SELECTing only some of the columns

In SQL, SELECT keyword is used to select / all some of the columns.

Dataset:

For this tutorial we shall make use of employee_performance:

CREATE TABLE employee_performance(
employee_id int,
department varchar(20),
education_level varchar(20) ,
gender char(1),
no_of_trainings int,
age int,
previous_year_rating numeric(2,1),
length_of_service int,
KPIs_met char(1),
avg_training_score numeric(5,2),
promoted_or_not varchar(3))

INSERT INTO
employee_performance(employee_id,department,education_level,gender,no_of_trainings,age,previous_year_rating,length_of_service,KPIs_met,avg_training_score,promoted_or_not)
VALUES
(1001,'Marketing','Graduate','M',2,24,NULL,1,'Y',69.5,'N'),
(1002,'Analytics','Post-Graduate','M',5,32,4.5,7,'Y',89.5,'Y'),
(1003,'R&D','Graduate','F',5,44,5,10,'Y',87,'Y'),
(1004,'HR','Graduate','M',1,32,3,3,'Y',54,'N'),
(1005,'Marketing','PhD','M',0,38,4,7,'N',79.5,'N'),
(1006,'IT','Graduate','F',2,23,NULL,0,'Y',83.5,'N'),
(1007,'Analytics','Post-Graduate','M',2,28,5,4,'Y',78.5,'Y'),
(1008,'Sales','Graduate','F',5,22,NULL,0,'Y',69.5,'N'),
(1009,'Marketing','PhD','F',2,48,3,7,'Y',98,'Y'),
(10010,'Marketing','PhD','M',10,39,5,4,'Y',71,'N'),
(10011,'Finance','Post-Graduate','M',3,35,3.5,10,'N',55,'N'),
(10012,'Sales','Post-Graduate','F',2,55,4,19,'Y',87,'Y'),
(10013,'HR','Graduate','M',0,48,2.5,21,'Y',72.5,'N'),
(10014,'IT','Graduate','F',2,24,3.5,12,'Y',64,'N');

Viewing all the columns

A * along with SELECT keyword represents retrieving all the columns.

SELECT * FROM employee_performance;

Selecting only specific columns

SELECT employee_id,department FROM employee_performance;

SELECTing DISTINCT values from specific columns

In SQL, DISTINCT keywords helps in ascertaining unique records from our data. Since our data had only 7 unique departments thus resultant query leads to 7 rows.

SELECT DISTINCT department FROM employee_performance;

SELECTing DISTINCT values on the basis of multiple columns:

To get unique values on the basis of multiple columns, all the column names combinations for which unique values are needed follow the DISTINCT keyword.

SELECT DISTINCT department,education_level FROM employee_performance;