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;

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.

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