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Tuesday, 3 January 2012

Oracle Interview FAQ


Oracle Sample Questions
The Oracle Database is the product of Oracle Corporation. Oracle is released in 1979.
It is a multi-user relational database management system for small as well as large systems.
Oracle is in the same market space as DB2, Sybase, SQL Server and a few other products.

Oracle SQL
  1. What is SQL?
Answer: Structured Query Language (SQL) is a language that provides an interface to relational database systems.
In common usage SQL also encompasses DML (Data Manipulation Language), for INSERTs, UPDATEs, DELETEs and DDL (Data Definition Language), used for creating and modifying tables and other database structures.
  1. What is SELECT statement?
Answer: The SELECT statement lets you select a set of values from a table in a database.The values selected from the database table would depend on the various conditions that are specified in the SQL query.
  1. How can you compare a part of the name rather than the entire name?
Answer: SELECT * FROM people WHERE empname LIKE '%ab%'Would return a recordset with records consisting empname the sequence 'ab' in empname.
  1. What is the INSERT statement?
Answer: The INSERT statement lets you insert information into a database.
  1. How do you delete a record from a database?
Answer: Use the DELETE statement to remove records or any particular column values from a database.
  1. How could I get distinct entries from a table?
Answer: The SELECT statement in conjunction with DISTINCT lets you select a set of distinct values from a table in a database.The values selected from the database table would of course depend on the various conditions that are specified in the SQL query.ExampleSELECT DISTINCT empname FROM emptable
  1. How to get the results of a Query sorted in any order?
Answer: You can sort the results and return the sorted results to your program by using ORDER BY keyword thus saving you the pain of carrying out the sorting yourself.The ORDER BY keyword is used for sorting.SELECT empname, age, city FROM emptable ORDER BY empname
  1. How can I find the total number of records in a table?
Answer: You could use the COUNT keyword , exampleSELECT COUNT(*) FROM emp WHERE age>40
  1. What is GROUP BY?
Answer: The GROUP BY keywords have been added to SQL because aggregate functions (like SUM) return the aggregate of all column values every time they are called.Without the GROUP BY functionality, finding the sum for each individual group of column values was not possible.
  1. What is the difference among "dropping a table", "truncating a table" and "deleting all records" from a table.
Answer: Dropping : (Table structure + Data are deleted), Invalidates the dependent objects ,Drops the indexes Truncating: (Data alone deleted), Performs an automatic commit, Faster than deleteDelete : (Data alone deleted), Doesn't perform automatic commit

  1. What are the Large object types suported by Oracle?
Answer: Blob and Clob.
  1. Difference between a "where" clause and a "having" clause.
Answer: Having clause is used only with group functions whereas Where is not used with.
  1. What's the difference between a primary key and a unique key?
Answer: Both primary key and unique enforce uniqueness of the column on which they are defined.But by defaultprimary key creates a clustered index on the column, where are unique creates a nonclustered index by default.Another major difference is that, primary key doesn't allow NULLs, but unique key allows one NULL only.
  1. What are cursors? Explain different types of cursors.What are the disadvantages of cursors? How can you avoid cursors?
Answer: Cursors allow row-by-row prcessing of the resultsets.Types of cursors: Static, Dynamic, Forward-only, Keyset-driven.See books online for more information.Disadvantages of cursors: Each time you fetch a row from the cursor, it results in a network roundtrip, where as a normal SELECT query makes only one rowundtrip, however large the resultset is.Cursors are also costly because they require more resources and temporary storage (results in more IO operations).Furthere, there are restrictions on the SELECT statements that can be used with some types of cursors.Most of the times, set based operations can be used instead of cursors.
  1. What are triggers? How to invoke a trigger on demand?
Answer: Triggers are special kind of stored procedures that get executed automatically when an INSERT, UPDATE or DELETE operation takes place on a table.Triggers can't be invoked on demand.They get triggered only when an associated action (INSERT, UPDATE, DELETE) happens on the table on which they are defined.Triggers are generally used to implement business rules, auditing.Triggers can also be used to extend the referential integrity checks, but wherever possible, use constraints for this purpose, instead of triggers, as constraints are much faster.
  1. What is a join and explain different types of joins.
Answer: Joins are used in queries to explain how different tables are related.Joins also let you select data from a table depending upon data from another table.Types of joins: INNER JOINs, OUTER JOINs, CROSS JOINs.OUTER JOINs are further classified as LEFT OUTER JOINS, RIGHT OUTER JOINS and FULL OUTER JOINS.
  1. What is a self join?
Answer: Self join is just like any other join, except that two instances of the same table will be joined in the query.
  1. How can I eliminate duplicates values in a table?
Answer: SQL> DELETE FROM table_name A WHERE ROWID > (SELECT min(rowid) FROM table_name B WHERE A.key_values = B.key_values);
  1. How can one examine the exact content of a database column?
Answer: SELECT DUMP(col1)
FROM tab1
WHERE cond1 = val1;
DUMP(COL1)
Typ=96 Len=4: 65,66,67,32
For this example the type is 96, indicating CHAR, and the last byte in the column is 32, which is the ASCII code for a space.This tells us that this column is blank-padded.
  1. How can I change my Oracle password?
Answer: From SQL*Plus type: ALTER USER username IDENTIFIED BY new_password

  1. What are the most important DDL statements in SQL?
Answer: 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
  1. What are the different SELECT statements?
Answer: The SELECT statements are:
    • SELECT column_name(s) FROM table_nameSELECT 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
  1. What are the INSERT INTO Statements?
Answer: The INSERT INTO statements are:
    • INSERT INTO table_name VALUES (value1, value2,....)
    • INSERT INTO table_name (column1, column2,...) VALUES (value1, value2,....)
  1. Write theUpdate Statement?
Answer: UPDATE table_name SET column_name = new_value WHERE column_name = some_value
  1. What are the Delete Statements?
Answer: These are:
    • DELETE FROM table_name WHERE column_name = some_value
Delete All Rows:
    • DELETE FROM table_name
    • DELETE * FROM table_name
  1. how we can sort the Rows?
Answer: There are mainly three types:
    • 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
  1. Explain IN operator.
Answer: 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,..)
  1. Explain BETWEEN...AND
Answer: SELECT column_name FROM table_name WHERE column_name BETWEEN value1 AND value2 The values can be numbers, text, or dates.





Oracle Sample Questions : Oracle PL/SQL
  1. What is PL/SQL?
Answer: PL/SQL is Oracle's Procedural Language extension to SQL.PL/SQL's language syntax, structure and data types are similar to that of ADA.
The language includes object oriented programming techniques such as encapsulation, function overloading, information hiding.
  1. How can I protect my PL/SQL source code?
Answer: PL/SQL V2.2, available with Oracle7.2, implements a binary wrapper for PL/SQL programs to protect thesource 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.plb
  1. Can one read/write files from PL/SQL?
Answer: 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.
Before Oracle 7.3 the only means of writing a file was to use DBMS_OUTPUT with the SQL*Plus SPOOL command.
DECLARE
fileHandler UTL_FILE.FILE_TYPE;
BEGIN
fileHandler := UTL_FILE.FOPEN('/home/oracle/tmp', 'myoutput', 'W');
UTL_FILE.PUTF(fileHandler, 'Value of func1 is %s\n', func1(1));
UTL_FILE.FCLOSE(fileHandler);
END;
  1. Can one use dynamic SQL within PL/SQL?
Answer: From PL/SQL V2.1 one can use the DBMS_SQL package to execute dynamic SQL statements.
Eg:
CREATE OR REPLACE PROCEDURE DYNSQL AS
cur integer;
rc integer;
BEGIN
cur := DBMS_SQL.OPEN_CURSOR;
DBMS_SQL.PARSE(cur, 'CREATE TABLE X (Y DATE)', DBMS_SQL.NATIVE);
rc := DBMS_SQL.EXECUTE(cur);
DBMS_SQL.CLOSE_CURSOR(cur);
END;
  1. How does one get the value of a sequence into a PL/SQL variable?
Answer: select sq_sequence.NEXTVAL into :i from dual;
  1. Is there a PL/SQL Engine in SQL*Plus?
Answer: No.All your PL/SQL are send directly to the database engine for execution.
  1. Is there a limit on the size of a PL/SQL block?
Answer: 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'

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