the following query if the initial value of 'sal' is 10000 UPDATE EMP SET SAL = SAL + 1000, COMM = SAL*0.1;?
sal = 11000, comm = 1000.
Ques 56. Which command displays the SQL command in the SQL buffer, and then executes it?
RUN.
Ques 57. What command is used to get back the privileges offered by the GRANT command?
REVOKE.
Ques 58. 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.
Ques 59. Which date function is used to find the difference between two dates?
MONTHS_BETWEEN.
Ques 60. What operator performs pattern matching?
LIKE operator.
Ques 61. What is the use of the DROP option in the ALTER TABLE command?
It is used to drop constraints specified on the table.
Ques 62. What operator tests column for the absence of data?
IS NULL operator.
Ques 63. 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.
Ques 64. Which function is used to find the largest integer less than or equal to a specific value?
FLOOR.
Ques 65. Which is the subset of SQL commands used to manipulate Oracle Database structures, including tables?
Data Definition Language (DDL).
Ques 66. 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.
Ques 67. What command is used to create a table by copying the structure of another table?
CREATE TABLE .. AS SELECT command
Explanation:
To copy only the structure, the WHERE clause of the SELECT command should contain a FALSE statement as in the following.
CREATE TABLE NEWTABLE AS SELECT * FROM EXISTINGTABLE WHERE 1=2;
If the WHERE condition is true, then all the rows or rows satisfying the condition will be copied to the new table.
Ques 68. TRUNCATE TABLE EMP; DELETE FROM EMP; Will the outputs of the above two commands differ?
Both will result in deleting all the rows in the table EMP..
Ques 69. What is the output of the following query SELECT TRUNC(1234.5678,-2) FROM DUAL;?
1200.
Ques 70. What are the wildcards used for pattern matching.?
_ for single character substitution and % for multi-character substitution.
Ques 71. What is the parameter substitution symbol used with INSERT INTO command?
&
Ques 72. What's an SQL injection?
SQL Injection is when form data contains an SQL escape sequence and injects a new SQL query to be run.
Ques 73. What is difference between TRUNCATE & DELETE ?
TRUNCATE commits after deleting entire table i.e., cannot be rolled back. Database triggers do not fire on TRUNCATE
DELETE allows the filtered deletion. Deleted records can be rolled back or committed. Database triggers fire on DELETE.
Ques 74. What is a join? Explain the different types of joins?
Join is a query, which retrieves related columns or rows from multiple tables.
Self Join - Joining the table with itself.
Equi Join - Joining two tables by equating two common columns.
Non-Equi Join - Joining two tables by equating two common columns.
Outer Join - Joining two tables in such a way that query can also retrieve rows that do not have corresponding join value in the other table.
Ques 75. What is the sub-query?
Sub-query is a query whose return values are used in filtering conditions of the main query.
Ques 76. What is correlated sub-query?
Correlated sub-query is a sub-query, which has reference to the main query.
Ques 77. Explain CONNECT BY PRIOR?
Retrieves rows in hierarchical order eg.
select empno, ename from emp where.
Ques 78. 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.
Ques 79. Explain UNION, MINUS, UNION ALL and INTERSECT?
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.
Ques 80. 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.
Ques 81. What is the fastest way of accessing a row in a table?
Using ROWID.
CONSTRAINTS
Ques 82. What is an integrity constraint?
Integrity constraint is a rule that restricts values to a column in a table.
Ques 83. 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.
Ques 84. 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.
Ques 85. What is ON DELETE CASCADE?
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.
Ques 86. What are the data types allowed in a table?
CHAR, VARCHAR2, NUMBER, DATE, RAW, LONG and LONG RAW.
Ques 87. 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.
Ques 88. 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.
Ques 89. 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.
Ques 90. Where the integrity constraints are stored in data dictionary?
The integrity constraints are stored in USER_CONSTRAINTS.
Ques 91. How will you activate/deactivate integrity constraints?
The integrity constraints can be enabled or disabled by ALTER TABLE ENABLE CONSTRAINT / DISABLE CONSTRAINT.
Ques 92. 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.
Ques 93. What is a database link?
Database link is a named path through which a remote database can be accessed.
Ques 94. 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.
Ques 95. 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.
Ques 96. 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.
Ques 97. 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.
Ques 98. 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.
Ques 99. 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.
Ques 100. 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.
Ques 101. 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.
Ques 102. Which three of the following are implicit cursor attributes?
1. %found
2. %too_many_rows
3. %notfound
4. %rowcount
5. %rowtype
Ques 103. If left out, which of the following would cause an infinite loop to occur in a simple loop?
1. LOOP
2. END LOOP
3. IF-THEN
4. EXIT
Ques 104. 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.
Ques 105. 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.
Ques 106. 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
The answer is: It loads the current row values into variables.
Ques 107. 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
Ques 108. 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
Ques 109. Which Oracle supplied package can you use to output values and messages from database triggers, stored procedures and functions within SQL*Plus?
1. DBMS_DISPLAY
2. DBMS_OUTPUT
3. DBMS_LIST
4. DBMS_DESCRIBE
Ques 110. 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.
Ques 111. 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?
1. DBMS_DDL
2. DBMS_DML
3. DBMS_SYN
4. DBMS_SQL
Ques 112. 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',
'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ',
'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX'),'X')
FROM dual;
It returns 0 if it is a number, 1 if it is not.
Ques 113. 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
Ques 114. 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:
SELECT a.*
FROM ter.ter_master a
WHERE NOT EXISTS (SELECT b.repno FROM ermast b
WHERE a.repno=b.repno) AND
(a.brepno = 'ALL' or a.repno > a.brepno)
ORDER BY a.repno;
Ques 115. 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.
Ques 116. 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.
Ques 117. What does the OLTP stands for ?
OLTP stands for On Line Transaction Processing
Ques 118. What is the most important requirement for OLTP ?
OLTP requires real time response.
Ques 119. 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.
Ques 120. Why is the most of the processing done at the sever ?
To reduce the network traffic and for application sharing and implementing business rules.
Ques 121. 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.
Ques 122. What does one do when one is rightsizing ?
With rightsizing, one would move applications to the most appropriate server platforms.
Ques 123. What does the term downsizing refer to ?
A host based application is re-engineered to run in smaller or LAN based environment.
Ques 124. What is event trigger ?
An event trigger, a segment of code which is associated with each event and is fired when the event occurs.
Ques 125. 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.
Ques 126. What are the types of processes that a server runs ?
Foreground process and Background process.
Ques 127. What is a event handler ?
An event handler is a routine that is written to respond to a particular event.
Ques 128. 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.
Ques 129. 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.
Ques 130. 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.
Ques 131. Why are the integrity constraints preferred to database triggers ?
Because it is easier to define an integrity constraint than a database trigger.
Ques 132. 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.
Ques 133. What are the three components of a client server model ?
A Client,
A Server and
A Network/Communication software.
Ques 134. 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.
Ques 135. What are the disadvantages of the client/server model ?
Heterogeneity of the system results in reduced reliability. May not be suitable for all applications. Managing and tuning networks becomes difficult.
Ques 136. What are the different topologies available for network ?
Star,
Bus,
Ring.
Ques 137. What is the first work of Client process ?
A client process at first establishes connection with the Server.
Ques 138. What are the responsibilities of a Server ?
1. Manage resources optimally across multiple clients.
2. Controlling database access and security.
3. Protecting the database and recovering it from crashes.
4. Enforcing integrity rules globally.
Ques 139. 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.
Ques 140. Give some examples of standard API??
Open Database Connectivity (ODBC),
Integrated Database Application Programming Interface (IDAPI),
XOpen
SQL/CLI
Ques 141. 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.
Ques 142. What is the main disadvantage of developing an application using an API ?
The application cannot use any special features of the backend server.
Ques 143. 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.
Ques 144. What are the four types of events ?
1. System Events.
2. Control Events
3. User Events
4. Other Events.
Ques 145. 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.
Ques 146. 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.
Ques 147. What are the two components of ODBC ?
1. An ODBC manager/administrator and
2. ODBC driver.
Ques 148. What is the function of a ODBC manager ?
The ODBC Manager manages all the data sources that exists in the system.
Ques 149. What is the function of a ODBC Driver ?
The ODBC Driver allows the developer to talk to the back end database.
Ques 150. 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.
Ques 151. What are stored procedures? How is it useful?
A stored procedure is a set of statements/commands which reside in the database. The stored procedure is pre-compiled and saves the database the effort of parsing and compiling sql statements every time a query is run. Each database has its own stored procedure language, usually a variant of C with a SQL preproceesor. Newer versions of db's support writing stored procedures in Java and Perl too. Before the advent of 3-tier/n-tier architecture it was pretty common for stored procs to implement the business logic( A lot of systems still do it). The biggest advantage is of course speed. Also certain kind of data manipulations are not achieved in SQL. Stored procs provide a mechanism to do these manipulations. Stored procs are also useful when you want to do Batch updates/exports/houseKeeping kind of stuff on the db. The overhead of a JDBC Connection may be significant in these cases.
Experienced / Expert level questions & answers
Ques 152. How to Retrieve Warnings?
SQLWarning objects are a subclass of SQLException that deal with database access warnings. Warnings do not stop the execution of an application, as exceptions do; they simply alert the user that something did not happen as planned. A warning can be reported on a Connection object, a Statement object (including PreparedStatement and CallableStatement objects), or a ResultSet object. Each of these classes has a getWarnings method, which you must invoke in order to see the first warning reported on the calling object
SQLWarning warning = stmt.getWarnings();
if (warning != null)
{
while (warning != null)
{
System.out.println("Message: " + warning.getMessage());
System.out.println("SQLState: " + warning.getSQLState());
System.out.print("Vendor error code: ");
System.out.println(warning.getErrorCode());
warning = warning.getNextWarning();
}
}
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