Database Interview Questions - 4

Tuesday, May 4, 2010

Q: Can dual table be deleted, dropped or altered or updated or inserted?
A: Yes

Q: If content of dual is updated to some value computation takes place or not?
A: Yes

Q: If any other table same as dual is created would it act similar to dual?
A: Yes

Q: For which relational operators in where clause, index is not used?
A: <> , like '% ...' is NOT functions, field +constant, field || ''

Q: Assume that there are multiple databases running on one machine. How can you switch from one to another?
A: Changing the ORACLE_SID

Q: What are the advantages of Oracle?

·         Portability : Oracle is ported to more platforms than any of its competitors, running on more than 100 hardware platforms and 20 networking protocols.

·         Market Presence : Oracle is by far the largest RDBMS vendor and spends more on R & D than most of its competitors earn in total revenue. This market clout means that you are unlikely to be left in the lurch by Oracle and there are always lots of third party interfaces available.

·         Backup and Recovery : Oracle provides industrial strength support for on-line backup and recovery and good software fault tolerence to disk failure. You can also do point-in-time recovery.

·         Performance : Speed of a 'tuned' Oracle Database and application is quite good, even with large databases. Oracle can manage > 100GB databases.

·         Multiple database support : Oracle has a superior ability to manage multiple databases within the same transaction using a two-phase commit protocol.

Q: What is a forward declaration? What is its use?
A: PL/SQL requires that you declare an identifier before using it. Therefore, you must declare a subprogram before calling it. This declaration at the start of a subprogram is called forward declaration. A forward declaration consists of a subprogram specification terminated by a semicolon.

Q: What are actual and formal parameters?

·         Actual Parameters : Subprograms pass information using parameters. The variables or expressions referenced in the parameter list of a subprogram call are actual parameters. For example, the following procedure call lists two actual parameters named emp_num and amount:
Eg. raise_salary(emp_num, amount);

·         Formal Parameters : The variables declared in a subprogram specification and referenced in the subprogram body are formal parameters. For example, the following procedure declares two formal parameters named emp_id and increase:
Eg. PROCEDURE raise_salary (emp_id INTEGER, increase REAL) IS current_salary REAL;

Q: What are the types of Notation?
A: Position, Named, Mixed and Restrictions.

Q: What all important parameters of the init.ora are supposed to be increased if you want to increase the SGA size?
A: In our case, db_block_buffers was changed from 60 to 1000 (std values are 60, 550 & 3500) shared_pool_size was changed from 3.5MB to 9MB (std values are 3.5, 5 & 9MB) open_cursors was changed from 200 to 300 (std values are 200 & 300) db_block_size was changed from 2048 (2K) to 4096 (4K) {at the time of database creation}.
The initial SGA was around 4MB when the server RAM was 32MB and The new SGA was around 13MB when the server RAM was increased to 128MB.

Q: If I have an execute privilege on a procedure in another users schema, can I execute his procedure even though I do not have privileges on the tables within the procedure?
A: Yes

Q: What are various types of joins?
A: Equijoins, Non-equijoins, self join, outer join

Q: What is a package cursor?
A: A package cursor is a cursor which you declare in the package specification without an SQL statement. The SQL statement for the cursor is attached dynamically at runtime from calling procedures.

Q: If you insert a row in a table, then create another table and then say Rollback. In this case will the row be inserted?
A: Yes. Because Create table is a DDL which commits automatically as soon as it is executed. The DDL commits the transaction even if the create statement fails internally (eg table already exists error) and not syntactically.

Q: What are the various types of queries?

·         Normal Queries

·         Sub Queries

·         Co-related queries

·         Nested queries

·         Compound queries

Q: What is a transaction?
A: A transaction is a set of SQL statements between any two COMMIT and ROLLBACK statements.

Q: What is implicit cursor and how is it used by Oracle?
A: An implicit cursor is a cursor which is internally created by Oracle. It is created by Oracle for each individual SQL.

Q: Which of the following is not a schema object : Indexes, tables, public synonyms, triggers and packages ?
A: Public synonyms

Q: What is the difference between a view and a snapshot?

Q: What is PL/SQL?
A: PL/SQL is Oracle's Procedural Language extension to SQL. The language includes object oriented programming techniques such as encapsulation, function overloading, information hiding (all but inheritance), and so, brings state-of-the-art programming to the Oracle database server and a variety of Oracle tools.

Q: Is there a PL/SQL Engine in SQL*Plus?
A: No. Unlike Oracle Forms, SQL*Plus does not have a PL/SQL engine. Thus, all your PL/SQL are send directly to the database engine for execution. This makes it much more efficient as SQL statements are not stripped off and send to the database individually.

Q: Is there a limit on the size of a PL/SQL block?
A: Currently, the maximum parsed/compiled size of a PL/SQL block is 64K and the maximum code size is 100K. You can run the following select statement to query the size of an existing package or procedure.
SQL> select * from dba_object_size where name = 'procedure_name'

Q: Can one read/write files from PL/SQL?
A: Included in Oracle 7.3 is a UTL_FILE package that can read and write files. The directory you intend writing to has to be in your INIT.ORA file (see UTL_FILE_DIR=... parameter). Before Oracle 7.3 the only means of writing a file was to use DBMS_OUTPUT with the SQL*Plus SPOOL command.
fileHandler := UTL_FILE.FOPEN('/home/oracle/tmp', 'myoutput','W');
UTL_FILE.PUTF(fileHandler, 'Value of func1 is %sn', func1(1));

Q: How can I protect my PL/SQL source code?
A: PL/SQL V2.2, available with Oracle7.2, implements a binary wrapper for PL/SQL programs to protect the source code. This is done via a standalone utility that transforms the PL/SQL source code into portable binary object code (somewhat larger than the original). This way you can distribute software without having to worry about exposing your proprietary algorithms and methods. SQL*Plus and SQL*DBA will still understand and know how to execute such scripts. Just be careful, there is no "decode" command available.
The syntax is:
wrap iname=myscript.sql oname=xxxx.yyy

Q: Can one use dynamic SQL within PL/SQL? OR Can you use a DDL in a procedure? How?
A: From PL/SQL V2.1 one can use the DBMS_SQL package to execute dynamic SQL statements.

cur integer;
rc integer;
rc := DBMS_SQL.EXECUTE(cur);


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