SQL Query Interview Questions and Answers

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SQL Optimization

SQL Statements are used to retrieve data from the database. We can get same results by writing different SQL queries. But use of the best query is important when performance is considered. So you need to SQL query tuning based on the requirement. Here is the list of queries which we use regularly and how these SQL queries can be optimized for better performance.

Actual column names instead of *

The SQL query becomes faster if you use the actual column names in SELECT statement instead of *.

Write the query as

SELECT P_Id, FirstName, LastName, Address, City
FROM Persons

Instead of

SELECT *
FROM Persons

HAVING clause is like a filter

HAVING clause is used to filter the rows after all the rows are selected. It is just like a filter. Do not use HAVING clause for any other purposes.

Write the query as

SELECT FirstName, COUNT(FirstName) FROM Persons WHERE FirstName <> 'John' AND FirstName <> 'Kate' GROUP BY FirstName

Instead of

SELECT FirstName, COUNT(FirstName) FROM Persons GROUP BY FistName HAVING FirstName <> 'John' AND FirstName <> 'Kate'

The number of subquery block in query

Sometimes you may have more than one subqueries in your main query. Try to minimize the number of subquery block in your query.

Write the query as

SELECT Name FROM Employees WHERE (Salary, Age ) = (SELECT MAX(Salary), MAX(Age) FROM EmployeeDetails) AND Dept = 'Electronics'

Instead of

SELECT Name FROM Employees WHERE Salary = (SELECT MAX(Salary) FROM EmployeeDetails) AND Age = (SELECT MAX(Age) FROM EmployeeDetails) AND Dept = 'Electronics';

Efficient use of EXISTS and IN

Use operator EXISTS, IN and table joins appropriately in your query.

Usually IN has the slowest performance;IN is efficient when most of the filter criteria is in the subquery;EXISTS is efficient when most of the filter criteria is in the main query.

Write the query as

SELECT * FROM Products WHERE EXISTS (select * from OrderItems WHERE Product_Id = Product_Id_p)

Instead of

SELECT * FROM Products WHERE Product_Id IN (SELECT Product_Id FROM OrderItems)

Using EXISTS instead of DISTINCT

Use EXISTS instead of DISTINCT when using joins which involves tables having one-to-many relationships.

Write the query as

SELECT D.Dept_Id, D.Dept FROM Dept.D WHERE EXISTS (SELECT 'X' FROM Employees 

WHERE E.Dept = D.Dept)

Instead of

SELECT DISTINCT D.Dept_Id, D.Dept FROM Dept.D, Employees WHERE E.Dept = E.Dept

UNION ALL in place of UNION

Try to use UNION ALL in place of UNION.

Write the query as

SELECT Id, FirstName FROM Students UNION ALL SELECT Id, FirstName FROM SportsTeam

Instead of

SELECT Id, FistName, Subject FROM Students UNION SELECT Id, FirstName FROM SportsTeam

Conditions in WHERE clause

Be careful while using conditions in WHERE clause.

Write the query as

SELECT Id, FirstName, Age FROM Persons WHERE Age > 25

Instead of

SELECT Id, FirstName, Age FROM Persons WHERE Age <> 25

Write the query as

SELECT Id, FirstName, Age FROM Persons WHERE FirtName LIKE 'Chan%'

Instead of

SELECT Id, FirstName, Age FROM Persons WHERE SUBSTR(FirstName, 1, 3) = 'Cha'

Use non-column expression on one side of the query because it will be processed earlier.

Write the query as

SELECT Id, Name, Salary FROM Employees WHERE Salary < 25000

Instead of

SELECT Id, Name, Salary FROM Employees WHERE Salary + 10000 < 35000

Use DECODE to avoid the scanning of same rows

Use DECODE to avoid the scanning of same rows or joining the same table repetitively. DECODE can also be made used in place of GROUP BY or ORDER BY.

Write the query as

SELECT Id FROM Employees WHERE Name LIKE 'Ramesh%' AND Location = 'Bangalore'

Instead of

SELECT DECODE(Location, 'Bangalore', Id, NULL) Id FROM Employees WHERE Name LIKE 'Ramesh%';

Storing large binary objects

To store large binary objects, first place them in the file system and add the file path in the database.

General SQL Standard Rules

To write queries which provide efficient performance follow the general SQL standard rules:

Use single case for all SQL verbs;Begin all SQL verbs on a new line;Separate all words with a single space;Right or left aligning verbs within the initial SQL verb.

SQL Query to find second highest salary of Employee


SELECT max(salary) FROM Employee 
WHERE salary NOT IN (SELECT max(salary) FROM Employee);

SELECT max(salary) FROM Employee 
WHERE salary < (SELECT max(salary) FROM Employee);

SELECT TOP 1 salary 
FROM ( SELECT TOP 2 salary FROM employees ORDER BY salary DESC) AS emp 
ORDER BY salary ASC


SQL Query to find Max Salary from each department.

SELECT DeptID, MAX(Salary) FROM Employee  GROUP BY DeptID.


Write SQL Query to find duplicate rows in a database? and then write SQL query to delete them?

SELECT * 
FROM emp a 
WHERE rowid = (SELECT MAX(rowid) FROM EMP b WHERE a.empno=b.empno)

--nth Highest Salary
select min(Emp_Sal) from Employee_Test where Emp_Sal in
(select distinct top n Emp_Sal from Employee_Test order by Emp_Sal desc)

--3rd Highest Salary
select min(Emp_Sal) from Employee_Test where Emp_Sal in
(select distinct top 3 Emp_Sal from Employee_Test order by Emp_Sal desc)

row number ----
SELECT name, sal, row_number() over(order by sal desc) rownum_by_sal
FROM EMPLOYEE o

;with T as
(
 select * , row_number() over (partition by Emp_ID order by Emp_ID) as rank
 from employee_test1
)

delete
from T
where rank > 1

SELECT contact_id,
CASE
  WHEN website_id = 1 THEN 'TechOnTheNet.com'
  WHEN website_id = 2 THEN 'CheckYourMath.com'
  ELSE 'BigActivities.com'
END
FROM contacts;

SELECT contact_id,
CASE website_id
  WHEN 1 THEN 'TechOnTheNet.com'
  WHEN 2 THEN 'CheckYourMath.com'
END
FROM contacts;

Oracle 

1. To fetch ALTERNATE records from a table. (EVEN NUMBERED)select * from emp where rowid in (select decode(mod(rownum,2),0,rowid, null) from emp);
2. To select ALTERNATE records from a table. (ODD NUMBERED)select * from emp where rowid in (select decode(mod(rownum,2),0,null ,rowid) from emp);
3. Find the 3rd MAX salary in the emp table.select distinct sal from emp e1 where 3 = (select count(distinct sal) from emp e2 where e1.sal <= e2.sal);
4. Find the 3rd MIN salary in the emp table.select distinct sal from emp e1 where 3 = (select count(distinct sal) from emp e2where e1.sal >= e2.sal);
5. Select FIRST n records from a table.select * from emp where rownum <= &n;
6. Select LAST n records from a tableselect * from emp minus select * from emp where rownum <= (select count(*) - &n from emp);
7. List dept no., Dept name for all the departments in which there are no employees in the department.select * from dept where deptno not in (select deptno from emp);  
   alternate solution:  select * from dept a where not exists (select * from emp b where a.deptno = b.deptno);
   altertnate solution:  select empno,ename,b.deptno,dname from emp a, dept b where a.deptno(+) = b.deptno and empno is null;
8. How to get 3 Max salaries ?select distinct sal from emp a where 3 >= (select count(distinct sal) from emp b where a.sal <= b.sal) order by a.sal desc;
9. How to get 3 Min salaries ?select distinct sal from emp a  where 3 >= (select count(distinct sal) from emp b  where a.sal >= b.sal);
10. How to get nth max salaries ?
    select distinct hiredate from emp a where &n =  (select count(distinct sal) from emp b where a.sal >= b.sal);
11. Select DISTINCT RECORDS from emp table.select * from emp a where  rowid = (select max(rowid) from emp b where  a.empno=b.empno);
12. How to delete duplicate rows in a table?delete from emp a where rowid != (select max(rowid) from emp b where  a.empno=b.empno);
13. Count of number of employees in  department  wise.select count(EMPNO), b.deptno, dname from emp a, dept b  where a.deptno(+)=b.deptno  group by b.deptno,dname;
14.  Suppose there is annual salary information provided by emp table. How to fetch monthly salary of each and every employee?
    select ename,sal/12 as monthlysal from emp;
15. Select all record from emp table where deptno =10 or 40.
    select * from emp where deptno=30 or deptno=10;
16. Select all record from emp table where deptno=30 and sal>1500.
    select * from emp where deptno=30 and sal>1500;
17. Select  all record  from emp where job not in SALESMAN  or CLERK.
    select * from emp where job not in ('SALESMAN','CLERK');
18. Select all record from emp where ename in 'BLAKE','SCOTT','KING'and'FORD'.
    select * from emp where ename in('JONES','BLAKE','SCOTT','KING','FORD');
19. Select all records where ename starts with ‘S’ and its lenth is 6 char.
    select * from emp where ename like'S____';
20. Select all records where ename may be any no of  character but it should end with ‘R’.
    select * from emp where ename like'%R';
21. Count  MGR and their salary in emp table.
    select count(MGR),count(sal) from emp;
22. In emp table add comm+sal as total sal  .
    select ename,(sal+nvl(comm,0)) as totalsal from emp;
23. Select  any salary <3000 from emp table. 
    select * from emp  where sal> any(select sal from emp where sal<3000);
24. Select  all salary <3000 from emp table. 
    select * from emp  where sal> all(select sal from emp where sal<3000);
25. Select all the employee  group by deptno and sal in descending order.
    select ename,deptno,sal from emp order by deptno,sal desc;
26. How can I create an empty table emp1 with same structure as emp?
    Create table emp1 as select * from emp where 1=2;
27. How to retrive record where sal between 1000 to 2000?
    Select * from emp where sal>=1000 And  sal<2000
28. Select all records where dept no of both emp and dept table matches.
    select * from emp where exists(select * from dept where emp.deptno=dept.deptno)
29. If there are two tables emp1 and emp2, and both have common record. How can I fetch all the recods but common records only once?
    (Select * from emp) Union (Select * from emp1)
30. How to fetch only common records from two tables emp and emp1?
    (Select * from emp) Intersect (Select * from emp1)
31.  How can I retrive all records of emp1 those should not present in emp2?
    (Select * from emp) Minus (Select * from emp1)
32. Count the totalsa  deptno wise where more than 2 employees exist.
    SELECT  deptno, sum(sal) As totalsal
    FROM emp
    GROUP BY deptno
    HAVING COUNT(empno) > 2