Advanced SQL Queries Interview Questions and Answers

Advanced SQL Queries Interview Questions and Answers:



1. Select employee details from employee table if data exists in incentive table ?

select * from EMPLOYEE where exists (select * from INCENTIVES)

Explanation : Here "exists" statement helps us to do the job of If statement. Main query will get executed if the sub query returns at least one row. So we can consider the sub query as "If condition" and the main query as "code block" inside the If condition. We can use any SQL commands (Joins, Group By , having etc) in sub query. This command will be useful in queries which need to detect an event and do some activity.




2. How to fetch data that are common in two query results ?



select * from EMPLOYEE where EMPLOYEE_ID INTERSECT select * from EMPLOYEE where EMPLOYEE_ID < 4

Explanation : Here "INTERSECT" command is used to fetch data that are common in 2 queries. In this example, we had taken EMPLOYEE table in both the queries.We can apply INTERSECT command on different tables. The result of the above query will return employee details of "ROY" because, employee id of ROY is 3, and both query results have the information about ROY.




3. Get Employee ID's of those employees who didn't receive incentives without using sub query ?

select EMPLOYEE_ID from EMPLOYEE

MINUS

select EMPLOYEE_REF_ID from INCENTIVES

Explanation : To filter out certain information we use MINUS command. What MINUS Command odes is that, it returns all the results from the first query, that are not part of the second query. In our example, first three employees received the incentives. So query will return employee id's 4 to 8.




4. Select 20 % of salary from John , 10% of Salary for Roy and for other 15 % of salary from employee table

SELECT FIRST_NAME, CASE FIRST_NAME WHEN 'John' THEN SALARY * .2 WHEN 'Roy' THEN SALARY * .10 ELSE SALARY * .15 END "Deduced_Amount" FROM EMPLOYEE

Explanation : Here, we are using "SQL CASE" statement to achieve the desired results. After case statement, we had to specify the column on which filtering is applied. In our case it is "FIRST_NAME". And in then condition, specify the name of filter like John, Roy etc. To handle conditions outside our filter, use else block where every one other than John and Roy enters.




5. Select Banking as 'Bank Dept', Insurance as 'Insurance Dept' and Services as 'Services Dept' from employee table

SQL Queries in Oracle, SELECT distinct DECODE (DEPARTMENT, 'Banking', 'Bank Dept', 'Insurance', 'Insurance Dept', 'Services', 'Services Dept') FROM EMPLOYEE

SQL Queries in SQL Server and MySQL, SELECT case DEPARTMENT when 'Banking' then 'Bank Dept' when 'Insurance' then 'Insurance Dept' when 'Services' then 'Services Dept' end FROM EMPLOYEE

Explanation : Here "DECODE" keyword is used to specify the alias name. In oracle we had specify, Column Name followed by Actual Name and Alias Name as arguments. In SQL Server and MySQL, we can use the earlier switch case statements for alias names.




6. Delete employee data from employee table who got incentives in incentive table

delete from EMPLOYEE where EMPLOYEE_ID in (select EMPLOYEE_REF_ID from INCENTIVES)

Explanation : Trick about this question is that we can't delete data from a table based on some condition in another table by joining them. Here to delete multiple entries from EMPLOYEE table, we need to use Subquery. Entries will get deleted based on the result of Subquery.




7. Insert into employee table Last Name with " ' " (Single Quote - Special Character)

Use another single quote before special character

Insert into employee (LAST_NAME) values ('Test''')




8. Select Last Name from employee table which contain only numbers

Select * from EMPLOYEE where lower(LAST_NAME)=upper(LAST_NAME)

Explanation : In order to achieve the desired result, we use "ASCII" property of the database. If we get results for a column using Lower and Upper commands, ASCII of both results will be same for numbers. If there is any alphabets in the column, results will differ.




9. Write a query to rank employees based on their incentives for a month

select FIRST_NAME,INCENTIVE_AMOUNT,DENSE_RANK() OVER (PARTITION BY INCENTIVE_DATE ORDER BY INCENTIVE_AMOUNT DESC) AS Rank from EMPLOYEE a, INCENTIVES b where a.EMPLOYEE_ID=b.EMPLOYEE_REF_ID

Explanation : In order to rank employees based on their rank for a month, "DENSE_RANK" keyword is used. Here partition by keyword helps us to sort the column with which filtering is done. Rank is provided to the column specified in the order by statement. The above query ranks employees with respect to their incentives for a given month.




10. Update incentive table where employee name is 'John'

update INCENTIVES set INCENTIVE_AMOUNT='9000' where EMPLOYEE_REF_ID=(select EMPLOYEE_ID from EMPLOYEE where FIRST_NAME='John' )

Explanation : We need to join Employee and Incentive Table for updating the incentive amount. But for update statement joining query wont work. We need to use sub query to update the data in the incentive table. SQL Query is as shown below.




"SQL Join" Interview Questions:




1. Select first_name, incentive amount from employee and incentives table for those employees who have incentives



Select FIRST_NAME,INCENTIVE_AMOUNT from employee a inner join incentives B on A.EMPLOYEE_ID=B.EMPLOYEE_REF_ID




2. Select first_name, incentive amount from employee and incentives table for those employees who have incentives and incentive amount greater than 3000

Select FIRST_NAME,INCENTIVE_AMOUNT from employee a inner join incentives B on A.EMPLOYEE_ID=B.EMPLOYEE_REF_ID and INCENTIVE_AMOUNT >3000



3. Select first_name, incentive amount from employee and incentives table for all employes even if they didn't get incentives

Select FIRST_NAME,INCENTIVE_AMOUNT from employee a left join incentives B on A.EMPLOYEE_ID=B.EMPLOYEE_REF_ID



SQL Interview Questions on "SQL Table Scripts":




1. Write create table syntax for employee table

CREATE TABLE EMPLOYEE (

EMPLOYEE_ID NUMBER,

FIRST_NAME VARCHAR2(20 BYTE),

LAST_NAME VARCHAR2(20 BYTE),

SALARY FLOAT(126),

JOINING_DATE TIMESTAMP (6) DEFAULT sysdate,

DEPARTMENT VARCHAR2(30 BYTE) )

SQL Server -CREATE TABLE EMPLOYEE(

EMPLOYEE_ID int NOT NULL,

FIRST_NAME varchar(50) NULL,

LAST_NAME varchar(50) NULL,

SALARY decimal(18, 0) NULL,

JOINING_DATE datetime2(7) default getdate(),

DEPARTMENT varchar(50) NULL)



2. Write syntax to delete table employee



DROP table employee;



3. Write syntax to set EMPLOYEE_ID as primary key in employee table

ALTER TABLE EMPLOYEE add CONSTRAINT EMPLOYEE_PK PRIMARY KEY(EMPLOYEE_ID)




4. Write syntax to set 2 fields(EMPLOYEE_ID,FIRST_NAME) as primary key in employee table

ALTER TABLE EMPLOYEE add CONSTRAINT EMPLOYEE_PK PRIMARY KEY(EMPLOYEE_ID,FIRST_NAME)




5. Write syntax to drop primary key on employee table

Alter TABLE EMPLOYEE drop CONSTRAINT EMPLOYEE_PK;




6. Write Sql Syntax to create EMPLOYEE_REF_ID in INCENTIVES table as foreign key with respect to EMPLOYEE_ID in employee table

ALTER TABLE INCENTIVES ADD CONSTRAINT INCENTIVES_FK FOREIGN KEY (EMPLOYEE_REF_ID) REFERENCES EMPLOYEE(EMPLOYEE_ID)



7. Write SQL to drop foreign key on employee table

ALTER TABLE INCENTIVES drop CONSTRAINT INCENTIVES_FK;



8. Write SQL to create Orcale Sequence

CREATE SEQUENCE EMPLOYEE_ID_SEQ START WITH 0 NOMAXVALUE MINVALUE 0 NOCYCLE NOCACHE NOORDER;




9. Write Sql syntax to create Oracle Trigger before insert of each row in employee table

CREATE OR REPLACE TRIGGER EMPLOYEE_ROW_ID_TRIGGER

BEFORE INSERT ON EMPLOYEE FOR EACH ROW

DECLARE

seq_no number(12);

BEGIN

select EMPLOYEE_ID_SEQ.nextval into seq_no from dual ;

:new EMPLOYEE_ID :=seq_no;

END;

SHOW ERRORS;



10. What is SQL Injection ?

SQL Injection is one of the the techniques uses by hackers to hack a website by injecting SQL commands in data fields.



"SQL Union" Query Interview Questions:

1. Select First_Name,LAST_NAME from employee table as separate rows

select FIRST_NAME from EMPLOYEE union select LAST_NAME from EMPLOYEE



2. What is the difference between UNION and UNION ALL ?

Both UNION and UNION ALL is used to select information from structurally similar tables. That means corresponding columns specified in the union should have same data type. For example, in the above query, if FIRST_NAME is DOUBLE and LAST_NAME is STRING above query wont work. Since the data type of both the columns are VARCHAR, union is made possible. Difference between UNION and UNION ALL is that , UNION query return only distinct values.