Primary and Foreign Key in SQL

Syntax to set EMPLOYEE_ID  as primary key in employee table.

ALTER TABLE EMPLOYEE add CONSTRAINT EMPLOYEE_PK PRIMARY KEY(EMPLOYEE_ID)

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)

Syntax to drop primary key on employee table.

ALTER TABLE EMPLOYEE drop CONSTRAINT EMPLOYEE_PK

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)

Syntax to drop foreign key on employee table.

ALTER TABLE INCENTIVES drop CONSTRAINT INCENTIVES_FK

Top N Salary – SQL

Query to select TOP 2 salary from employee table.

select * from employee order by salary desc limit 2

Query to select TOP N salary from employee table.

select * from employee order by salary desc limit N

Query to select 2nd Highest salary from employee table.

select min(SALARY) from 
(select * from employee order by salary desc limit 2) a

Query to select Nth Highest salary from employee table.

select min(SALARY) from 
(select * from employee order by salary desc limit N) a

Query to select First_Name, Last_Name from employee table as separate rows.

select FIRST_NAME from EMPLOYEE union select LAST_NAME from EMPLOYEE

Joins in SQL

Query to 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

Query to 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

Query to select first_name, incentive amount from employee and incentives table for all employees 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

Query to select first_name, incentive amount from employee and incentives table for all employees even if they didn’t get incentives and set incentive amount as 0 for those employees who didn’t get incentives.

select FIRST_NAME, IFNULL(INCENTIVE_AMOUNT,0) from employee A left join incentives B on A.EMPLOYEE_ID=B.EMPLOYEE_REF_ID

Query to select first_name, incentive amount from employee and incentives table for all employees who got incentives using right join.

select FIRST_NAME, IFNULL(INCENTIVE_AMOUNT,0) from employee A right join incentives B on A.EMPLOYEE_ID=B.EMPLOYEE_REF_ID

Query to select max incentive with respect to employee from employee and incentives table using sub query.

select DEPARTMENT,(select IFNULL(max(INCENTIVE_AMOUNT),0) from INCENTIVES where EMPLOYEE_REF_ID=EMPLOYEE_ID) Max_incentive from EMPLOYEE

Useful SQL Queries – 1

Query to select employee details from employee table if data exists in incentive table.

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

Query to fetch data that are common in two query results.

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

Query to 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

Query to 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*.1
ELSE SALARY*.15
END "Deducted_Amount" 
from EMPLOYEE

Query to select Banking as ‘Bank Dept’, Insurance as ‘Insurance Dept’ and Services as ‘Services Dept’ from employee table.

select case DEPARTMENT
when 'Banking' then 'Bank Dept'
when 'Insurance' then 'Insurance Dept'
when 'Services' then 'Services Dept'
end 
from employee

Query to 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)

Insert into employee table Last Name with ” ‘ ” ( Single Quote – Special Character)

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

Query to select last name from employee table which contain only numbers.

select * from employee where 
lower(LAST_NAME)=upper(LAST_NAME)

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

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')