Interview SQL queries

1.   Find the Duplicate records in table.
 
   1. SELECT email, COUNT(email) AS NumOccurrences
FROM T 
GROUP BY email HAVING ( COUNT(email) > 1 ) ;
  
 2. SELECT * FROM emp a
     WHERE rowid = ( SELECT max(rowid) FROM emp
                            WHERE empno = a.empno
                            GROUP BY empno HAVING count(*) >1)

2.   Delete Duplicate records from table.
    1.  DELETE FROM table_name A 
WHERE ROWID > (SELECT min(rowid) 
FROM table_name B  
WHERE A.key_val = B.key_val);


    2. 


1.   Find the Duplicate records in table.





















Consider the below DEPT and EMPLOYEE table and answer the below queries.
DEPT
DEPTNO (NOT NULL , NUMBER(2)),
DNAME (VARCHAR2(14)),
LOC (VARCHAR2(13)

EMPLOYEE
EMPNO (NOT NULL , NUMBER(4)),
ENAME (VARCHAR2(10)),
JOB (VARCHAR2(9)),
MGR (NUMBER(4)),
HIREDATE (DATE),
SAL (NUMBER(7,2)),
COMM (NUMBER(7,2)),
DEPTNO (NUMBER(2))

MGR is the EMPno of the Employee whom the Employee reports to.DEPTNO is a foreign key
1. List all the Employees who have at least one person reporting to them.</span><br /> 

SELECT ENAME FROM EMPLOYEE WHERE EMPNO IN (SELECT MGR FROM EMPLOYEE); 

2. List the highest salary paid for each job.

SELECT JOB, MAX(SAL) FROM EMPLOYEE GROUP BY JOB

3. In which year did most people join the company? Display the year and the number of Employees 
SELECT TO_CHAR(HIREDATE,'YYYY') "YEAR", COUNT(EMPNO) "NO. OF EMPLOYEES"
FROM EMPLOYEE
GROUP BY TO_CHAR(HIREDATE,'YYYY')
HAVING COUNT(EMPNO) = ( SELECT MAX(COUNT(EMPNO))
FROM EMPLOYEE
GROUP BY TO_CHAR(HIREDATE,'YYYY'));
  
4.  Write a correlated sub-query to list out the Employees who earn more than the average salary of their department. 
SELECT ENAME,SAL
FROM EMPLOYEE E
WHERE SAL &gt; (SELECT AVG(SAL)
FROM EMPLOYEE F
WHERE E.DEPTNO = F.DEPTNO);

5. Find the nth maximum salary.  
SELECT ENAME, SAL
FROM EMPLOYEE A
WHERE &N = (SELECT COUNT (DISTINCT(SAL))
FROM EMPLOYEE B
WHERE A.SAL <=B.SAL);  

6. Select the duplicate records (Records, which are inserted, that already exist) in the EMPLOYEE table.  
SELECT * FROM EMPLOYEE A
WHERE A.EMPNO IN (SELECT EMPNO                                        
FROM EMPLOYEE                                        
GROUP BY EMPNO                                          
HAVING COUNT(EMPNO)> 1)
AND A.ROWID!=MIN (ROWID));


7.  Write a query to list the length of service of the Employees (of the form n years and m months).
SELECT ENAME "EMPLOYEE",
        TO_CHAR(TRUNC(MONTHS_BETWEEN(SYSDATE,HIREDATE)/12))
||' YEARS '|| TO_CHAR(TRUNC(MOD(MONTHS_BETWEEN
(SYSDATE, HIREDATE),12)))||' MONTHS ' "LENGTH OF SERVICE"
FROM EMPLOYEE;

Joseph Problem...

Joseph likes taking part in programming contests. His favorite problem is, of course, Joseph's problem. It is stated as follows.

Problem : There are n persons numbered from 0 to n - 1 standing in a circle. The person number k, counting from the person number 0, is executed. After that the person number k of the remaining persons is executed, counting from the person after the last executed one. The process continues until only one person is left. This person is a survivor. The problem is, given n and k detect the survivor's number in the original circle.

  Solution: You can easily write a recursive write:
 Example:(20, 3) = 4+17-1 =20 Find J (20, 3), assume that you have to know the J (19, 3) = 17, then remove the number 3 20 people who left with 19 people, and this 19 starting from the number 4, 17 the last person is left, so J (20, 3) = 4 +17-1 = 20
 

public class JoshephProblem {
    public static void main(String[] args) {
        System.out.println(">>> " + J(5, 3));
    }

    public static int J(int n, int m) {
        if (n == 2) {
            if (m % 2 == 1)
                return 2;
            else
                return 1;
        } else {
            int temp = (J(n - 1, m) + m) % n;
            if (temp == 0)
                return n;
            else
                return temp;
        }
    }
}

Why Marker Interface ?

he main purpose to have marker interfaces is to create special types in those cases where the types themselves have no behavior particular to them. If there is no behavior then why to have an interface? Because the implementer of the class might only need to flag that it belongs to that particular type and everything else is handled/done by some other unit - either internal to Java (as in the case of Java supplied standard marker interfaces) or an app specific external unit.

Let's understand this by two examples - one in which we will discuss the purpose of a standard Java interface (Cloneable) and then another user-created marker interface.

When JVM sees a clone() method being invoked on an object, it first verifies if the underlying class has implemented the 'Cloneable' interface or not. If not, then it throws the exception CloneNotSupportedException. 
Assuming the underlying class has implemented the 'Cloneable' interface, JVM does some internal work (maybe by calling some method) to facilitate the cloning operation.
So, effectively marker interfaces kind of send out a signal to the corresponding external/internal entity (JVM in case of Cloneable) for them to arrange for the necessary functionality. 

public Object clone() throws CloneNotSupportedException {

 if (this implements Cloneable)

     return nativeCloneImpl();

 else

     throw new CloneNotSupportedException();

}

 
Can anyone tell why and when do we get 'CloneNotSupportedException' exception at compile-time itself? Well... that's no trick. If you see the signature of the 'Object.clone()' method carefully, you will see a throws clause associated with it. I'm sure how can you get rid of it: (i) by wrapping the clone-invocation code within appropriate try-catch (ii) throwing the CloneNotSupportedException from the calling method.

What purpose does a user-defined marker interface serve? It can well serve the same purpose as by any standard marker interface, but in that case the container (the module controlling the execution of the app) has to take the onus of making sure that whenever a class implements that interface it does the required work to support the underlying behavior - the way JVM does for Cloneable or any other standard marker interface for that matter.


user-defined marker interface in Java:-
to be continued........ 

DB Indexes

Q. What's DB indexes ?
A. basically used to sort data in a table.
There are basically 2 types of DB indexes:- Clustered and non-clusterd

Q. What's Mean By Cluster?
A. Oracle cluster supply an optional way of storing table data. In a cluster a group of tables share the same data blocks. The tables are grouped together because they share identical columns and are often used together. For example, the employee and department table use the depth no column. When you cluster the employee and department tables Oracle Database physically stores all rows for each department from both the employee and department tables in the same data blocks.
A cluster is used in database so that to keep all the rows of all data tables together and they can easily be accessed by the help of shared cluster key.

Because clusters store relevant rows of dissimilar tables together in the one data blocks, correctly used clusters offer two primary benefits:
■ Disk I/O is condensed and access time improves for joins of clustered tables.
■ The cluster key is the column, or collection of columns, that the clustered tables have
in same. You identify the columns of the cluster key when creating the cluster.
You next specify the same columns when creating every table added to the cluster. Each cluster key value is saved only single time each in the cluster and the cluster index, no concern how many rows of different tables hold the value.

Q. What's Cluster and Non-Cluster Indexes ?

A clustered Index sort the data in a table physically.
A non-clustered Index doesn't sort the data physically.

As clustered Index sort the data physically, So there can be only one Clustered index per table.
where as there can be many non-clustered index in table.
(with a maximum of 255 such indexes per table a very authentic possibility. i.e. every single column can be utilize for non-cluster indexing )

Q. when you can use a clustered or a non clustered index????
A. The clustered is a safe bet if you are working with a table having a large number of unique data, such as a table containing the employee IDs for the different members of an organization. These IDs are always unique, and using a clustered index is a good way of retrieving this large volume of unique data rapidly. On the other hand, a non clustered index is the best approach if you have a table that does not contain too many such unique values. These are just some of the differences between the two. You can read in detail about these differences by putting a search on the Internet for the differences between the two.

- - - - - -- - - - -- - - -

BST to Doubly Link List

To solve this problem, First we will analyze the different Cases :-
  Case 1: Leaf Node - left & right child is Null
  Case 2: No Left Subtree   - left Child is Null
  Case 2: No Right Subtree  - right Child is Null
  Case 2: No Left Subtree   - left Child is Null

Java 'ArrayList' Pseudocode ....

protected int modCount = 0;

ArrayList(int initialSize)

        IF  initialSize<0  THEN
                       Throw exception "Invalid Size"
       else
             arrayElement= new Object[initialSize];
        

ArrayList() 

  Call ArrayList(defaultSize); // default-size is 10

ArrayList(Collection<? extends E> C)
     arrayElement = C.toArray();
     size = arrayElement.length;
     IF arrayElement.getClass() != Object[].class THEN
                 arrayElement = Arrays.copyOf(arrayElement, size, Object[].class);

ensureCapacity(int newCapacity) 

   modCount++
   oldCapacity  = arrayElement.length;
    IF newCapacity > oldCapacity THEN
         Object[] oldData = arrayElement;
         newCap = (oldCap *3)/2 +1;
         IF newCap < newCapacity THEN
                   newCap=newCapacity;

         ENDIF
    END IF
  arrayElement =Arrays.copyOf(arrayElement, newCap); // My own implementation is at the end.

add(E data)
    ensureCapacity(size + 1);  // Increments modCount!!
    elementData[size++] = data;


add(int index, E element)
 IF index > size OR  index < 0 THEN
        throw new IndexOutOfBoundsException("Index: "+index+", Size: "+size);
 ENDIF
 ensureCapacity(size+1);  // Increments modCount!!
 System.arraycopy(elementData, index, elementData, index + 1, size - index); //shifting the right hand side elements by 1.
 elementData[index] = element;
 size++;

RangeCheck(int index)
   IF index >= size THEN
        throw new IndexOutOfBoundsException( "Index: "+index+", Size: "+size);

 remove(int index)

  RangeCheck(index)
  modCount++;   // modification count
  Object oldVal = elementData.get(index);
  int numMoved = size - index - 1;

addAll(Collection<? extends E> c)

Object[] a = c.toArray();
length = a.length;
ensureCapacity(size + numNew);
System.arraycopy(a, 0, elementData, size, numNew);
 size = size + numNew;
return numNew != 0;




  ........... to be continued (will add other details later)

Advantages of Iterator over forEach

1. Iterator is a design pattern and is a light-weight container.
2. Iterator doesnt need to know what the collection is (i.e. a List or sth else )
3. Using iterator ,we can remove the current element. The for-each loop hides the iterator, so you cannot call remove. Therefore, the for-each loop is not usable for filtering.
4. Similarly, forEach is not usable for loops where you need to replace elements in a list or array as you traverse it, but Iterator can be used.
5 .Finally, forEach is not usable for loops that must iterate over multiple collections in parallel.