Unix_Commands

For finding a file in current directory and all subdirectory

find . -name "file_name.File_to_be_found" -print



File display with line numbers:
cat -n file_name_with_full_path

disk free space:
df -h
du -h

Network Programming

http://en.wikipedia.org/wiki/Computer_network_programming


http://msdn.microsoft.com/en-us/library/4as0wz7t.aspx


http://beej.us/guide/bgnet/output/print/bgnet_A4.pdf - C network programming

http://www.kohala.com/start/rstevensfaq.html - Unix Network Programming - Guru


http://download.oracle.com/javase/tutorial/networking/urls/index.html - Java Network Programming

List Vs Linked List

http://stackoverflow.com/questions/169973/when-should-i-use-a-list-vs-a-linkedlist

Linked lists provide very fast insertion or deletion of a list member. Each member in a linked list contains a pointer to the next member in the list so to insert a member at position i:

• update the pointer in member i-1 to point to the new member
• set the pointer in the new member to point to member i

The disadvantage to a linked list is that random access is not possible. Accessing a member requires traversing the list until the desired member is found.

.NET CORE

Difference Between HashTable and Dictionary:

http://stackoverflow.com/questions/876656/difference-between-dictionary-and-hashtable

Simply, Dictionary<TKey,TValue> is a generic type, allowing:

• static typing (and compile-time verification)
• use without boxing

If you are .NET 2.0 or above, you should prefer Dictionary<TKey,TValue> (and the other generic collections)

A subtle but important difference is that Hashtable supports multiple reader threads with a single writer thread, while Dictionary offers no thread safety. If you need thread safety with a generic dictionary, you must implement your own synchronization or (in .NET 4.0) useConcurrentDictionary<TKey, TValue>.

SQL - Interview Questions

Can one retrieve only the Nth row from a table?
SELECT * FROM (
   SELECT name,ROWNUM RN FROM test_employee WHERE ROWNUM < 8 )
 WHERE  RN = 7;


http://www.orafaq.com/wiki/ROWNUM


Any other efficient way:
--returns the 7th row, assuming minimum 7 rows exists
SELECT rownum,name,id,mid FROM test_employee a
 GROUP BY rownum,name,id,mid HAVING rownum = 7;


--returns the fourth row, assuming minimum 4 rows exists
SELECT * FROM test_employee WHERE rownum=1 AND rowid NOT IN
(SELECT rowid FROM test_employee WHERE rownum <= 3);


--returns 8th row, assuming, min of 8 rows exists
SELECT * FROM test_employee t1
WHERE  rowid = (
        SELECT rowid FROM test_employee t1
        WHERE  rownum <= 8
        MINUS
        SELECT rowid FROM test_employee t1
        WHERE  rownum < 8);


--Adding a column after a specific column
ALTER TABLE tablename ADD columnname AFTER columnname;


CREATE TABLE tab1 AS SELECT 0 AS col1, col1 AS col2 FROM tab1_old; .
alter table test_employee add  dept_id number;
alter table test_employee add  sex varchar2(1);


--to find the males and females in a company
select  sum(  decode(sex,'M',1,0)) MALE,
             sum(  decode(sex,'F',1,0)) FEMALE,
             count(decode(sex,'M',1,'F',1)) TOTAL
  from test_employee;


--To find the number of males, females department wise
select dept_id, sum(  decode(sex,'M',1,0)) MALE,
             sum(  decode(sex,'F',1,0)) FEMALE,
             count(decode(sex,'M',1,'F',1)) TOTAL
  from test_employee
 group by dept_id;


--2nd max from the table
select max(id) from test_employee where id <
(select max(id) from test_employee);


Implementing IF-THEN-ELSE login in select statement:
One can use the CASE statement or functions like DECODE, NVL, NVL2, NULLIF, COALESCE, etc. 

SQL - Update Statements

Scenario: (Uses Oracle-SQL)


HR asks to increment the salary


1. <= 3000, increase by 1000;
2. <= 4000, increase by 500;
3. <= 5000 increase by 250;


Example:
Current Salary: 3000 then New Salary should be 3000 + 1000 + 500 + 250 = 4750


Solution:





create table emp_salary
(
  id               number,
  name             varchar2(20),
  current_sal      number(20,2),
  new_sal          number(20,2)
);

insert into EMP_SALARY (id, name, current_sal)
values (1, 'Mani', 3000);
insert into EMP_SALARY (id, name, current_sal)
values (2, 'abhishek', 4000);
insert into EMP_SALARY (id, name, current_sal)
values (3, 'ajay', 5000);
insert into EMP_SALARY (id, name, current_sal)
values (4, 'arun', 2000);
commit;




1. Not efficient:



update emp_salary
set new_sal = current_sal + 1000 + 500 + 250
where current_sal > 0 and current_sal <= 3000;
update emp_salary
set new_sal = current_sal +  500 + 250
where current_sal >= 3001 and current_sal <= 4000;
update emp_salary
set new_sal = current_sal +  250
where current_sal >= 4001 and current_sal <= 5000;
commit;


2. Efficient:


update test_emp_sal
set current_sal =
   case
      when current_sal > 0 AND current_sal <= 3000 then current_sal + 1000 + 500 + 250
      when current_sal > 3001 AND current_sal <= 4000 then current_sal +  500 + 250
      when current_sal > 4001 AND current_sal <= 5000 then current_sal +  250
   end
;
commit;


==================================================================================

update orders
set order_mode_num =
   decode(order_mode,
             'direct',100,
             'online',200,
             'walmart',300,
             'amazon',400,
          0)
;
==================================================================================

update orders
set order_mode_num =
   case
      when order_mode = 'direct' and
           customer_id < 102 then 500
      when order_mode = 'direct' then 100
      when order_mode = 'online' then 200
      when order_mode = 'walmart' then 300
      when order_mode = 'amazon' then 400
      else 0
   end
;
=================================================================================

CAREER!!!

What one skill, if you developed and practiced it consistently in an excellent fashion, would have the greatest positive impact on your career?   ----Brian Tracy.