Database Interview questions for Java Developers

Question 1: SQL Query to find second highest salary of Employee

Answer: There are many ways to find second highest salary of Employee in SQL, you can either use SQL Join or Subquery to solve this problem. Here is SQL query using Subquery:

select MAX(Salary) from Employee WHERE Salary NOT IN (select MAX(Salary) from Employee );

Question 2: SQL Query to find Max Salary from each department.

Answer: You can find the maximum salary for each department by grouping all records by DeptId and then using MAX function to calculate maximum salary in each group or each department.

SELECT DeptID, MAX(Salary) FROM Employee GROUP BY DeptID.
These questions become more interesting if Interviewer will ask you to print department name instead of department id, in that case, you need to join Employee table with Department using foreign key DeptID, make sure you do LEFT or RIGHT OUTER JOIN to include departments without any employee as well. Here is the query

SELECT DeptName, MAX(Salary) FROM Employee e RIGHT JOIN Department d ON e.DeptId = d.DeptID GROUP BY DeptName;

In this query, we have used RIGHT OUTER JOIN because we need the name of the department from Department table which is on the right side of JOIN clause, even if there is no reference of dept_id on Employee table.
Question 3: Write SQL Query to display the current date.

Answer: SQL has built-in function called GetDate which returns the current timestamp. This will work in Microsoft SQL Server, other vendors like Oracle and MySQL also has equivalent functions.

SELECT GetDate;

Question 4: Write an SQL Query to check whether date passed to Query is the date of given format or not.

Answer: SQL has IsDate function which is used to check passed value is a date or not of specified format, it returns 1(true) or 0(false) accordingly. Remember ISDATE is an MSSQL function and it may not work on Oracle, MySQL or any other database but there would be something similar.

It will return 0 because passed date is not in correct format.

Question 5: Write an SQL Query to print the name of the distinct employee whose DOB is between to .

Answer: This SQL query is tricky, but you can use BETWEEN clause to get all records whose date fall between two dates.

SELECT DISTINCT EmpName FROM Employees WHERE DOB BETWEEN ‘01/01/1960’ AND ‘31/12/1975’;

Question 6: Write an SQL Query find number of employees according to gender

SELECT COUNT(*), sex sex;

Question 7: Write an SQL Query to find an employee whose Salary is equal or greater than 10000.

SELECT EmpName FROM Employees WHERE Salary>=10000;

Question 8: Write an SQL Query to find name of employee whose name Start with ‘M’

SELECT * FROM Employees WHERE EmpName like 'M%';

Question 9: find all Employee records containing the word "Joe", regardless of whether it was stored as JOE, Joe, or joe.

SELECT * from Employees WHERE UPPER(EmpName) like '%JOE%';

Question 10: Write an SQL Query to find the year from date.

SELECT YEAR(GETDATE) as "Year";

Question 11: Write SQL Query to find duplicate rows in a database? and then write SQL query to delete them?
Answer: You can use the following query to select distinct records:

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

DELETE FROM emp a WHERE rowid != (SELECT MAX(rowid) FROM emp b WHERE a.empno=b.empno);

Question 12: There is a table which contains two column Student and Marks, you need to find all the students, whose marks are greater than average marks i.e. list of above average students.
Answer: This query can be written using subquery as shown below:

SELECT student, marks from table where marks > SELECT AVG(marks) from table)

SELECT e.name, m.name FROM Employee e, Employee m WHERE e.mgr_id = m.emp_id;
this will show employee name and manager name in two column e.g.

name manager_name
John David

One follow-up is to modify this query to include employees which don't have a manager. To solve that, instead of using the inner join, just use left outer join, this will also include employees without managers.

Question 14: You have a composite index of three columns, and you only provide the value of two columns in WHERE clause of a select query? Will Index be used for this operation? For example if Index is on EmpId, EmpFirstName, and EmpSecondName and you write query like