SQL Queries SET 3

 

SQL Query Interview Questions

 

Consider the below two tables for most of the questions asked here.

 

Table – EmployeeDetails

 

EmpId FullName ManagerId DateOfJoining

121 John Snow 321 01/31/2014

321 Walter White 986 01/30/2015

421 Kuldeep Rana 876 27/11/2016

 

Table – EmployeeSalary

 

EmpId Project Salary

121 P1 8000

321 P2 1000

421 P1 12000

SQL Queries Asked in Interviews with Answers

 

Write a SQL query to fetch the count of employees working in project ‘P1’.

Ans. Here, we would be using aggregate function count() with the SQL where clause-

SELECT COUNT(*)

FROM EmployeeSalary

WHERE Project = 'P1';

 

Write a SQL query to fetch employee names having a salary greater than or equal to 5000 and less than or equal 10000.

Here, we will use BETWEEN in the ‘where’ clause to return the EmpId of the employees with salary satisfying the required criteria and then use it as subquery to find the fullName of the employee form EmployeeDetails table.

SELECT FullName FROM EmployeeDetails WHERE EmpId IN (SELECT EmpId FROM EmpolyeeSalary WHERE Salary BETWEEN 5000 AND 10000);

 Write a SQL query to fetch project-wise count of employees sorted by project’s count in descending order.

The query has two requirements – first to fetch the project-wise count and then to sort the result by that count. For project-wise count, we will be using GROUPBY clause and for sorting, we will use ORDER BY clause on the alias of the project-count.

SELECT Project, count(EmpId) EmpProjectCount FROM EmployeeSalary GROUP BY Project ORDER BY EmpProjectCount DESC;

Write a query to fetch only the first name(string before space) from the FullName column of EmployeeDetails table.

In this question, we are required to first fetch the location of the space character in the FullName field and then extract the first name out of the FullName field. For finding the location we will use LOCATE method in MySQL and CHARINDEX in SQL SERVER and for fetching the string before space, we will use SUBSTRING OR MID method.mySQL- Using MID

SELECT MID(FullName, 0, LOCATE(' ',FullName)) FROM EmployeeDetails;

SQL Server-Using SUBSTRING

SELECT SUBSTRING(FullName, 0, CHARINDEX(' ',FullName)) FROM EmployeeDetails;

Also, we can use LEFT which returns the left part of a string till a specified number of characters.

SELECT LEFT(FullName, CHARINDEX(' ',FullName) - 1) FROM EmployeeDetails;

Write a query to fetch employee names and salary records. Return employee details even if the salary record is not present for the employee.

Here, we can use left join with EmployeeDetail table on the left side.

SELECT E.FullName, S.Salary  FROM EmployeeDetails E LEFT JOIN EmployeeSalary S ON E.EmpId = S.EmpId;

Write a SQL query to fetch all the Employees who are also managers from EmployeeDetails table.

Here, we have to use Self-Join as the requirement wants us to analyze the EmployeeDetails table as two different tables, each for Employee and manager records.

SELECT DISTINCT E.FullNameFROM EmpDetails EINNER JOIN EmpDetails M

ON E.EmpID = M.ManagerID;

Write a SQL query to fetch all employee records from EmployeeDetails table who have a salary record in EmployeeSalary table.

Using ‘Exists’-

SELECT * FROM EmployeeDetails E WHERE EXISTS (SELECT * FROM EmployeeSalary S WHERE  E.EmpId = S.EmpId);

 Write a SQL query to fetch duplicate records from a table.

In order to find duplicate records from the table, we can use GROUP BY on all the fields and then use HAVING clause to return only those fields whose count is greater than 1 i.e. the rows having duplicate records.

SELECT EmpId, Project, Salary, COUNT(*)FROM EmployeeSalaryGROUP BY EmpId, Project, SalaryHAVING COUNT(*) > 1;

Write a SQL query to remove duplicates from a table without using a temporary table.

Ans. Using Group By and Having clause-

DELETE FROM EmployeeSalary  WHERE EmpId IN (SELECT EmpId FROM EmployeeSalary       

GROUP BY Project, SalaryHAVING COUNT(*) > 1));

Write a SQL query to fetch only odd rows from the table.

Ans. This can be achieved by using Row_number in SQL server-

SELECT E.EmpId, E.Project, E.Salary FROM (SELECT *, Row_Number() OVER(ORDER BY EmpId) AS RowNumber FROM EmployeeSalary) EWHERE E.RowNumber % 2 = 1

 Write a SQL query to fetch only even rows from the table.

Ans. Using the same Row_Number() and checking that the remainder when divided by 2 is 0-

SELECT E.EmpId, E.Project, E.Salary

FROM (SELECT *, Row_Number() OVER(ORDER BY EmpId) AS RowNumber FROM EmployeeSalary) EWHERE E.RowNumber % 2 = 0

Write a SQL query to create a new table with data and structure copied from another table.

Ans. Using SELECT INTO command-

SELECT * INTO newTable FROM EmployeeDetails;

Write a SQL query to create an empty table with the same structure as some other table.

Ans. Using SELECT INTO command with False ‘WHERE’ condition-

SELECT * INTO newTable FROM EmployeeDetails WHERE 1 = 0;

Write a SQL query to fetch common records between two tables.

Ans. Using INTERSECT-

SELECT * FROM EmployeeSalaryINTERSECTSELECT * FROM ManagerSalary

Write a SQL query to fetch records that are present in one table but not in another table.

Ans. Using MINUS-

SELECT * FROM EmployeeSalaryMINUSSELECT * FROM ManagerSalary

Write a SQL query to find the current date-time.

Ans. MySQL-

SELECT NOW();

SQL Server-

SELECT getdate();

Oracle-

SELECT SYSDATE FROM DUAL;

Write a SQL query to fetch all the Employees details from EmployeeDetails table who joined in the Year 2016.

Ans. Using BETWEEN for the date range ’01-01-2016′ AND ’31-12-2016′-

SELECT * FROM EmployeeDetails WHERE DateOfJoining BETWEEN '01-01-2016' AND date '31-12-2016';

 

Write a SQL query to fetch top n records?

Ans. In MySQL using LIMIT-

SELECT *  FROM EmployeeSalary ORDER BY Salary DESC LIMIT N 

In SQL server using TOP command-

SELECT TOP N *  FROM EmployeeSalary  ORDER BY Salary DESC

In Oracle using ROWNUM-

SELECT * FROM (SELECT * FROM EmployeeSalary ORDER BY Salary DESC) WHERE ROWNUM <= 3;

Write SQL query to find the nth highest salary from table.

Ans. Using Top keyword (SQL Server)-

SELECT TOP 1 Salary FROM (SELECT DISTINCT TOP N Salary FROM Employee ORDER BY Salary DESC ) ORDER BY Salary ASC

Using limit clause(MySQL)-

SELECT Salary FROM Employee ORDER BY Salary DESC LIMIT N-1,1;

Write SQL query to find the 3rd highest salary from table without using TOP/limit keyword.

Ans. The below SQL query makes use of correlated subquery wherein in order to find the 3rd highest salary the inner query will return the count of till we find that there are two rows that salary greater than other distinct salaries.

SELECT Salary FROM EmployeeSalary Emp1

WHERE 2 = (

                SELECT COUNT( DISTINCT ( Emp2.Salary ) )

                FROM EmployeeSalary Emp2

                WHERE Emp2.Salary > Emp1.Salary

            )

 

For nth highest salary-

SELECT Salary FROM EmployeeSalary Emp1

WHERE N-1 = (

                SELECT COUNT( DISTINCT ( Emp2.Salary ) )

                FROM EmployeeSalary Emp2

                WHERE Emp2.Salary > Emp1.Salary

            )