SQL Scenario based Interview Questions

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1. How to select UNIQUE records from a table using a SQL Query?

Consider below EMPLOYEE table as the source data

CREATE TABLE EMPLOYEE (
	EMPLOYEE_ID NUMBER(6,0), 
	NAME VARCHAR2(20), 
	SALARY NUMBER(8,2) 
);

INSERT INTO EMPLOYEE(EMPLOYEE_ID,NAME,SALARY) VALUES(100,'Jennifer',4400);
INSERT INTO EMPLOYEE(EMPLOYEE_ID,NAME,SALARY) VALUES(100,'Jennifer',4400);
INSERT INTO EMPLOYEE(EMPLOYEE_ID,NAME,SALARY) VALUES(101,'Michael',13000);
INSERT INTO EMPLOYEE(EMPLOYEE_ID,NAME,SALARY) VALUES(101,'Michael',13000);
INSERT INTO EMPLOYEE(EMPLOYEE_ID,NAME,SALARY) VALUES(101,'Michael',13000);
INSERT INTO EMPLOYEE(EMPLOYEE_ID,NAME,SALARY) VALUES(102,'Pat',6000);
INSERT INTO EMPLOYEE(EMPLOYEE_ID,NAME,SALARY) VALUES(102,'Pat',6000);
INSERT INTO EMPLOYEE(EMPLOYEE_ID,NAME,SALARY) VALUES(103,'Den',11000);



SELECT * FROM EMPLOYEE;
EMPLOYEE_IDNAMESALARY
100Jennifer4400
100Jennifer4400
101Michael13000
101Michael13000
101Michael13000
102Pat6000
102Pat6000
103Den11000

METHOD-1: Using GROUP BY Function

GROUP BY clause is used with SELECT statement to collect data from multiple records and group the results by one or more columns. The GROUP BY clause returns one row per group. By applying GROUP BY function on all the source columns, unique records can be queried from the table.

Below is the query to fetch the unique records using GROUP BY function.

Query:

SELECT EMPLOYEE_ID, 
       NAME, 
       SALARY 
FROM EMPLOYEE 
GROUP BY EMPLOYEE_ID, NAME, SALARY;

Result:

EMPLOYEE_IDNAMESALARY
100Jennifer4400
101Michael13000
102Pat6000
103Den11000

METHOD-2: Using ROW_NUMBER Analytic Function

The ROW_NUMBER Analytic function is used to provide consecutive numbering of the rows in the result by the ORDER selected for each PARTITION specified in the OVER clause. It will assign the value 1 for the first row and increase the number of the subsequent rows.

Using ROW_NUMBER Analytic function, assign row numbers to each unique set of records.

Query:

SELECT EMPLOYEE_ID, 
       NAME, 
       SALARY, 
       ROW_NUMBER() OVER(PARTITION BY EMPLOYEE_ID,NAME,SALARY ORDER BY EMPLOYEE_ID) AS ROW_NUMBER
FROM EMPLOYEE;

Result:

EMPLOYEE_IDNAMESALARYROW_NUMBER
100Jennifer44001
100Jennifer44002
101Michael130001
101Michael130002
101Michael130003
102Pat60001
102Pat60002
103Den110001

Once row numbers are assigned, by querying the rows with row number 1 will give the unique records from the table.

Query:

SELECT EMPLOYEE_ID, NAME, SALARY 
FROM( SELECT 
       EMPLOYEE_ID, 
       NAME, 
       SALARY, 
       ROW_NUMBER() OVER(PARTITION BY EMPLOYEE_ID,NAME,SALARY ORDER BY EMPLOYEE_ID) AS ROW_NUMBER 
      FROM EMPLOYEE)
WHERE ROW_NUMBER = 1;

Result:

EMPLOYEE_IDNAMESALARY
101Michael13000
100Jennifer4400
102Pat6000
103Den11000

Related Article: SQL Analytic Functions Interview Questions


2. How to delete DUPLICATE records from a table using a SQL Query?

Consider the same EMPLOYEE table as source discussed in previous question

METHOD-1: Using ROWID and ROW_NUMBER Analytic Function

An Oracle server assigns each row in each table with a unique ROWID to identify the row in the table. The ROWID is the address of the row which contains the data object number, the data block of the row, the row position and data file.

STEP-1: Using ROW_NUMBER Analytic function, assign row numbers to each unique set of records. Select ROWID of the rows along with the source columns

Query:

SELECT ROWID,
       EMPLOYEE_ID,
       NAME,SALARY, 
       ROW_NUMBER() OVER(PARTITION BY EMPLOYEE_ID,NAME,SALARY ORDER BY EMPLOYEE_ID) AS ROW_NUMBER 
FROM EMPLOYEE;

Result:

ROWIDEMPLOYEE_IDNAMESALARYROW_NUMBER
AAASnBAAEAAACrWAAA100Jennifer44001
AAASnBAAEAAACrWAAB100Jennifer44002
AAASnBAAEAAACrWAAC101Michael130001
AAASnBAAEAAACrWAAD101Michael130002
AAASnBAAEAAACrWAAE101Michael130003
AAASnBAAEAAACrWAAF102Pat60001
AAASnBAAEAAACrWAAG102Pat60002
AAASnBAAEAAACrWAAH103Den110001

STEP-2: Select ROWID of records with ROW_NUMBER > 1

Query:

SELECT ROWID FROM( 
      SELECT ROWID,
             EMPLOYEE_ID,
             NAME,
             SALARY, 
             ROW_NUMBER() OVER(PARTITION BY EMPLOYEE_ID,NAME,SALARY ORDER BY EMPLOYEE_ID) AS ROW_NUMBER 
      FROM EMPLOYEE)
WHERE ROW_NUMBER > 1;

Result:

ROWID
AAASnBAAEAAACrWAAB
AAASnBAAEAAACrWAAD
AAASnBAAEAAACrWAAE
AAASnBAAEAAACrWAAG

STEP-3: Delete the records from the source table using the ROWID values fetched in previous step

Query:

DELETE FROM EMP WHERE ROWID IN (
    SELECT ROWID FROM(
        SELECT ROWID,
        ROW_NUMBER() OVER(PARTITION BY EMPLOYEE_ID,NAME,SALARY ORDER BY EMPLOYEE_ID) AS ROW_NUMBER 
    FROM EMPLOYEE)
WHERE ROW_NUMBER > 1);

Result:

The table EMPLOYEE will have below records after deleting the duplicates

ROWIDEMPLOYEE_IDNAMESALARY
AAASnBAAEAAACrWAAA100Jennifer4400
AAASnBAAEAAACrWAAC101Michael13000
AAASnBAAEAAACrWAAF102Pat6000
AAASnBAAEAAACrWAAH103Den11000

METHOD-2: Using ROWID and Correlated subquery

Correlated subquery is used for row-by-row processing. With a normal nested subquery, the inner SELECT query runs once and executes first. The returning values will be used by the main query. A correlated subquery, however, executes once for every row of the outer query. In other words, the inner query is driven by the outer query.

In the below query, we are comparing the ROWIDs’ of the unique set of records and keeping the record with MIN ROWID and deleting all other rows.

Query:

DELETE FROM EMPLOYEE A WHERE ROWID > (SELECT MIN(ROWID) FROM EMPLOYEE B WHERE B.EMPLOYEE_ID = A.EMPLOYEE_ID );

Result:

The table EMPLOYEE will have below records after deleting the duplicates

ROWIDEMPLOYEE_IDNAMESALARY
AAASnBAAEAAACrWAAA100Jennifer4400
AAASnBAAEAAACrWAAC101Michael13000
AAASnBAAEAAACrWAAF102Pat6000
AAASnBAAEAAACrWAAH103Den11000

The opposite of above discussed case can be implemented by keeping the record with MAX ROWID from the unique set of records and delete all other duplicates by executing below query.

Query:

DELETE FROM EMPLOYEE A WHERE ROWID < (SELECT MAX(ROWID) FROM EMPLOYEE B WHERE B.EMPLOYEE_ID = A.EMPLOYEE_ID );

Result:

The table EMPLOYEE will have below records after deleting the duplicates

ROWIDEMPLOYEE_IDNAMESALARY
AAASnBAAEAAACrWAAA100Jennifer4400
AAASnBAAEAAACrWAAC101Michael13000
AAASnBAAEAAACrWAAF102Pat6000
AAASnBAAEAAACrWAAH103Den11000

3. How to read TOP 5 records from a table using a SQL query?

Consider below table DEPARTMENTS as the source data

CREATE TABLE Departments(
   Department_ID number,
   Department_Name varchar(50)
);

INSERT INTO DEPARTMENTS VALUES('10','Administration');
INSERT INTO DEPARTMENTS VALUES('20','Marketing');
INSERT INTO DEPARTMENTS VALUES('30','Purchasing');
INSERT INTO DEPARTMENTS VALUES('40','Human Resources');
INSERT INTO DEPARTMENTS VALUES('50','Shipping');
INSERT INTO DEPARTMENTS VALUES('60','IT');
INSERT INTO DEPARTMENTS VALUES('70','Public Relations');
INSERT INTO DEPARTMENTS VALUES('80','Sales');

SELECT * FROM Departments;
DEPARTMENT_IDDEPARTMENT_NAME
10Administration
20Marketing
30Purchasing
40Human Resources
50Shipping
60IT
70Public Relations
80Sales

ROWNUM is a “Pseudocolumn” that assigns a number to each row returned by a query indicating the order in which Oracle selects the row from a table. The first row selected has a ROWNUM of 1, the second has 2, and so on.

Query:

SELECT * FROM Departments WHERE ROWNUM <= 5;

Result:

DEPARTMENT_IDDEPARTMENT_NAME
10Administration
20Marketing
30Purchasing
40Human Resources
50Shipping

4. How to read LAST 5 records from a table using a SQL query?

Consider the same DEPARTMENTS table as source discussed in previous question.

In order to select the last 5 records we need to find (count of total number of records – 5) which gives the count of records from first to last but 5 records.

Using the MINUS function we can compare all records from DEPARTMENTS table with records from first to last but 5 from DEPARTMENTS table which give the last 5 records of the table as result.

MINUS operator is used to return all rows in the first SELECT statement that are not present in the second SELECT statement.

Query:

SELECT * FROM Departments

MINUS

SELECT * FROM Departments WHERE ROWNUM <= (SELECT COUNT(*)-5 FROM Departments);

Result:

DEPARTMENT_IDDEPARTMENT_NAME
40Human Resources
50Shipping
60IT
70Public Relations
80Sales

5. What is the result of Normal Join, Left Outer Join, Right Outer Join and Full Outer Join between the tables A & B?

Table_A

COL
1
1
0
null

Table_B

COL
1
0
null
null

Normal Join:

Normal Join or Inner Join is the most common type of join. It returns the rows that are exact match between both the tables.

The following Venn diagram illustrates a Normal join when combining two result sets:

Query:

SELECT a.COL as A, 
       b.COL as B 
FROM TABLE_A a JOIN TABLE_B b 
ON a.COL = b.COL;

Result:

AB
11
11
00

Left Outer Join:

The Left Outer Join returns all the rows from the left table and only the matching rows from the right table. If there is no matching row found from the right table, the left outer join will have NULL values for the columns from right table.

The following Venn diagram illustrates a Left join when combining two result sets:

Query:

SELECT a.COL as A, 
       b.COL as B 
FROM TABLE_A a LEFT OUTER JOIN TABLE_B b 
ON a.COL = b.COL;

Result:

AB
11
11
00
NULLNULL

Right Outer Join:

The Right Outer Join returns all the rows from the right table and only the matching rows from the left table. If there is no matching row found from the left table, the right outer join will have NULL values for the columns from left table.

The following Venn diagram illustrates a Right join when combining two result sets:

Query:

SELECT a.COL as A, 
       b.COL as B 
FROM TABLE_A a RIGHT OUTER JOIN TABLE_B b 
ON a.COL = b.COL;

Result:

AB
11
11
00
NULLNULL
NULLNULL

Full Outer Join:

The Full Outer Join returns all the rows from both the right table and the left table. If there is no matching row found, the missing side columns will have NULL values.

The following Venn diagram illustrates a Full join when combining two result sets:

Query:

SELECT a.COL as A, 
       b.COL as B 
FROM TABLE_A a FULL OUTER JOIN TABLE_B b 
ON a.COL = b.COL;

Result:

AB
11
11
00
NULLNULL
NULLNULL
NULLNULL

NOTE: NULL do not match with NULL


6. How to find the employee with second MAX Salary using a SQL query?

Consider below EMPLOYEES table as the source data

CREATE TABLE Employees(
	EMPLOYEE_ID NUMBER(6,0), 
	NAME VARCHAR2(20 BYTE), 
	SALARY NUMBER(8,2)
        );
        
INSERT INTO EMPLOYEES(EMPLOYEE_ID,NAME,SALARY) VALUES(100,'Jennifer',4400);
INSERT INTO EMPLOYEES(EMPLOYEE_ID,NAME,SALARY) VALUES(101,'Michael',13000);
INSERT INTO EMPLOYEES(EMPLOYEE_ID,NAME,SALARY) VALUES(102,'Pat',6000);
INSERT INTO EMPLOYEES(EMPLOYEE_ID,NAME,SALARY) VALUES(103,'Den', 11000);
INSERT INTO EMPLOYEES(EMPLOYEE_ID,NAME,SALARY) VALUES(104,'Alexander',3100);
INSERT INTO EMPLOYEES(EMPLOYEE_ID,NAME,SALARY) VALUES(105,'Shelli',2900);
INSERT INTO EMPLOYEES(EMPLOYEE_ID,NAME,SALARY) VALUES(106,'Sigal',2800);
INSERT INTO EMPLOYEES(EMPLOYEE_ID,NAME,SALARY) VALUES(107,'Guy',2600);
INSERT INTO EMPLOYEES(EMPLOYEE_ID,NAME,SALARY) VALUES(108,'Karen',2500);

SELECT * FROM Employees;
EMPLOYEE_IDNAMESALARY
100Jennifer4400
101Michael13000
102Pat6000
103Den11000
104Alexander3100
105Shelli2900
106Sigel2800
107Guy2600
108Karen2500

METHOD-1: Without using SQL Analytic Functions

In order to find the second MAX salary, employee record with MAX salary needs to be eliminated. It can be achieved by using below SQL query.

Query:

SELECT MAX(salary) AS salary FROM Employees WHERE salary NOT IN (
SELECT MAX(salary) AS salary FROM Employees);

Result:

SALARY
11000

The above query only gives the second MAX salary value. In order to fetch the entire employee record with second MAX salary we need to do a self-join on Employee table based on Salary value.

Query:

WITH
TEMP AS(
  SELECT MAX(salary) AS salary FROM Employees WHERE salary NOT IN (
  SELECT MAX(salary) AS salary FROM Employees)
)
SELECT a.* FROM Employees a JOIN TEMP b on a.salary = b.salary

Result:

EMPLOYEE_IDNAMESALARY
103Den11000

METHOD-2: Using SQL Analytic Functions

Query:

The DENSE_RANK is an analytic function that calculates the rank of a row in an ordered set of rows starting from 1. Unlike the RANK function, the DENSE_RANK function returns rank values as consecutive integers.

SELECT Employee_Id, 
       Name, 
       Salary 
FROM(
   SELECT Employees.*, 
          DENSE_RANK() OVER(ORDER BY Salary DESC) as SALARY_RANK 
   FROM Employees)
WHERE SALARY_RANK =2

Result:

EMPLOYEE_IDNAMESALARY
103Den11000

By replacing the value of SALARY_RANK, any highest salary rank can be found easily.

Related Article: SQL Analytic Functions Interview Questions


7. How to find the employee with third MAX Salary using a SQL query without using Analytic Functions?

Consider the same EMPLOYEES table as source discussed in previous question

In order to find the third MAX salary, we need to eliminate the top 2 salary records. But we cannot use the same method we used for finding second MAX salary (not a best practice). Imagine if we have to find the fifth MAX salary. We should not be writing a query with four nested sub queries.

STEP-1:

The approach here is to first list all the records based on Salary in the descending order with MAX salary on top and MIN salary at bottom. Next, using ROWNUM select the top 2 records.

Query:

SELECT salary FROM(
SELECT salary FROM Employees ORDER BY salary DESC)
WHERE ROWNUM < 3;

Result:

Salary
13000
11000

STEP-2:

Next find the MAX salary from EMPLOYEE table which is not one of top two salary values fetched in the earlier step.

Query:

SELECT MAX(salary) as salary FROM Employees WHERE salary NOT IN (
    SELECT salary FROM(
    SELECT salary FROM Employees ORDER BY salary DESC)
    WHERE ROWNUM < 3
);

Result:

SALARY
6000

STEP-3:

In order to fetch the entire employee record with third MAX salary we need to do a self-join on Employee table based on Salary value.

Query:

WITH 
TEMP AS(
  SELECT MAX(salary) as salary FROM Employees WHERE salary NOT IN (
      SELECT salary FROM(
      SELECT salary FROM Employees ORDER BY salary DESC)
      WHERE ROWNUM < 3)
)
SELECT a.* FROM Employees a join TEMP b on a.salary = b.salary

Result:

EMPLOYEE_IDNAMESALARY
102Pat6000

In order to find the employee with nth highest salary, replace the rownum value with n in above query.

Related Articles:

  • Top Advanced SQL Interview Questions
  • SQL Scenario based Interview Questions
  • SQL Analytical Functions Interview Questions

13 thoughts on “SQL Scenario based Interview Questions”

  1. Hello,
    The questions as well as the answers are really clear. Thank you so much for putting up this content. It was really helpful, while preparing for technical rounds.

    Reply

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