Advanced SQL Queries Interview Questions and Answers
49. 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.
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.
select EMPLOYEE_ID from EMPLOYEE
select EMPLOYEE_REF_ID from INCENTIVES
SELECT FIRST_NAME, CASE FIRST_NAME WHEN 'John' THEN SALARY * .2 WHEN 'Roy' THEN SALARY * .10 ELSE SALARY * .15 END "Deduced_Amount" FROM EMPLOYEE
53. 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.
54. 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.
55. Insert into employee table Last Name with " ' " (Single Quote - Special Character)
Tip - Use another single quote before special character
Insert into employee (LAST_NAME) values ('Test''')
56. 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.
57. 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.
58. 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.