SQL

In this section, you will have a lot of SQL questions. So, if you think you are good with SQL, then do try these to become even better.

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For the next 7-8 questions, we will be referring to the below table

Emp. IDNameSalary
123Amit50000
453Sumit50000
232Rakshit30000
124Aman40000
543Rahul30000

1. What is ROW_NUMBER() function in SQL?
– The ROW_NUMBER() ranking returns a unique and sequential number to
each row of the table without repeating or skipping any number. If there are two rows which are partitioned and holds the same value as Amit and Sumit, then the row number will be given randomly.

2. Write the syntax to create a new column using Row Number over the Salary column

SELECT *, ROW_NUMBER() OVER (Order By Salary) as Row_Num
FROM Employee

Output

Emp. IDNameSalaryRow_Num
232Rakshit300001
543Rahul300002
124Aman400003
123Amit500004
453Sumit500005

3. What is PARTITION BY clause?
PARTITION BY clause is used to create a partition of ranking in a table. If you partition by Salary in the above table, then it will provide a ranking based on each unique salary. Example below:-

SELECT *, ROW_NUMBER() OVER (PARTITION BY Salary ORDER BY Salary) as Row_Num

Emp. IDNameSalaryRow_Num
232Rakshit300001
543Rahul300002
124Aman400001
123Amit500001
453Sumit500002

4. What is a RANK() function? How is it different from ROW_NUMBER()?
– RANK() function gives ranking to a row based on the value on which you want to base your ranking. If there are equal values, then the rank will be repeated and the row following the repeated values will skip as many ranks as there are repeated values row. Confused?? Try out the example below:-

SELECT *, RANK() OVER (ORDER BY Salary) as Row_Num
FROM Employee

Output

Emp. IDNameSalaryRow_Num
232Rakshit300001
543Rahul300001
124Aman400003
123Amit500004
453Sumit500004

As you can see, the rank 2 has been skipped because there were two employees with the same Salary and the result is ordered in ascending order by default.

5. How to use PARTITION BY clause in RANK() function?
– There is no point using PARTITION BY clause with RANK() function. The ranking result will have no meaning, as the rank will be done according to Salary values per each partition, and the data will be partitioned according to the Salary values. And due to the fact that each partition will have rows with the same Salary values, the rows with the same Salary values in the same partition will be ranked with the value equal to 1. 

SELECT *, RANK() OVER (PARTITION BY Salary ORDER BY Salary) as Row_Num

OUTPUT

Emp. IDNameSalaryRow_Num
232Rakshit300001
543Rahul300001
124Aman400001
123Amit500001
453Sumit500001

6. What is Dense Ranking?
– DENSE_RANK() is similar to the RANK() function but it does not skip any rank, so if there are two equal values then both will be termed as 1, the third value will be termed as 3 and not 2.

Syntax:-
SELECT *, DENSE_RANK() OVER (PARTITION BY Salary ORDER BY Salary) as Row_Num
FROM Employee

Output:-

Emp. IDNameSalaryRow_Num
232Rakshit300001
543Rahul300001
124Aman400003
123Amit500004
453Sumit500004
432Nihar600006

7. What is NTILE() function?
-NTILE() is similar to percentile NTILE(3) will divide the data in 3 parts. 

SELECT *, NTILE() OVER (ORDER BY Salary) as Ntile
FROM Employee

The number of rows should be 6/3 = 2, therefore we need to divide the 2 rows for each percentile

Emp. IDNameSalaryNtile
232Rakshit300001
543Rahul300001
124Aman400002
123Amit500002
453Sumit500003
432Nihar600003

8. How to get the second highest salary from a table?
Select MAX(Salary) 
from Employee 
Where Salary NOT IN (SELECT MAX(Salary) from Employee) 

9. Find the 3rd Maximum salary in the employee table
-Select distinct sal
from emp e1
where 3 = ((select count(distinct sal) from emp e2 where e1.sal <= e2.sal);

10. Get all employee detail from EmployeeDetail table whose “FirstName” not start with any single character between ‘a-p’
– SELECT * 
FROM EmployeeDetail
WHERE FirstName like ‘[^a-p]%’

11. How to fetch only even rows from a table?
-The best way to do it is by adding a row number using ROW_NUMBER() and then pulling the alternate row number using row_num%2 = 0

Suppose, there are 3 columns in a table i.e. student_ID, student_Name, student_Grade. Pull the even rows

SELECT * 
FROM ( SELECT *, ROW_NUMBER() OVER (ORDER BY student_ID) as row_num FROM student) x
WHERE x.row_num%2=0 

12. How to fetch only odd rows from the same table?
-Simply apply the x.row_num%2 <> 0 to get the odd rows

SELECT * 
FROM ( SELECT *, ROW_NUMBER() OVER (ORDER BY student_ID) as row_num FROM student) x
WHERE x.row_num%2 <> 0 

13. How to find the minimum salary using subquery?
-SELECT * 
FROM employee 
WHERE salary = (select MIN(salary) from employee);

14. How to find the second minimum salary?
– SELECT *
FROM employee
WHERE salary = (SELECT MIN(salary) FROM employee > SELECT MIN(salary) FROM employee)

Similarly, find the third minimum salary

– SELECT *
FROM employee
WHERE salary = (SELECT MIN(salary) FROM employee > SELECT MIN(salary) FROM employee > SELECT MIN(salary) FROM employee)

15. The above query is too lengthy, write a query to get the third minimum salary with some other method.

– SELECT DISTINCT (salary) 
FROM emp e1 where 3 = (SELECT COUNT(DISTINCT salary) FROM emp e2 WHERE e1.sal >= e2.sal); 

16. How to get 3 Min salaries?
-SELECT DISTINCT salary FROM emp a WHERE 3 >= (SELECT COUNT(DISTINCT salary) FROM emp b WHERE a.salary >= b.salary);

17. Some basic SQL Select questions
– SELECT 125
125
-SELECT ‘Ankit’+’1’
Ankit1
-SELECT ‘Ankit’+1
Error
– SELECT ‘2’+2
4
-SELECT SUM(‘1’)


18. 
SELECT CURDATE(); à It returns the current date(MySQL)
SELECT NOW(); à It returns the current date and time(MySQL)
SELECT getdate(); à It returns the current date and time(SQL Server Query)
SELECT SYSDATE FROM DUAL à This Oracle query returns the current date and time 

19. Explain Scalar functions.
Scalar functions are those functions which are used to return a single value based on the input. Following are the commonly used scalar functions in SQL:-
1. UCASE() – Convert to Upper Case
2. LCASE() – Convert to Lower Case
3. MID() – It extracts a substring from a string. SELECT MID(“ABCDEFGH”,3,6) AS ExtractedString
It will extract 6 characters starting from 3rd character i.e. C,D,E,F,G,H
4. FORMAT() – It specifies the display format
5. LEN() – Gives the length of a text field
6. ROUND() – Rounds up the decimal field in a number 


20. Write a generic method to fetch the nth highest salary without TOP or Limit

SELECT Salary
FROM Worker W1
WHERE n-1 = (
 SELECT COUNT( DISTINCT ( W2.Salary ) )
 FROM Worker W2
 WHERE W2.Salary >= W1.Salary
 ); 

21. What is the order of execution in a query?

The order of query goes like this:-
FROM – Choose and join tables to get the raw data
WHERE – First filtering condition
GROUP BY – Aggregates the base data
HAVING – Apply condition on the base data
SELECT – Return the final data
ORDER BY – Sort the final data
LIMIT – Apply limit to the returned data

22. What is wrong with the following query?
SELECT Id, Year(PaymentDate) as PaymentYear
FROM Bill_Table
WHERE PaymentYear > 2018;

Though the variable PaymentYear has already been defined in the first line of the query, but this is not the correct logical process order. The correct query will be 
SELECT Id, Year(PaymentDate) as PaymentYear
FROM Bill_Table
WHERE Year(PaymentDate) > 2018;

23. What is Normalization? How many Normalization forms are there?

Normalization is used to organize the data in such manner that data redundancy will never occur in the database and avoid insert, update and delete anomalies.
There are 5 forms of Normalization
First Normal Form (1NF): It removes all duplicate columns from the table. Creates a table for related data and identifies unique column values
Second Normal Form (2NF): Follows 1NF and creates and places data subsets in an individual table and defines the relationship between tables using a primary key
Third Normal Form (3NF): Follows 2NF and removes those columns which are not related through the primary key
Fourth Normal Form (4NF): Follows 3NF and do not define multi-valued dependencies. 4NF also known as BCNF

More SQL questions in the links given below:
1. SQL interview Questions
2. SQL basics to kick off
3. SQL one table, 30 Questions
4. SQL Theoretical Questions

Also, Visit the interview Questions to find more questions on the same line.

Keep Learning 🙂

The Data Monk